Zmienność układu koryt rzecznych w dolinie Narwi na odcinku między Łapami a Żółtkami w II połowie XX wieku

The water conditions in the River Narew valley began to change in the 1970s and 1980s owing to anthropogenic and natural factors. The former included the regulation of a part of the river valley below the village of Rzędziany. As a result of these measures, the river course was shortened and its gradient increased, leading to lower water levels, shortened surface flooding periods and a gradual lowering of the groundwater level in the whole of the valley. In consequence, many active river channels are disappearing due to their overgrowing and silting. The impact of the natural factors is slower and can be observed in the whole of the valley. From 1981 onwards, precipitation levels gradually fell. Mild winters and shorter snow cover periods resulted in reduced spring floodings. The water flow can also be significantly affected by the vegetation. Due to the overgrown banks plants can obstruct the water flow, leading to the damming of the river and consequently to the overgrowing of the river channel or avulsion. The existing vegetation also gradually leads to the narrowing of the river channels. The paper analyses the changes in the river channel network which took place in the River Narew valley between the villages of Łapy and Zółtki in the second half of the 20th century. It also describes how the natural and anthropogenic factors have influenced the channel network and characteristics of the River Narew. The research period covers the years 1966- 1997; the analysis was made using maps of the river channel network. The first map, developed on the basis of a series of panchromatic aerial photographs, shows the situation in 1966, while the second map, prepared on the basis of a spectrozonal orthophotomap, shows the location of the river channels in 1997. These maps were compared to illustrate the extent and directions of the changes that took place in the river channel network in the researched area over the past 31 years. To this end, two differential maps were prepared using ESRI’s ArcGIS 10.0 Two river channel types were distinguished: active and cut off. The first map shows all potential river channel transformations and those channels which did not undergo any changes in the researched area. This helped identify the overall maximum area occupied by the river channels in the period in question and the extent of the unchanged river channels. The second map shows six directions of changes, excluding the channels where no changes could be detected. In 1966 the natural River Narew network was very well developed, with the channels occupying the larger part of the river valley. In 1997 the river channels had changed considerably in comparison to 1966. A part of the researched area lying within the boundaries of the River Narew National Park had a relatively well-developed river channel network, although with a visible increase of cut-off channels. At the same time, in the section below Kurowo, it was found that the number of active channels had decreased, resulting in the dominance of the single-channel system, particularly well visible in the artificially created channel in the vicinity of the village of Rzędziany, reaching as far as the village of Żółtki. Changes in the river channel network could be observed in about 50 per cent of the area of the researched River Narew 16 valley floor. The dominant direction of change (50 per cent) has been the disappearance of active channels, mainly found in the southern part of the area in question. The year 1966 saw many floodings here. In the northern part of the researched area, strong vegetation growth could be observed, which led to the overgrowing of the river channels. The other significant direction of change (observable in 25 per cent of the researched area) has been the transformation of active channels into cutoff ones. Such changes were found mostly in the northern part of the area in question, between the villages of Radule and Żółtki. The third change type, covering about 17 per cent of the researched area, has been the formation of active channels, also found in the northern part of the area in question, in the so-called buffer zone of the Narew National Park. Similar changes were also observed in the vicinity of Waniewo and Radule, with few changes found in the remaining river channel categories. To sum up, it should be concluded that the changes which took place in the area in question were both varied and significant. Most notably, they included the disappearance of active channels in the southern part of the area in question and the transformation of active channels into cut-off ones in the northern part. The extent of the changes was the smallest in the latitudinal section of the valley, between Topilec and Waniewo; this is an area with a well-developed water network, characterised by stable river channels. Since 1996, work has been under way to restore the River Narew valley and recreate the water conditions existing here before the land drain age.