Schneeschmelzabfluß, Aufeis und fluviale Morphodynamik in periglazialen Flußgebieten NW-Kanadas

Abstract

This paper deals with the results of field studies in some periglacial drainage areas within the Richardson Mountains concerning modalities of snowmelt run-off and associated fluvial morphodynamics. The results are based especially on measurements of snowmelt run-off, active layer thawing, snow ablation, water temperatures, icing behaviour, and sediment transport during the spring melt period in the basin of Andy Creek (13 km²) in 1986. Before the beginning of the snowmelt in spring 1986, the floors of most of the valleys dissecting the intermontane pediments of central Richardson Mountains were filled with aufeis which was 0.5-1.5 m thick. The snowmelt run off took place within a span of 14 days. It started on the surface of the aufeis and then concentrated on an ice channel in the aufeis with flow velocities up to 3-5 m/s. In valleys with aufeis, three stages of snowmelt run-off can be distinguished: During the first stage in which the valley floor is covered almost completely with aufeis, the run-off is characterized by a very high magnitude of kinetic energy and a low magnitude of thermal energy; nevertheless, fluvial erosion does not take place because the valley floor is protected by aufeis. During the second stage, the channel has been carved through the aufeis body approxi mately over the whole length of the valley, and the flooding attains such a high kinetic as well as thermal energy as to pick up and transport the available sediment easily. During the third stage, the icing body is nearly melted and water temperatures and availability of material are high. Now however, small flow volume and low flow velocity are the limiting factors of fluvial erosion. In summary, on the one hand, due to low friction, the stream ice is responsible for increasing flow velocity and fluvial transport capacity of the snowmelt run-off. On the other hand, the stream ice protects the valley floor against fluvial erosion because the peak discharge passes off before the icing body has melted.