Formation and evolution of river valleys in large melt-out depressions in the North Podlasie Lowland

In the Holocene, the Narew and Biebrza Rivers formed their valleys in large melt-out depressions in the area of the North Podlasie Lowland. However, these rivers developed their valleys in various ways. The activity of rivers was limited only to a part of melt-out depressions. At the end of the Pleistocene, the Narew and the Biebrza were braided rivers. In the older Holocene the Narew River ran between Suraż and Tykocin, and erosion with subsequent lowering of valley bottom caused its later swamping. The braided Narew River built up the inflowing cone near the connection with the Biebrza Lower Basin that was eroded throughout the whole Holocene era. The remaining fragments of the flood terrace are the effects of this process. The Biebrza River in the Lower Basin began to create its valley in the younger Holocene among growing peats. This process led to the formation of the valley as a mineral dam among peatlands. The change of hydroecological conditions in the latter period resulted in the transformation of the alluvial Biebrza valley into a muddy valley