The trend of gravel bars' degradation in the last two decades is perceived positively by society

Abstract

River management and regulations can significantly impact the dynamic nature of gravel bars in rivers. The aim of this research is to identify changes in gravel bar morphodynamics including vegetation cover succession in three Czech rivers: the Odra and two of its tributaries. The spatiotemporal analysis focuses on the most recent years 2000-2020 and compares them with historical conditions (1937-1994 in the Odra and from the 1950s in the Olše and Ostravice). Additionally, we aimed to determine how society perceives gravel bars as features in river channels. We analysed gravel bar spatiotemporal morphodynamics using orthophotos and associated hydrological data. The survey of public perception included two sets of photographs depicting regulated and natural environments at different stages of vegetation growth, and assessed perceptions of naturalness, recreation, aesthetics, and vegetation. The historical state of gravel bars showed naturally dynamic gravel bars with great variability of vegetation cover throughout the years. However, the recent period has demonstrated an increasing trend in vegetation cover with disruptions following the 2010 and 2014 floods and leading to temporary decreases of vegetation, notably in the two gravel-bed tributaries. The meandering Odra river conversely exhibited an uninterrupted upward trend. Additionally, the regulated sections of the Odra featured significantly less gravel bar area compared to natural sections. In the public survey, 239 respondents expressed a preference for gravel bars in natural river section. They particularly preferred those with abundant vegetation cover, across all assessed criteria. Unvegetated gravel bars were indicated to be preferable if they were either vegetated or removed from the river channel. Our research emphasizes the unfavourable viewpoint of society towards gravel bars with limited or no vegetation and preference of highly vegetated bars that is a direct reflection of recent trend toward a substantial increase in vegetation-covered gravel bars or loss of gravel bars. These findings highlight the loss or degradation of ecologically valuable habitats and society's misconception about gravel bars, which may affect future conservation efforts of these features.