Drainage and valley asymmetry in the Tertiary Hills of Lower Bavaria, Germany

Valley asymmetry with steeper west-facing valley sides is a common feature in the Tertiary Hills of Lower Bavaria. Several previous studies have attributed these forms to microclimatic differences under periglacial conditions during the Pleistocene glacials. However, little attention has been given to the pattern of drainage networks in this area as a means of explaining valley asymmetry. Drainage networks show asymmetry both in length and number of tributaries on either side of the main stream. Drainage asymmetry is very likely primarily the reason of a drainage development in competitive situations. Four possible causes have been identified: (1) the position of an initial channel in relation to its adjacent parallel or subparallel drainage lines, (2) different rates of headward erosion of tributaries on either side of an inter-stream divide, (3) drainage development oblique to an initial terrain slope and (4) tilting of a landsurface and the resulting preferential headward erosion of consequent running streams. Thus, asymmetric drainage development causes differences in slope dimensions and an imbalance in run-off and sediment yield on opposite valley sides leading to the development of asymmetric valleys.