Assessment of dam-induced changes in ecogeomorphological behaviour and fluvial functionality in the Damodar River, West Bengal, India

There has been a long-running debate pertaining to the commencement of the Anthropocene and whether to include it in the geological timescale or not (Monastersky 2015). Aside from this, researchers from various fields of study agree that human actions (both directly and indirectly) place significant signals of human dominance as a geological agent on every sphere of the Earth's atmosphere. With increasing anthropogenic pressure on the natural environment, an urge for integrating different disciplines has often been seen among researchers for conserving the environment (Thmas & Parsons 2002). As a part of environmental conservation philosophy, the fluvial systems, mainly river basins, were being started thinking from an ecosystem perspective as well as from an aspect of eco-geomorphology (Thmas & Parsons 2002; Tare et al. 2017). Here, apart from the physical, chemical, and ecological components, the longitudinal, lateral, and vertical components of a river and its floodplain were also considered as components useful for conserving the river ecosystem (Ward 1989).

Considering only the natural flow arteries of water, the direct impoundments within its regime (transverse intervention) during the latter half of the 20th century jumped up to a certain high level where each day registered two newly constructed dams that started functioning (Richter & Thomas 2007). Although these dams and other river water storage infrastructure have been contributing largely to society in terms of substantial benefits, it has come at a great cost of degrading geomorphic, hydrological, and ecological health (Resh et al. 1988). The immediate and long-term effect of dam construction is that the river becomes unstable to maintain hydrogeomorphic equilibrium with time for that human-induced threshold (Ghosh & Guchhait 2014a). The loss of geomorphic connectivity due to the construction of dams and barrages between the upstream and the downstream parts of a fluvial system has largely affected ecology (both channel and riparian) (Ligon et al. 1995; Kingsford 2000; Schmutz & Moog 2018), sediment budgeting (Vinh et al. 2014), water chemistry (Zhong & Power 1996), and flow pulses and floodplain processes (Graf 2006; Wiejaczka et al. 2014; Biswas & Pani 2021), while converting a natural hydrological regime to a regulated one. As a cumulative term, it alters or modifies the functioning of the physical entity.

The Damodar, a tropical river, flowing over the Rarh plains in the eastern part of India, bears a century-long history of human regulation (both transverse and lateral). Primarily, the human impoundments were aimed mainly at flood control and agricultural development (Bhattacharyya 2011), but later, a comprehensive enactment of river-related planning in the Damodar River Basin was introduced to develop its vast depository of resources in an integrated way. It was a comprehensive plan aimed at multipurpose management where the trunk channel and its tributaries were tamed and regulated for utilizing its water resources. This large-scale taming was done using river training engineering structures such as dams, barrages, weirs, sluices, and embankments. The works of Ghosh & Guchhait (2014b, 2016) and Singh et al. (2020) show significant alterations in the flow condition of Damodar and a shift from its morphological stability as a direct consequence of such impoundments.

Although scientific research on the impact of dams and barrages on the fluvial environment started increasing simultaneously with an increase in the number of dams and barrages getting installed globally, only since the late 1990s have several researchers started focusing on the ecological impact of these structures (Opperman et al. 2019). Several works have indicated the degradation of ecological health and ecosystem services due to dam construction as it regulates the extreme flow condition (both low and high). This, in turn, veers the system away from its naturality, both geomorphologically and in terms of ecosystem services. The deviations in the fluvial processes of the Damodar River from its naturality in terms of its geomorphological and ecological functionality are one of the less-studied aspects calling for a comprehensive understanding. A venture of this sort requires such a methodological outlook where the extent and intensity of naturality loss could be grasped through the integration of morphological, sedimentological, hydrological, and ecological characteristics existing within the floodplain and active channel belt. In this study, the fluvial process and its response to direct channel impoundment have been assumed to be existing in close proximity with the ecosystem services and riverine ecology. Thus, it integrates floodplain development and the nature of ongoing fluvial processes with the ecological characteristics of the active channel belt.

The Damodar River is one of most important rivers in the eastern Indian states of Jharkhand and West Bengal, covering the terrain of the eastern Chhotanagpur Plateau, the lateritic Rarh plain and Quaternary to Recent alluvial floodplains of the Bengal Basin (Figure 1). The Damodar River Basin (DRB) is the sub-basin of the river Ganges, having a funnel-shaped basin. The total catchment area of this funnel-shaped basin is about 23,371 km², covering the states of Jharkhand (73.7%) and West Bengal (26.3%) (Majumder et al. 2010). The Damodar and its main tributary, the Barakar, cover about 40% area of the Damodar valley region, which is the core area of the Damodar Valley Corporation (DVC). The Barakar joins the Damodar after traversing about 241 km. The other tributaries and sub-tributaries of this river are Konar, Bokaro, Haharo, Jamunia, Ghari, Guaia, Khadia, and Bhera. Once the Damodar River was known as the sorrow of Bengal, as several devastating floods occurred in the lower Damodar Basin due to torrential rainfall (<1,300 mm year⁻¹) and siltation in the channel area, but now the ‘river of sorrow’ has turned into a ‘river of agony’ due to escalating human interventions and pressure and environmental degradation in the Anthropocene (see section 4.1.3). It is a major rainfed river of eastern India, influenced by south-west monsoonal regimes and tropical cyclones. This rainfed river rises from the Khamarpet hill near Chandwa village in Latehar district of Jharkhand approximately at 23 °37′ N latitude and 84 °41′ E longitude (Sen 1985; Chandra 2003; Bhattacharyya 2011; Ghosh 2014). The total length of this river is about 592 km, and the average discharge rate of this river is about 296 m³/s (measured at Rhondia Weir, Purba Barddhaman). It is one of the most polluted rivers in India, as the basin is home to heavy industries, mining activity, and populated regions (Senapati et al. 2020).

The geographical extension of this basin is between 22°15′ and 24°30′ N latitude and 84°30′ and 88°15′ E longitude. According to Ghosh et al. (2016), the entire Damodar valley is divided into three segments, namely, the upper, middle, and lower valleys, on the basis of the gradient of the river. The river slope is about 1.89 m km⁻¹ in the first 241.35 km, about 0.568 m km⁻¹ in the next 160.9 km, and about 0.189 m km⁻¹ in the last 144.8 km. The undulating upper plateau terrain and the middle valleys are wider than the flat lower alluvial valley. The Damodar River in its upper reaches flows over a plateau followed by a flat alluvial plain in the southeast and eastward toward the Bay of Bengal. The region is richly endowed with varied mineral resources. The upper and middle catchment areas cover about 4/5th of the total catchment area, which is a hilly terrain with a steep slope, while the lower valley is narrow, gently dipped, and flat. During heavy monsoonal rain in the upper valley, the river has a natural tendency of overflowing the excess runoff water in the lower alluvial plain. Four key dams (concrete gravity and composite earth – concrete dams; height range 30.18–47.85 m) are functional to regulate water flow in the fluvial system of Damodar: (1) Tilaiya on the Barakar River (Koderma, Jharkhand), (2) Konar on the Konar River (Hazaribagh, Jharkhand), (3) Maithon on the Barakar River (Dhanbad, Jharkhand), and (4) Panchet on the Damodar River (Dhanbad, Jharkhand). Another dam (55 m height), Tenughat, on the Damodar River, was built in 1978 to control the runoff volume of the upper catchment. The flood management capacity of these dams varies from 38 to 434 million m³ and the storage level of the reservoirs ranges between 365 and 6,777 million m³. At Durgapur, a barrage is functional, having 64 gates and a length of 692 m, to maintain flood flows and irrigational canal flows in the downstream floodplains of Purba Bardhaman district.

Quaternary to recent floodplains and the channel bed of the Damodar River (West Bengal, India) have been gradually developed by the complex interactions of fluvial hydrogeomorphic processes and anthropogenic processes, but their characteristics and genetic evolution are essentially the product of stream power, channel dimensions, and sediment character (Ghosh 2016). Empirical observation of these processes and landforms is an approach by which geomorphologists practice their knowledge and use their experience to identify the assemblage of landforms or planform features that make up rivers, develop hypotheses to interpret the processes responsible for those landforms, determine how those features have adjusted and changed over time, and, finally, place this understanding in its spatial and temporal context (Fryirs & Brierley 2013; Ghosh 2016). In this context, geomatics is applied to deal with the quantitative analysis of channel dimensions and the geomorphic classifications of streams and floodplains with the help of GIS (the geographic information system) and remote sensing software (i.e., Erdas Imagine 2014, ArcMap 10.4, and Global Mapper 21.0) and Google Earth Pro Engine. The basic spatial information of the Damodar Basin was collected from the topographic sheets of scale RF 1:50,000 (i.e., 73 M/5, M/7, M/11, M/12, M/15, and M/16). The main satellite image information (WGS84 datum and UTM projection) was gathered and processed from the multispectral bands of Landsat MSS (collected from https://earthexplorer.usgs.gov/; path/row – 149/44; date of image: 22-02-1973), IRS Resources at 2 LISS III (https://bhuvan-app3.nrsc.gov.in/data/download/index.php; path/row – 106/55, and 177/55, date of image: 06-01-2019) and Sentinal 2A (collected from https://earthexplorer.usgs.gov/; tile number – T45QWF and T45QWG; date of image: 06-08-2021). The digital elevation model (DEM) was created using SRTM 30 m (collected from https://earthexplorer.usgs.gov/; date: 23-09-2014) and Cartosat-1 10 m version 2 (collected from the data repository of NRSC-ISRO, Hyderabad; date: 27-12-2014) for an analysis of the channel dimensions. The present study uses the digital elevation model (DEM, 10 m resolution) of Cartosat-1 (IRS P5 satellite) which has elevation accuracy of ±1.98 m in the alluvial plains of India (Agarwal et al. 2020). For the study of daily, monthly, and annual discharge variability (stations: Durgapur Barrage and Rhondia Weir), the pre-dam and post-dam flow data were collected from the works of Bhattacharyya (2011), Verma et al. (2015, 2017), Ghosh & Guchhait (2016), and Mitra & Singh (2018).

The genetic classification of natural channels and floodplains is an essential geomorphic tool for the analysis of process-form dynamic interrelationship in different spatial scale, and the basic belief is that the fluvial forms imply processes” (Buffington & Montgomery 2013). The classification scheme is used to simplify the complex river continuum of fluvial processes and conditions within a landscape unit by identifying places that function in a similar manner. In this context, the main goal is to classify channels and floodplains for predicting the response to anthropogenic disturbance (i.e., dams and barrage, hindrance in organism movement, longitudinal delinking of the micro-fluvial system, and discontinuation of the riverine ecosystem) and designing a river-style framework. The spatial scale of the study varies from 10¹ m (reach system) to 10³ m (stream system), having an average time frame of 50 years (1970–2021). The sample survey reach of Damodar is 47 km long from Mejia (23° 34′ 39.16″ N, 87° 06′ 42.16″ E) and Rhondia (23° 22′ 09.35″ N, 87° 28′ 17.29″ E), having a river impoundment of barrages at the middle. The riverbed elevation difference between two extreme points is 23 m, and the estimated average channel gradient is 0.489 m km⁻¹. One of the most widely used hierarchical channel classification schemes, developed by Rosgen (1994), is applied here taking into consideration of physical processes (e.g., bed load transport, bank erosion, etc.), morphology (e.g., the sinuosity index (SI), the entrenchment ratio (ER), the width–depth (W/D) ratio, etc.), and biological inventories (e.g., vegetation, aquatic organisms, etc.) (USDA 2007; Buffington & Mongomery 2013; Meehan & O'Brien 2019). In this scheme, the channels are subdivided into 94 minor channel types as a function of slope and grain size. In short, Rosgen classification generates four levels of channels – (a) Level I geomorphic characterization (stream types ‘A through G’); (b) Level II morphological description (stream types ‘A1–A6’ through ‘G1–G6’); (c) Level III stream ‘state’ or condition; and (d) Level IV validation level. Another genetic classification scheme, developed by Nanson & Croke (1992), is applied to recognize the floodplain type, considering the hypothesis – the floodplains are formed by a complex interaction of fluvial processes, but character and evolution are essentially the products of stream power and sediment character. Based on specific stream power, sediment type, erosion and deposition processes, landforms, channel planform, and environment, 15 types of floodplains can be identified. Both classification schemes of Damodar serve three purposes of the Anthropocene River – (a) prediction of the river's behaviour from its appearance; (b) development of specific hydraulic and sediment relations for a given morphological channel type and state; and (c) getting an idea of river metamorphosis.

The key channel dimensions, namely, cross-section and longitudinal profile, are measured both manually and digitally using survey instruments and GIS. The cross sections of river and short long profile of river bed were measured using the Leica Sprinter 250 m laser level survey kit to record reduced level (RL) elevations and distances across the entire valley (Figure 2), specifically identifying the edge of the terrace, flood-prone area, bankfull height and thalweg (Figure 3). Using geomatics, the cross-profile of the channel is derived from the Cartosat-1 10 m DEM with Global Mapper 21 3D path profile tool settings (Figures 3 and 4). The elevation corridor type is set to keep the maximum elevation perpendicular (the sample number along the corridor is maximum, so that it captures the full 10 m resolution of DEM), and the distance from the path is set to a large distance of 1,000 m. The elevation profile dataset is transformed into a grid format, and the data are exported to MIKE 21 hydrologic software for estimating the mean cross-sectional area of the channel (A_bc, assuming that the channel follows the form of a trapezoid). In each study unit (i.e., sample reach), five cross-profiles are prepared to obtain an average value of area, and the volumetric flow rate (Q_max) of the trapezoid channel is calculated by using MIKE 21 software. The value of Q_max is measured at the bankfull stage, which is the elevation of the floodplain adjacent to the active channel, and the value reflects the maximum flood flow (i.e., the maximum carrying capacity) passing through the channel. The reach is morphologically analyzed in two parts (taking nine sections) – (a) upstream of the Durgapur Barrage reservoir (0–25.09 km) and (b) downstream of the Durgapur Barrage reservoir (25.09–47 km) to trace the impact of flow regulation and sediment load variation. In each section (S1–S9), the following morphological parameters are estimated using Cartosat 110 m DEM and Sentinal 2A image (10 m resolution) in the GIS platform of ArcMap 10.4 and Global Mapper 21: (1) maximum channel depth (D_max), (2) average channel depth (D_avg), (3) bankfull channel width (W_b), (4) channel bottom width (W_bt), (5) flood-prone width (W_f), (6) channel cross-sectional area at the bankfull stage (A_bc), (7) hydraulic radius at the bankfull stage (R), (8) channel W/D ratio, (9) ER, (10) SI, (11) braid-channel ratio (BR), and (12) channel gradient(s) (Table 1).

The methodology of the fluvial functionality index (FFI) contains a holistic approach (Siligardi & Cappelletti 2006; Siligardi 2007; Roy & Majumder 2017; Roy 2018; Roy et al. 2018) to include key environmental factors (biotic association, water quality, biological diversity, anthropogenic impact, etc.) as well as hydrogeomorphic factors (floodplain width, bed roughness, flow variability, etc.) for the restoration of the fluvial ecosystem and riparian flora and fauna that are in harmony with humans. The FFI report card is applied in nine channel sections or units of the Damodar River, from Mejia to Rhondia (47 km stretch), to characterize the expected ecological functioning level using current Sentinal 2A image (7th June, 2021), Google EarthPro Engine and field database. The field survey was conducted from 10th to 13th February, 2021, considering the right and left floodplains of each unit from Mejia to Rhondia. The final report card comprises 14 pertinent questions concerning the hydrogeomorphic and ecohydrological characteristics of the channels, which may be controlled or influenced by the Durgapur Barrage and the surrounding urban–industrial complex (Table 2). In the FFI report card, each of the questions’ replies is assigned weighted numbers grouped into four classes (a maximum score of 40 and a minimum score of 1) that reflect the functional differences between individual answers (Siligardi 2007). The FFI score obtained by adding the partial scores relevant to each question can range from a smallest value of 14 to a maximum 300 (Siligardi 2007). The final score of each reach is matched with the five functionality levels (having conventional colour codes), expressed as I (best situation) to V (worst situation). After this, it is necessary to have a thematic outlook of the FFI score to get a holistic perspective of fluvial functional zones under the current conditions. The FFI can be used as an instrument for the scale management of a river stretch within the domain of landscape ecology, which focuses mainly on ecological structure (spatial relations between distinct ecosystems), function (interaction between spatial elements such as energy flows, materials, and individuals), and change (alteration in structures and function of ecomosaic) (Siligardi & Cappelletti 2006).

The main desktop methods of environmental flow assessment (EFA) encompass the Tennant method of mean annual runoff (MAR) at a site (a threshold of 10% MAR is a prerequisite for an aquatic ecosystem), the range of the variability approach with 32 hydrological parameters, the flow duration curve (FDC) (environmental flow referred to 95 percentiles on the FDC), and a desktop reserve model (the measuring level of ecological protection and scoring system) (Smakhtin & Anputhas 2007). In this study, FDC is selected to design the environmental flow in the Damodar River, which has extreme low flows during the lean period (December to May), and the ecosystem manages the severity of flow (from high to low flows) very well. The method of FDC is adapted from the works of Verma et al. (2015, 2017) and Mitra & Singh (2018). FDC is a graphical representation of the observed historical variation of stream flows with different time resolutions, namely, daily, weekly, monthly, and seasonal at the site that show that the percent of time-specified discharges will be equaled or exceeded over different time scales of interest (Verma et al. 2017). Two main approaches of FDC are applied here: (a) the period of record FDC by inserting the cumulative density function of different 1-, 7-, 30-, and 60-day time-series data, a unique ranking number m to each flow (total n), and the corresponding probability P of exceeding individual i (P_i = m 100/n + 1) in a log-normal plot, and (b) a stochastic FDC is used by daily discharge or every 5% probability of exceedance, estimating the FDC of the 1-, 2-, 5-. 10-, 20-, 50-, and 100-year return periods and also estimating the probability value of exceedance equal to 95% (Q95) (Verma et al. 2015, 2017).

It is expected that the alluvial river, Damodar, exists as a continuum state of interlinked processes of physical and biological components at a variable time-scale frame on the transfers (longitudinal, lateral, and vertical distribution) of energy, material, and biota, but anthropogenic interventions (like dams) can break the state of continuum for a certain period, promoting disruption mainly in longitudinal connectivity (Casado 2013). Schumm (1969) elucidated the fluvial metamorphosis in response to a disruption in the equilibrium of the fluvial hydrosystem. River metamorphosis can be referred to as the expectable transformation or alternation of channel patterns, floodplain landforms, annual discharge variability, sediment type, and load due to either anthropogenic activities or past climatic changes (Schumm, 1985; Miller & Miller 2007). Schumm (1985) also explained that fluvial metamorphosis can be attributed to sediment and water detention in the reservoirs, and it can change four elements of a river: (a) the ability of sediment transport to downstream, (b) sediment amount available to transport, (c) quality of running water, and (d) hindarance of organism movement along the channel (Casado 2013). In this case, the impressive works of Brandt (2000), Graf (2006) and Petts & Gurnell (2005) provide an exhaustive information on the upstream and downstream effects of river impoundment. With reference to the dam-controlled Damodar River, more than one possibility of metamorphosis in the alluvial river is expected: (a) the straight channel changing to become sinuous or braided, (2) the braided channel changing to become straight or meandering, and (3) the meandering channel changing to become straight or braided. This assessment is performed here to know the present state of metamorphosis observed in the Damodar River, mainly controlled by the Durgapur Barrage Reservoir.

An analysis of channel dimension results (Table 3) reveals that the upstream part (t) of the Durgapur Barrage reflects relatively high in-channel sedimentation, developing numerous mid-channels and point bars and islands (Figure 3). The value of D_max increases from 7.070 to 10.568 m at the barrage, and it escalates downstream up to 11.938 m, which may be attributed to fluvial incision of low sediment load peak discharge from the barrage. The mean channel gradient (s) of the upstream section is 0.0318%, and it escalates to 0.0412% at the downstream section. The flood-prone width (W_f) of the Damodar River is quite low at the upstream reaches (836–4,283 m), but the downstream W_f ranges in between 3,135 and 4,806 m, showing a wider river oscillation within wider floodplains (Figure 5). The W/D ratio, by Rosgen (1994), is defined as the ratio of W_f to the maximum channel depth of the bankfull stage (D_max). The W/D is the key parameter to realize the distribution of the available kinetic energy within an alluvial channel and the ability of various discharges occurring within the channel to move sediments. A high W/D indicates a broad valley with fluvial terraces, an abundance of sediment supply, a slightly entrenched channel, and an active lateral adjustment. W/D varies from 300 (upstream) to below 150 (downstream), and it signifies a high bankfull width compared with channel depth at the upstream sections due to impoundment of the barrage (Table 3). This can be attributed to the variable character of sediment sizes in the channel.

Entrenchment is the vertical containment of a river to understand the tendency of valley incision within the wider floodplain (like entrenched meander) (Rosgen 1994). In general, high entrenchment has negative impacts (observed in the study area), namely, accelerated bank erosion, land loss, loss of aquatic habitat, loss of land productivity, lowering of the water table, and sedimentation of the river downstream. The ER, by Rosgen (1994), is the ratio of the flood-prone width to the surface width of the bankfull channel (measured at twice the maximum depth of the bankfull channel). It is observed from the Sentinal 2A image (2021) that at the upstream of the barrage, the ER varies widely from 1.202 to 2.862 (mean 1.650), which signifies a highly to moderately entrenched channel in a well-developed floodplain. Significantly, the river turns into a slightly entrenched channel below the barrage because the value varies from 1.810 to 2.497 (mean 2.275). Surprisingly, the river sinuosity does not vary too much in the study area, i.e., a SI (Friend & Sinha 2013) range of 1.036–1.135 (tending more to the straight channel). The strong topographic linearity of Damodar may be attributed to two factors: (a) the river follows a W–E lineament of the Bengal Basin crossing three prominent basement faults (namely, Chhotanagpur Foothill Fault, Khandaghosh-Garhmayna Fault, and Pingla Fault) (Ghosh 2022) and (b) the dam-controlled river is jacketed by elevated embankments at both banks to stop oscillation in the floodplain (i.e., anthropogenic confinement) (Bhattacharyya 2011). The braiding nature of the river reflects a threshold level of sediment load or slope to maintain a steep gradient throughout the long profile. The high tendency of braiding means an abundant supply of sediments from the upstream, flow blockage by dam impoundment, in-channel sedimentation (low flushing due to damming), and rapid and infrequent discharge. The braid-channel ratio (BR), by Friend & Sinha (2013), reflects a typical pattern in the Damodar valley. The BR is estimated as 2.173 at the initial survey section (S1), and it escalates up to 4.316 at the Durgapur Barrage section (S5) due to the development of more linguoid bars and islands. Below the barrage, the BR drops to 1.811 up to a 10- km stretch, and then, it again increases up to 3.525. The main reason is the fluvial incision and channel narrowing at just the downstream of the barrage, and further, the eroded sediments are deposited in the downstream channel.

The bankfull cross-sectional area (A_bc) (assuming the channel geometry as a trapezoid form), mass flow rate, and bankfull discharge are estimated using DEM and MIKE 21 hydrological software in nine reach sections (taking five cross-section samples from each reach). The parameter A_bc is estimated at the bankfull stage to get an idea of flood discharge variability at the maximum level (Q_max). At the upstream of the barrage, the value of A_bc varies highly from 2,835 to 19,957 m² (mean 9,345 m²), and at downstream, the value ranges in between 5,674 and 13,091 m² (mean 9,267 m²). The Manning's roughness coefficient (n_c) of the lower Damodar River is estimated as 0.025–0.035 by Singh et al. (2020). The estimated Q_max (i.e., potential bankfull discharge) varies widely from 4,143 to 53,155 m³/s in the upstream reaches of Damodar (having an average hydraulic radius of 2.932–7.216 m). Below the barrage, it ranges from 13,795 to 30,445 m³/s (having an average hydraulic radius of 4.820–6.572 m). In general, there is a reduction of the bankfull carrying capacity of the channel (below the barrage) to accommodate maximum flood flow during the monsoon season due to channel narrowing and terrace formation within the valley. It is observed that the minimum flow rate (during the lean season) varies from 528 to 1,287 m³/s at the upstream reaches, but due to flow regulation, it maintains in between 842 and 1,293 m³/s up to the Rhondia weir. This signifies a relatively good environmental flow, which is the minimum flow required for the sustainable maintenance of the riparian and aquatic ecosystem. The studies of Ghosh (2011), Bhattacharyya (2011), Ghosh & Guchhait (2016), Verma et al. (2017), and Karim & De (2019) revealed that four primary types of flow regulation were observed in the Damodar River, namely, (1) peak absorption (peak flows from catchment tributaries absorbed in reservoirs), (2) peak attenuation (reduced or delayed due to reservoir attenuation), (3) release manipulation (dam designed for flood control keeping the reservoir volume low), and (4) maintenance of environmental flow (3.4–31.48 m³/s) during the lean period. After the construction of DVC dams and the Durgapur Barrage reservoir (built before 70 years), the flow discharge (Table 4) became variable and reduced significantly, i.e., there was a reduction of the mean annual peak flow from 8,378 to 3,522 m³/s at the Rhondia weir. Before dam construction, the confidence limit of the annual peak discharge was recorded between 6,081 and 10,676 m³/s, but after 1958, the limit reduced to only 2,574–4,470 m³/s (Ghosh & Guchhait 2016).

The dam discharge of clean water has a strong erosion capacity, which can increase the erosion of the downstream river bed of the dam, coarsing the sediment size of the river bed rather than refining. On the contrary, the hydrodynamic force upstream of the dam is weakened due to the retention effect of the dam, resulting in the fining of sediment in the riverbed. Bed degradation is usually the most immediate channel adjustment after dam closure, although bed aggradation processes and bank stabilization by vegetation encroachment have also been widely documented (Casado 2013). Two prominent effects of the barrage are recognized: (1) at the downstream, the river becomes narrower due to the incision of sediment-free peak flow, and (2) at the upstream, the sediments are trapped in the channel to decrease the cross-sectional area and to increase the river width. The floodplain width is estimated at about 2–3 km wide at the downstream, but the active channel width is reduced to only 510–1,110 m at present (up to 14 km downstream stretch from the barrage). The previous oscillation of the active channel is compressed by the flow regulation of barrage, embankment rise, and encroachment of river islands for agriculture and settlement. The upstream section is controlled by the barrage flow obstruction and urban–industrial complex. The trapped sediments and low flow competence increase the number and size of longitudinal bars in the channel up to 17 km upstream. These elevated islands (locally known as ‘manas’) are now used for settlements and agricultural practices. The in-channel sedimentation is reflected from the increasing value of the BR (i.e., 2.173–4.316; tending more to the braided channel), and this morphological change reduces the in-channel-carrying capacity to accommodate the peak flood flow during the monsoon season.

More than half of the sediments from the controlled river basins are trapped by dams and about 25.30% of the sediments worldwide are intercepted by large dams (Bhattacharyya 2011). It is inevitable that due to unscientific coal mining, urbanization, extensive deforestation, soil erosion, and expansion of agriculture in the Chhotanagpur Plateau, the Damodar River transports an enormous amount of sand load during floods, leading to the formation of bars. The suspended sediment concentration was measured at the Damodar Bridge site (29 km downstream of the Panchet Dam), and the value decreased from 1.87 gm l⁻¹ (pre-dam) to 0.54 gm l⁻¹ (post-dam), promoting almost 72% reduction of sediment concentration in channel flow (Bhattacharyya 2011). The data reveal that the Maithon reservoir of the Barakar River lost about 27.4% of its overall storage capacity up to 2001, whereas the Panchet Reservoir of the Damodar River lost 15.9% of its overall storage capacity (WRIS 2021). In the Matithon reservior, the actual rate of deposition as stated in the plan was much greater than the designed mean siltation rate of 4.79 million m³ yr⁻¹. The siltation rate in the Maithon Reservoir will be reduced to about 1.5 million m³ yr⁻¹due to the installation of another dam on the Barakar River, Balpahari Dam (at 50 km upstream of Maithon). The estimated siltation rate of DVC reservoirs is depicted as follows (Table 5): (1) Konar – 1.743 million m³ yr⁻¹, (2) Maithon – 6.77 million m³ yr⁻¹, (3) Panchet – 6.92 million m³ yr⁻¹, (4) Tenughat – 3.210 million m³ yr⁻¹, and (5) Tilaiya – 2.748 million m³ yr⁻¹, respectively (Bhattacharyya & Singh 2019). The reduction of gross storage varies from 11.85 million m³ (1955) to 6.437 million m³ in the Durgapur Barrage Reservoir, reflecting a loss of 45.68% of live storage. At present, the average rate of siltation is 0.042 million m³ yr⁻¹ in the barrage (WRIS 2021).

The pertinent hydrogeomorphic research works of Ghosh (2011), Bhattacharyya (2011), Ghosh & Guchhait (2016), Pal et al. (2015), Verma et al. (2017), and Karim & De (2019) have revealed that the Damodar river's ability to transport the available sediment and the quality of running water simultaneously to trigger a series of adjustments until the fluvial system of the river either accommodates the anthropogenic disturbance or reaches a new equilibrium state (Figure 6). An alluvial river is a sensitive element of the Earth's surface, whereas any change or shift in external factors (e.g., climate change or tectonic upliftor river impoundment) instigates a rapid response from the fluvial system towards instability (i.e., disequilibrium) within the floodplains. It is hypothesized that the Damodar River is still situated in a phase of river instability or disequilibrium, which is generally considered to be that period during which river processes and forms readjust to changes (changes in land use and installation of dams since the 1950s) in sedimentological and hydrological regimes (Miller & Miller 2007). This situation can be considered within a framework of threshold and complex response (Schumm & Khan 1972) where the driving forces and resisting forces operating within the Damodar River were altered (due to dam constructions and the rise of embankments) to such a degree that the limits to equilibrium were exceeded. Due to this threshold event, the changing channel morphology and floodplain transformation have thrown up a scenario of river metamorphosis, which is depicted in the following paragraphs.

A threshold crossing event, namely, multipurpose large dam constructions by the DVC, occurred when the river system underwent a change from a state of natural balance to a temporary condition of disequilibrium, which is now being gradually corrected as the system develops a new state of balance adjusted to a different set of environmental conditions. After crossing the threshold, the alteration in channel forms and patterns (complex response) is rapid initially and reduces with the passage of time until a new equilibrium state is achieved, because large dams can change the local base-level conditions, forming several knickpoints in the longitudinal profile.

Base-level change promotes incision to a certain distance downstream, but the aggradation dominates further to develop terraces and bars. It is predicted that within a time frame of 100 years, changes in the lithology of sand-bed channels, bed configuration, cross-sectional form (W/D ratio), braiding or sinuosity, and channel confinement will become more evident. Within a time frame of 10⁰–10⁴ years, changes in meander wavelength, channel gradient, and profile concavity can be traced. Two prominent metamorphosis scenarios are observed in the Damodar River: (1) Downstream of the Barrage – a significant reduction of flow competence (Q⁻) and sediment load (L⁻), a dominance of fluvial incision (I⁺) overaggradation and terrace formation, an increase of slope (s⁺), an increase in depth (d⁺), a decrease in active channel width (w⁻), and an overall significant reduction in channel capacity (CC⁻); and (2) Upstream of the Barrage – a moderate reduction of flow competence (Q⁻) and unchanging sediment load (L^o), a dominance of fluvial aggradation over incision (I⁻) and longitudinal bar formation, a decrease of slope (s⁻), a decrease in depth (d⁻), an increase in active channel width (w⁺), and an overall moderate reduction in channel capacity (CC⁻).

Based on the geomorphic characterization (Level I) and morphological description (Level II), the reaches of the Damodar River can be classified (Table 6) according to the Rosgen Stream Classification Scheme (Rosgen 1994). The key parameters of Level I (e.g., channel slope, channel shape, and channel pattern) and Level II (e.g., ER, W/D ratio, SI, and channel materials) are presented in the table. Similarly, the genetic classification of the floodplain scheme of Nanson & Croke (1992) is applied to gauge the floodplain metamorphosis in relation to river impoundment. At the upstream of the Durgapur Barrage, it can be seen that the channel has transformed from A-type to B-type and then to D-type (Figure 7). After crossing the barrage, the channel has again reversed to B-type, and further downstream, it again changes to D-type. An A-type channel has the characteristics of a steep-entrenched channel with high energy/debris transport with erosional or depositional and Gondwana bedrock forms. A B-type channel is characterized as a moderately entrenched, riffle-dominated channel, with stable banks and point bars, and gently sloping valleys with occasional pools. A D-type channel is slightly entrenched with a braided pattern, longitudinal and transverse bars, a very wide channel with multiple threads, active lateral adjustment, and an abundance of sediment supply. With regard to floodplain classification, the following two sequences are observed:

1. Change from A3- to B1-type at the upstream and

2. again change from A3- to B1-type at the downstream.

An A3-type floodplain is characterized by unconfined to confined vertical accretion, a specific stream of 300–600 Wm⁻², sandy strata with inter-bedded muds, a sandy flat floodplain surface with single-thread channel wandering, occasional channel wandering, overbank vertical accretion, island deposition, and abandoned channel accretion with minor lateral accretion. A B1-type floodplain is categorized usually as a braided channel, a specific stream of 50–300 Wm⁻², floodplains with gravels, sand and silts in bed sediments, braided channel accretion and incision, overbank vertical accretion of islands and abandoned channel accretion, undulating floodplains of abandoned channels and bars, backswamps, and a relatively high sediment load. So, it can be said that the Durgapur Barrage Reservoir has managed to control the floodplain associations and characteristics of the Damodar River, changing from its intrinsically braided nature (mainly upstream of the barrage) to a single-thread entrenched channel.

The developed thematic FFI map of the Damodar River (Figure 8) enables the present status of functioning (individual stretch) to be grasped straightaway, and it can be a useful instrument for planning the reclamation of the fluvial environment. Based on the FFI evaluation, the Damodar River functionality level varies from category IV to II, i.e., poor to good–fair functionality level. The FFI score varies from 85 to 181, showing the prominent influence of transverse obstacles, eutrophication, anthropogenic bed modifications, and urban–industrial pollutants. Only, S2 section shows good–fair fluvial functionality Level II, showing a relative abundance of flora and fauna, morphological diversity of the river bed, and a proliferation of complex formations and riparian vegetation (strips greater than 30 m). A key problem is observed in the river stretch immediate to the upstream and downstream of the Durgapur Barrage reservoir, which is a very low FFI score of 61–100 (category IV poor functionality level). The observed factors of this low fluvial functionality are mainly categorized as follows: (a) a daily influx of sewage and industrial pollutant water from Durgapur–Waria–Raniganj townships, thermal power plants and heavy industries; (b) intensive agriculture in stabilized islands; (c) flow obstacles caused by the barrage sluices and linear transverse constructions; (d) sand mining with heavy machines and vehicles, and (e) the issue of reservoir siltation and in-channel sedimentation (Figure 9). At the downstream of the barrage, the river recovers its functionality to category III level (i.e., fair), but the complex formation and riparian structure are not proliferated in the sand-dominated alluvial valley mainly due to channel narrowing, discharge variability (environmental flow), and intensive in-channel sand mining and agricultural practices. The main aquatic vegetation communities of the barrage reservoir and river bed are identified as Eichhorniacrassipes, Salviniacuculata, Nelumbonucifera, Hydrillaverticillata, Ipomoea aquatic, LemnaSp, and Typhasp. The dominant floral species of a sandy river bed is Saccharumspontaneum, which blooms during the monsoon period. The surroundings of the barrage are now identified as Important Bird and Biodiversity Area (IBA) (Figure 9) by the Birdlife International (Cambridge, UK), which has a record of 253 species.

From the survey and FFI report card analysis, the key issues (the need for restoration or management) of this fluvial ecosystem are recognized as follows (Figure 9): (a) seasonal cultivation with an expansion of urban areas mainly in the left bank floodplain; (b) the absence of riparian formations but the presence of functional formations due to river bed metamorphosis, and bed morphology modifications due to frequent stream flow interruption and the loss of continuum state; (c) frequent flow disturbances with a lean flow of six dry months; (d) flood-stage river breadth occasionally being 2–3 times greater than a moderate flow river bed; (e) the absence of retention structures that enhance reed groves and hydrophytes; (f) artificial interventions largely leading to very low morphological diversity and bed stabilization in the river cross sections; (g) intensive sand mining and allied activities becoming a threat to the avian wildlife of the Damodar River; and (h) a very low density of tolerating riparian plant components, recognizable fibrous – pulpy detritus materials. The zone – in yellow (S1, S7, S8 and S9) – needs to restore the eco buffer in the perifluvial zone, taking measures to restrict the expansion of commercial activities and residential use. The zone – in orange (S3 to S6) – needs more protection from intense urbanization and decreasing ecological diversity by limiting interventions and boosting ecological quality. The results of the FFI at the reach level of the dam-controlled Damodar River provide us with the possibility of doing more research at the micro level to mitigate the impact or to reassess the quality of the fluvial environment in the era of the Anthropocene. There is a need for a fluvial zone of adequate ecological quality made up of well-established arboreal and shrubby riparian formations (acting as an eco buffer to the surrounding territory) that must be protected and properly maintained in the Damodar River.

The FDC is used to assess environmental flows to illustrate the hydrological condition of a river system (Suwal et al. 2020). It represents the proportion flow that exceeds the percentage of time at a particular river section. This exceedance curve is used to define a minimum threshold value for sustaining and maintaining riverine ecological integrity. This flow duration (Figure 10(a) and 10(b)) curve was constructed on the basis of the average monthly flow discharge data of the lean period (December to May) at Rhondia of the Damodar River during the pre-dam period (1934–1957) and the post-dam period (1958–1957). Based on the FDC, 97.8% exceedance probability was attained, and the corresponding flow value was 1 m³/s during the pre-dam period. Therefore, 1 m³/s flow was available throughout the months of the lean period during the pre-dam period. Whereas 98.8% exceedance probability is attained during the post-dam period and its corresponding flow value is 8 m³/s of flow. Hence, 8 m³/s flow is available throughout the months of the lean period during the post-dam period. So, the exceedance probability with its corresponding flow value slightly increased (from 1 to 8 m³/s) after the construction of dams and barrages.

The daily average flow at the Rhondia site of the Damodar River during the pre-dam (1940–1950) and post-dam (1993–2008) periods was analyzed to evaluate the environmental flow for making daily recommendations of flow for the aquatic environment. Based on this analysis, the flow discharge corresponding to 85% exceedance probability was 0.7 and 2 m³/s of flow in the pre- and post-dam periods, respectively. Whereas 1% exceedance probability to its corresponding flow was 3,575 and 2,460 m³/s in the pre- and post-dam periods, respectively. The corresponding flow of 1, 5, 10, 20, and 30% exceedance probability reduced, which meant that the probability of daily high flow reduced. Whereas 40, 50, 60, 70, 80, and 85% exceedance probability to its corresponding flow increased, which meant that the probability of daily low flow increased after the installation of dams and barrages. So, the variability of daily flow changed due to the regulation of flow by dams and barrages.

This flow duration (Figure 11) curve was constructed on the basis of the daily average flow discharge data at Rhondia of the Damodar River during the pre-dam period (1940–1950) and the post-dam period (1993–2008). According to the reference FDC, any shift of an FDC to the left means the loss (a part of the variability is lost) is due to the reduced quantum of monthly flows, i.e., the same flow will occur less frequently. The corresponding flow of 1, 5, 10, 20, and 30% exceedance probability reduces due to the flow regulation and the flow occurs less frequently. Whereas the corresponding flow of 40, 50, 60, 70, 80, and 85% exceedance probability increases and the flow occurs in high frequency. The FDC of the high-frequency flow (<30% exceedance probability of flows) shifts to the left and the lower-frequency flow (>40% exceedance probability of flows) curve shifts to the right after the construction of dams and barrages on the Damodar River (Figure 11). Hence, the analysis of the daily flow and FDC indicates that the variability is lost due to the reduced frequency of high flows and increased frequency of low flows in the river. So, the variability in daily flow changes due to the regulation of the flow by dams and barrages.

The concept of environmental flow suggests that a minimum flow is required to maintain and sustain the riverine ecosystem and ecology. According to the Tennat method (Table 7), a minimum (i.e., 10% of mean annual flow (MAF)) percentage of the MAF is a prerequisite for an aquatic habitat to maintain the biological integrity of the riverine ecosystem. The MAF of the pre-dam period was 1,001.33 m³/s, but due to flow regulation, it decreases to 700.04 m³/s in the post-dam period at the Rhondia site of the Lower Damodar River. The requirement of minimum flow for the riverine habitat was 100.13 m³/s in the pre-dam period, while it is 70 m³/s in the post-dam period. As the flow is regulated to maintain the minimum flow in the river system, the requirement of minimum flow for the aquatic habitat is also decreasing due to a reduced MAF. A threshold of 10% MAR for the aquatic ecosystem was not maintained in the lean season (December to May months) in the pre-dam period, whereas it is maintained in the post-dam period according to the average monthly data of the Rhondia site of the Lower Damodar River.

Most of the world's major rivers are being intensively modified (Goldenberg-Vilar et al. 2021) by water resource management tools such as dams and barrages through which an altered seasonal flow regime and peak flow discharge is obtained on a global scale. Flow discharge is a vital element that significantly impacts the aquatic habitat, river morphology, biotic life, river connectivity, and water quality (Mitra & Singh 2018). The frequency, duration, timing, and intensity of daily, monthly, and annual peak flows have a direct impact on riverine biota (Lin 2011), but a high range of variation in steam flow (Bunn & Arthington 2002) affects the habitat and aquatic biodiversity (Richter & Thomas 2007), reducing their self-purifying ability (Dudgeon 2000) and altering erosion and deposition (Verma Murthy & Tiwary 2015). The flow of the Damodar River is variable and has reduced significantly after the installation of DVC dams and barrages on it (Bhattacharyya 2011; Ghosh & Mistri 2013; Verma Murthy & Tiwary 2015). After the constructions, the mean annual peak flow has reduced from 8,378 to 3,522 m³/s at Rhondia. The alteration of flow discharge impacts physical processes (sediment transportation, bank erosion), morphology, channel structure, floodplain hydrology, and biodiversity (Krchnak & Thomas 2009; Lin 2011). The flow alteration changes the river metamorphosis as it changes the channel pattern, flood plain landforms, annual discharge variability, sediment type, and load. An analysis of the channel dimension reveals that the channel morphology has changed due to altering and diverting the flow by anthropogenic interventions. The fluvial functionality and eco-geomorphological behaviour of the Damodar River are largely influenced by flow discharge variability, and the destabilized fluvial functionality of the river impacts sustainable ecosystem functions and ecological behaviour in the active channel and floodplain. It is neccesary to mention here that a topographically controlled linearity (i.e., low SI and less degree of meadering) may be unfavourable for the prodigious growth of aquitaic flora and fauna due to high speed turbulence flow discharge during the peak monsoon period. Anthropogenic structural interventions such as dam, barrage, embankment, and weir on the Damodar River have diverted and altered the flow (Krchnak & Thomas 2009; Pal 2015; Vucijak et al. 2015), impacting the hydrological functionality and floodplain connectivity through palaeochannels and siltation of barrages and channels, resulting in the river losing its carrying capacity during bankfull discharge. Besides, other anthropogenic interventions, such as urban–industrial waste disposal with their effluents, off the bed and on the bed intensive agricultural practices with their effluents, and mining activities (like sand, coal), affect the fluvial functionality of river systems.

The FFI score indicates that the ecological functionality level of the Damodar River ranges from poor to good–fair (i.e., category IV–II) functionality level due to the effects of transverse obstacles, eutrophication, anthropogenic bed modifications, urban–industrial pollutants, mining activities, flow alteration and diversion, and agricultural and urban expansion. Although there is a spatial variation in the ecological functionality level in nine channel sections of the study area, the ecological functionality levels of the upper and immediately lower sections (S3–S6) of the Durgapur barrage are poor due to the impacts of the barrage as a fluvial response to river impoundment, the alteration flow regime (flow variability), and the other anthropogenic interferences referred to previously. The minimum flow rate in the upper reach of the Durgapur barrage ranges from 528 to 1,278 m³/s, while it ranges from 842 and 1,293 m³/s in the down reach during the lean season as flow is regulated. The altering and reserving of peak flow through the reservoir reduces the sediment load downstream by tapping and filtering sediments. The upstream sections (S3–S6) are controlled by the barrage and are influenced by the presence of the urban–industrial complex. The effect of the Durgapur barrage triggers a narrowing of the downstream channel width due to a sediment-free peak flow (more erosion), a widening of the upstream width, and a reduction of the cross-sectional area due to trapping sediments in the upstream channel and the reservoir. By trapping sediments in the in-channel, siltation reduces the mean channel gradient at the upstream. The mean channel gradient of the upstream is 0.0318%, whereas it is 0.0412% in the downstream. Many mid-channel and point bars and islands are formed due to the trapping of sediments and low flow capacity. These islands and bars (locally named as ‘mana’) are intensively used for making permanent settlements and for the purposes of intensive agriculture, thus encroaching the active part of the channel and learning to live with the constant threat of floods.

The ecological functionality level in sections S3–S6 is poor and the FFI score ranges from 61 to 100 due to flow alteration, low flow capacity, intensive agriculture on the bed, and urban–industrial effluents. The urban–industrial waste with their various intoxicants and pollutants discharges into the river in this section from the industrial and urban complex of Durgapur (De et al. 1980, 1985). Moreover, the urban–industrial effluents with numerous intoxicant pollutants discharge into the river in this section through the Nala. For example, the Tamla Nala (located immediately next to the Durgapur barrage) and the Singaran Nala (the upper Durgapur barrage) constantly contaminate the river and the barrage (De et al. 1980, 1985). The agricultural effluents with pollutants discharge directly into the river in this section as intensive agriculture is practiced on the mid-channel bar. The urban and agricultural (off the bed and on the bed) expansion with its increasing disposal and effluent discharge into the river leads to a decrease in the ecological functionality level directly or indirectly in these sections. Once upon a time, the river was known as the ‘river of sorrow’ (due to flood havoc and destruction in the past) and was a source of drinking water, but now it has turned into a ‘river of agony’ due to large-scale anthropogenic interventions and degradation of the river valley due to heavy discharge from the industrial, domestic, and mining sectors (Figure 12). From the downstream of the Panchet Dam, a 68- km-long stretch of Damodar passes through the main industrial belt of West Bengal, covering the industrial cities of Barakar, Kulti, Asansol, Raniganj, and Durgapur. In the river valley, 50 major industries and 400 subsidiary industries have been functioning since the 1990s. Among them, the following 10 industries have been identified as a major source of pollution: (1) Santaldih Thermal Power Station (STPS), (2) Bhojudih Coal Washeries, (3) Dishergarh Thermal Power Station, (4) Indian Iron and Steel Company Ltd (IISCO) Burnpur, (5) Bengal Paper Mill Co. Ltd Raniganj, (6) Alloy Steel Plant (ASP) Durgapur, (7) Durgapur Steel Plant (DSP), (8) Durgapur Chemicals Ltd (DCL), (9) Durgapur Thermal Power Station (DTPS), and (10) Hindustan Fertilizer Corporation Ltd (HFC) Durgapur (Hoque et al. 2022). It was reported that on 2nd April, 1990, about 200,000 litres of furnace oil spilled into the river water of Damodar, and it took 4 days to clear the water. Hoque et al. (2022) reported traces of environmental stress in the river because DO (dissolved oxygen) ranges from 1 to 7.9 mg l⁻¹ (critical limit 4 mg l⁻¹), and the BOD (biological oxygen demand) ranges between 3 and 48 mg l⁻¹ (critical limit 3 mg l⁻¹) between Asansol and Durgapur. The alluvial sediments of the river, between Raniganj and Panagarh, are polluted with the toxic effluents of industries (with elements like Cd, Cn, Hg, Pd, Zn, Ni, and Fe), and the water has now become unfit for drinking purposes.

The FFI has been applied to evaluate the ecological functionality level for the restoration of the riverine environment. This is expected to restore the fluvial ecosystem and ecology, which confer a lot of benefits to people. So, it can be used as a planning tool for the restoration or reclamation of the fluvial environment. An analysis of the ecological functionality level will help to restore the degraded fluvial ecosystem and riparian flora and fauna. The FFI can also be used as a management tool for assessing the ecological structure, function, and changes in the river stretch.

Structural interventions such as dams and barrages of the DVC on the Damodar river system have altered and diverted the flow regimes of this river system, which has affected eco-geomorphological behaviour and fluvial functionality. Other non-structural interventions such as urban–industrial and agricultural growth in the name of development have threatened the hydro-chemistry of the river system, which has also changed the fluvial functionality of the river system. The major findings of this study are a reduced mean annual peak flow from 8,378 to 3,522 m³/s, an increased minimum mean monthly flow discharge in the lean period (December to May) from 1 to 8 m³/s, and a loss of flow variability due to a reduction of the frequency of high flows, an increase in the frequency of low flows, and a reduced MAF from 1,001.33 to 700.04 m³/s in the post-dam period. This radical shift in the flow regimes has changed the channel morphology. It is observed that the maximum channel depth, mean channel gradient, flood-prone width, and W/D ratio are decreasing in the upstream sections, while they are increasing in the downstream section of the Durgapur barrage due to sedimentation in the upstream channel and the reservoir. The ER of the upstream ranges from 1.202 to 2.862 with a mean value of 1.65, while it is lower than the downstream (ranging from 1.810 to 2.497 with a mean value of 2.275). Although the bankfull cross-sectional area (A_bc) of the upstream varies from 2,835 to 19,957 m² with a mean value of 9,345 m², it is higher than that of the downstream section (varying from 5,674 and 13,091 m² with a mean value of 9,267 m²). The turbidity and sediment with its grain size concentration have reduced in the downstream channel, while they are increasing in the downstream channel due to trapping and filtering by the reservoir. Now, the average rate of siltation is 0.042 million m³ yr⁻¹ in the barrage.

The changing channel morphology and floodplain transformation have thrown up two prominent metamorphosis scenarios in the Damodar River: (1) Downstream of the Barrage – a significant reduction of flow competence (Q⁻) and sediment load (L⁻), a dominance of fluvial incision (I⁺) over aggradation and terrace formation, an increase of slope (s⁺), an increase in depth (d⁺), a decrease in active channel width (w⁻), and an overall significant reduction in channel capacity (CC⁻); and (2) Upstream of the Barrage – a moderate reduction of flow competence (Q⁻) and an unchanging sediment load (L^o), the dominance of fluvial aggradation over the incision (I⁻) and longitudinal bar formation, a decrease of slope (s⁻), a decrease in-depth (d⁻), an increase in active channel width (w⁺), and an overall moderate reduction in channel capacity (CC⁻).

It is observed that the Damodar river functionality level varies from poor to good–fair functionality level due to the changing eco-geomorphological behaviour of the river system through anthropogenic interventions. So, the big impacts of dams and development have changed the eco-geomorphological behaviour and fluvial functionality of the Damodar River.

This study describes the eco-geomorphological behaviour and fluvial functionality of the Damodar River by identifying the ecological functionality level that is not supportive for the sustainability of the river environment. Measuring the ecological functionality level of an aquatic system is important for protecting the aquatic ecosystem, and this is gaining more global importance, especially in developed nations. But the scenario is not so inspiring in developing nations like India. So, intensive research is needed in this regard. For example, an EFA is required for an aquatic environment. Hydrological and ecological data of a number of gauge stations are needed for assessing the environmental flow of the DRB. Recognizing the sources of diversion and alteration of flow concerning anthropogenic interventions is required. For aquatic biota, research is needed due to the alterations of flow. Besides, research may be carried out on sediment movement and deposition concerning the evolution of aquatic biota. The research should be done on these aspects in the context of West Bengal rivers as well as other rivers in the country. These aspects may also inspire other researchers and organizations from the various fields of earth sciences.

On behalf of all authors, the corresponding author states that there is no conflict of interest.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Not Applicable.

Data cannot be made publicly available; readers should contact the corresponding author for details.