Impact of channel incision on the hydraulics of flood flows: Examples from Polish Carpathian rivers

• Channel deepening may result from channel incision or river metamorphosis.
• Lowering of water stages associated with given discharges indicates channel incision.
• Hydraulic importance of river incision by a given absolute amount increases with decreasing river size.
• Hydraulic effects of channel incision depend on lateral stability/mobility of an incising river.

Channel deepening may result from channel incision or river metamorphosis changing a wide and shallow channel to a narrow and deep one. As only the first type of channel change leads to increased flow capacity of the channel, a lowering of water stage associated with a given discharge rather than a lowering of river bed should be used to identify channel incision. A lowering of minimum annual stage at gauging stations is typically used to assess the relative importance of channel incision along a river or within a particular region. Rivers of the Polish Carpathians incised by 0.5–3.8 m over the twentieth century, with the amount of incision being greater in their lower and middle courses than in the upper ones. Variability in the hydraulic importance of channel incision with increasing river size is analysed by comparing changes in the frequency of valley floor inundation at gauging stations located along the seventh-order Dunajec River. Despite a lower absolute amount of channel incision in the upper river course, here incision has increased channel conveyance and reduced the frequency of valley floor inundation considerably more than in the lower course. Hydraulic effects of channel incision depend also on lateral stability of an incising river. Low-energy rivers from the eastern part of the Polish Carpathians remained laterally stable during channel incision. This has resulted in substantial lowering of stages for low flood discharges and markedly smaller one for high-magnitude floods, whereas velocity of the flows conveyed over the highly elevated floodplains has decreased considerably. In high-energy rivers from the western part of the Polish Carpathians, alternation of incision and lateral channel migration has led to the formation of incised meander belts, with substantially lowered stages for all flood discharges and increased velocity of the flows conveyed over the newly-formed, low-lying floodplains.