Quantifying the Role of Hydropeaking Regimes on Reach-Scale Vegetation Recruitment in a Gravel Bed River

Abstract:

Flow regime alterations represent the major driving factor behind loss of riparian vegetation along river channels, controlling seed dispersal, germination, and early growth of plants. In particular, frequent and short water inundations of recruitment sites caused by hydropower production, i.e. hydropeaking, can impair colonization of several plant species because of an increase of water stress. While evidences of individual plant response to water level fluctuations are mounting, yet the impacts of hydropeaking regimes on vegetation establishment at the river-reach scale remain unknown.

Here we use a simple recruitment vegetation model, based on the window of opportunity (WoO) concept, to evaluate the influence of flow regime alteration on vegetation dynamics in a hydropeaking-impacted reach of the Alpine Rhine river (Switzerland). The model identifies successful recruitment events by comparing time series of water levels, obtained running 2D hydrodynamic numerical simulations with the software BASEMENT, and time-varying vegetation resistance to water stress. We calibrated the model on aerial image observations on gravel bars in the period 2005-2008 and used the model to predict changes in vegetation cover depending on different hydropeaking scenarios, which included variations in base flow discharge and hydropeaking amplitude and frequency.

Results show that an average increase of inundation levels during growing seasons reduces vegetation cover by limiting vegetation colonization at low bar elevations, where inundations periods are longer. In contrast, a reduction of inundation levels tends to increase the windows of opportunity for vegetation recruitment. More interestingly, we found that the effect of hydropeaking regimes on vegetation cover varies depending on both the magnitude and yearly seasonality of natural floods. Our results quantify the importance of human-induced versus natural flow variability for predicting success of riparian vegetation establishment on gravel bed rivers.