Impact of river regulation on gravel bar decline and vegetation expansion over recent decades

Abstract

Many gravel-bed rivers in the Carpathians have undergone extensive regulation that has completely changed their geomorphic regime. Formerly rich in gravel bars, these river systems are now deprived of geomorphologically effective flows and sediment supply, resulting in the disappearance of gravel bars or the loss of their natural dynamic behavior due to vegetation overgrowth.

This study examines the effects of regulations on the development of gravel bars by comparing the historical condition in the 1950s (with limited human influence) with the recent period (2000–2022), and by analyzing the effectiveness of hydrological events and the development of in-channel vegetation in the recent period, in two heavily regulated gravel-bed rivers: the dammed Ostravice and the free-flowing Olše.

Rivers with abundant gravel bars underwent extensive channel regulations in the 1950s, which significantly reduced bar widths and lengths, as well as the total area of gravel bars. Hydrologically effective flows during bar reworking were associated with large floods (1997, 2010) or high flows in 2014, an effect facilitated by the previous flood event and the lack of resistance of highly vegetated bar surfaces. The remainder of the study period experienced progressive vegetation succession or only partial bar reworking. Overall, the dammed river showed more profound shifts in gravel bar morphology towards progressive bar narrowing and elongation and rapid vegetation succession than the less regulated river. This study provides insights into the development of gravel bars in regulated rivers of Central Europe, and highlights, from a management perspective, the potential loss of ecologically valuable habitats.