Hydrogeomorphic effects of beaver dams on floodplain morphology: avulsion processes and sediment fluxes in upland valley floors (Spessart, Germany) [ Les effets hydro-géomorphologiques des barrages de castors sur la morphologie de la plaine alluviale : processus d'avulsions et flux sédimentaires des vallées intra-montagnardes (Spessart, Allemagne).

Abstract

In a two years study, several methods were used to visualise and analyse beaver-induced hydrogeomorphic changes of channel pattern, sediment fluxes and erosion processes on the upland valley floor of the 3rd order river Jossa (Spessart/Germany). There, the re-introduced population of Castor fiber albicus shows intensive dam-building activity Using GIS and aenal photography, different stages of river and floodplain morphology (1998-2001) were mapped In addition, length of water courses, areal extent of ponds and wetlands, sediment depths, volumes and sedimentation rates, as well as erosion rates and amounts of eroded matenal were calculated. The results revealed that beaver dams create large wetlands and greatly increase the area of open water surface and quiet reaches by damming-up ponds. Moreover, they enhance the total water flow length by diverting water onto the floodplain, resulting in a multi-channeled (anastomosing) drainage network Where pond drainage was concentrated on a few relative stable channels, headcut erosion led to the development of one single dominant channel. Five stages of this development to a channel relocation were observed, resembling stages of a gradual avulsion. This implies that beaver dams should be recognised as possible - or even as important - avulsion triggers Intensified deposition of predominantly sands and organic silts occurred within the beaver ponds. Moreover, extended avulsion deposits caused by permanent overbank flow revealed the multi-channeled drainage network to be an important depositional environment of beaver sites. The highest amounts of sediments were deposited within the channel-bed and adjacent anthropogenic ditches, smaller amounts were accumulated on the inundated floodplain. The results may provide a rationale for attributing beaver activities a high significance as a forcing factor of Quaternary floodplain development along low order rivers in Central Europe.