Gravel-bed channel changes upstream of a reservoir: The case of the Dunajec River upstream of the Czorsztyn Reservoir, southern Poland

Despite a significant increase in the number of dam reservoirs in the world in the last century, the effects of these structures on the evolution of river channels above them are poorly understood. Constructed in 1997 in the Polish Carpathians, the Czorsztyn Reservoir (CR) has raised the base level of the single-thread, gravel-bed Dunajec River. An analysis of five sets of aerial photographs and orthophotos from the period before (1977, 1982, 1994) and after (2003, 2009) the construction of the CR allowed me to investigate the temporal and spatial impact of the raised base level of the river upstream of the reservoir on the evolution of the river channel. Comparison of channel width changes in the backwater river section with changes observed in the control section unaffected by the presence of the reservoir has shown that in the pre-dam period differences in the evolution of the river channel in the backwater section and in the control section were not statistically different. In the periods without major floods, both channel sections were dominated by channel narrowing or a relative stability of channel width. In the post-dam period, a major flood in 1997 caused more than two times greater channel widening in the backwater section than in the control section. The extent of channel widening in the longitudinal channel profile reached farther upstream of the reservoir than the extent of the backwater itself. In the later part of the post-dam period without major floods, channel narrowing were similar in the backwater section and the control section, as had been the case in the period before the reservoir construction. The comparison of changes in the width of the channel in the backwater and control sections indicates that channel widening in the backwater, interpreted as an early stage of backwater channel adjustment, was reinforced by the flood of 1997. This study shows that the use of a control section makes it possible to isolate reservoir effects from flood effects in the context of channel dynamics.