In the process of high expansion of drainage systems and land cultivation, most natural watercourses in Lithuania were converted into drainage channels. They were deprived of all natural riparian vegetation and abiotic conditions that kept up biodiversity, diminished erosion and deflation, induced self-purification of water etc. Thus the re-naturalization of drainage channels is essential. Consequently, to determine the possibilities to combine re-naturalization processes in channels with their function-ability it was examined the tendencies of woody vegetation (WV) spreading and the hydraulic calculation formulas estimating the performance of overgrown channels as water recipients were suggested.

As the study results showed, self-restoration of WV was most progressive in forest and outskirt channels. In the field channels WV expansion takes place as well. However, it depends on the distance from a forest, i.e. WV density decreases sharply from 1.17 stems m−2 to background level (c. 0.26 stems m−2) within the space of 300 m. In the paper there is some positive influence of WV cover in view to drainage channel maintenance discussed referring to the results of former studies as well as the publications of other authors. It includes the prevention of bottom silting as well as slope erosion and slide. The formulas estimating conductivity of overgrown channels are based on the balance of forces occurring on the boundaries of overgrown strips of a stream: resistance of tree stems, friction on the slopes, shear stresses that appear when two streams with different velocities are in contact and the horizontal component of the gravity force causing water movement. Some methods of WV cover abundance control are discussed when having assumed self- or artificial restoration of WV in channels to be maintained as recipients is projected.