Modelling flow regulation effects on vegetation recruitment and survival on alternate bars in channelized rivers

Abstract

Many river reaches have transitioned to heavily vegetated systems following the regulation of their flow regime. Vegetation recruitment and survival are key to these transformations. We propose a novel predictive approach to detect how changes in the flow regime may affect conditions for successful vegetation recruitment and survival in channelized rivers with alternate bars. Our approach combines elements of existing vegetation recruitment models (Recruitment Box and Window of Opportunity) with simplified morphodynamic predictions of alternate bar migration. The approach is illustrated by applying it to a reach of the regulated Isère river, SE France, which has transitioned from migrating, unvegetated to steady, vegetated alternate bars over two decades following flow regulation associated with hydropower development. Our modelling approach identifies a strong impact of modified summer flows on vegetation recruitment since 1950 when major flow regulation began. An increase of up to 1 cm year⁻¹ (after 1990) has occurred in the vertical extent of vegetation recruitment bands on the bars. This has been accompanied by an almost 20 m total increase (since 1950) in the horizontal extent of the recruitment bands, and a 1.5 times reduction in the frequency of high flows capable of promoting bar migration. Our modelling also suggests viable flow restoration options to limit widespread vegetation colonisation. Comparing outcomes from our modelling approach with those from the Bertagni et al. (2018) model suggests that the Isère may have been highly prone to vegetation colonisation even before flow regulation and close to some threshold for a shift from an unvegetated to a vegetated state.