Gravel bar inundation frequency : an indicator fort he ecological potentiel of a river

In braiding river systems, gravel bars fulfill important ecological functions. At the River Sense, one of the last unregulated rivers in Switzerland, the frequency of gravel bar inundation of a 2 km long site maintaining indicator species such as Myricaria Germanica (German Tamarisk) and Chorthippus pullus (Gravel Grashopper) was studied. Based upon both detailed data collected in the field and a hydrological analysis of the site, a numerical two-dimensional model of the site was developed to investigate the inundation area and frequency of the parafluvial zone for a range in flow regimes. Results show that the free surface of the parafluvial zone is reduced significantly only when floods with a return period greater than one year occur. Three types of gravel bars were distinguished: gravel bars devoid of vegetation occur for return periods less than two years. The elevation of gravel bars that support Myricaria Germanica and Chorthippus pullus are at higher discharge elevations that coincide with discharge return frequencies between 2 to 5 years. Densely vegetated overstory and understory communities occur at floods greater than the bankfull return period of five years which also coincide with the floods principally responsible for altering the riverscape. Findings correlate well with the hypothesis that the sustainability of Myricaria Germanica and Chorthippus pullus is largely dependent upon a specific frequency and duration of intermittent flood inundations.