Determining the extent and condition of riparian zones in drinking water supply catchments in Sarawak, Malaysia

Land cover within eight drinking water catchments in Sarawak was classified into six categories using satellite imagery and GIS. The categories represented varying levels of vegetation modification from largely undisturbed or mature secondary vegetation (Category 1) through to bare, non-vegetated areas (Category 6). Rivers less than ∼10 m in width were usually ‘invisible’ on satellite images because of dense canopy cover. More than 70% of headwaters in all catchments, except one (Buri Bakong), were not visible, indicating the presence of dense riparian vegetation. For the river sections that were visible on satellite images, 5%–22% of the riparian buffer was highly degraded (completely lacking vegetation). The highest degree of riparian modification occurred in the lowlands, the midland catchment of Tingkas and the highland catchment of Trusan. Although Sarawak government policy requires buffer widths of 5–50 m of natural vegetation to be retained along all streams and rivers in drinking water supply catchments these guidelines were not met at nine of the eleven field sites surveyed. These results suggest that compliance with buffer guidelines is important to water quality in rivers, particularly in logging areas, oil palm plantations and near settlements.