Abstract
Five parallel channels of the Narew River (NE Poland) were hydromorphologically assessed with the River Habitat Survey (RHS) method. Four out of the five studied channels were partly separated from the functioning system of the anastomosing river by a dike for more than 20 years, and were subjected to gradual overgrowing and shallowing. In the 1990’s and in 2002, a number of renaturisation measures were implemented, restoring sufficient discharge for the channels to make them active again. The Habitat Modification Score (HMS) obtained as a result of the survey only classifies the fragment ecologically to the third class of streams with high HMS value in the case of the current main channel of the Narew River (largely formed in an artificial manner). This suggests substantial anthropogenic changes in the river channel and its vicinity. The aggregate values of the Habitat Quality Assessment (HQA) index suggest the preservation of numerous environmentally valuable elements at all of the five studied sites of the anastomosing Narew River system. This even concerns channels which remained almost separated and functioned as oxbow lakes for more than 20 years. The highest HQA values were recorded in the case of the channel constituting the main channel before the regulation, and a neighbouring regulated channel currently fulfilling the function of the main water course in this cross-section. In spite of long-term negative changes, the studied fragment of the ecosystem of the anastomosing river is distinguished by high resistance to “anthropogenic stress”, and high capacity for spontaneous restoration of a state close to the natural one. Although lotic ecosystems constitute elements of the natural environment particularly sensitive to anthropopressure, they retain the ability to regenerate for a long time, under the condition of providing them with relevant hydrological and ecological conditions.