Channel incision and flow increase of the upper Wisloka River, southern Poland, subsequent to the reafforestation of its catchment

Changes in channel geometry and flows following catchment afforestation are demonstrated for the upper Wisloka River, southern Poland. The montane part of its catchment was dramatically depopulated in the mid-1940s, resulting in considerable reduction of agricultural activity in the area. As a consequence, forest cover increased from 30 per cent in 1938 to 67 per cent in 1995, mainly due to natural expansion of forest on hillslopes. Reafforestation and the decrease of human activity have significantly limited sediment delivery to the stream channel. Stabilization of the river course by valley slopes in the montane reach, and due to channelization works in the foothill reach, has prevented the upper Wisloka from adjusting its transporting ability through an increase in channel sinuosity. Under such conditions, the excess stream power has been dissipated by bed scouring and more than 2 m of channel incision occurred by 1995. River incision has resulted in increased concentration of flood flows in a river cross-section, the increased velocity of the channel and floodplain flows, and the reduced potential for sediment storage in the floodplain area. Results from this stud indicate that successful channel management decisions must take into account changes in river regime resulting from alterations in catchment management. A considerable increase in mean annual discharge of the upper Wisloka, only partly explained by an increase in precipitation, has been recorded in the last few decades. The 'excessive' increase in streamflow does not reflect increased water runoff from the catchment. Rather, water that was transferred downvalley within the valley floor gravels prior to channel incision, and was not included in the gauged record, now forms a part of river flows. Copyright (C) 2002 John Wiley Suns, Ltd.