GEOMORPHIC AND SEDIMENTOLOGICAL EVIDENCE FOR DRAINAGE REORGANIZATION IN SOUTHWEST NORWAY’S DRIVA RIVER

The Driva river of southwestern Norway flows westward into Sunndal, a high relief glacial valley that transverses a 2,000km long near-coastal escarpment. Well-developed barbed tributaries with confluence angles systematically greater than 90^o suggest an eastward flowing paleo-network with headwaters in the high elevation regions surrounding the Driva’s current mouth. Many high elevation glacial channels have low-relief divides and irregular drainage geometry, such as small tributary networks that flow into a central point from all sides before draining into the trunk stream, forming a star-shaped drainage pattern. Other landscape abnormalities include isolated topographic highpoints, which are scattered throughout the watershed and constrained on all sides by underfit and overfit glacial valleys with high confluence angles. Near Oppdal, a sharp change in the trunk stream’s course is situated across from a major wind gap, where terraced sediment deposits of uniform and rounded sand grains were observed in the field, necessitating a past fluvial environment with much higher discharge and sediment supply than currently present in the low-relief headwater streams occupying the area. Our geomorphic analysis suggests an eastward flowing glacial paleo-network, in contrast to the Driva’s modern day westward flowing trunk stream, suggesting a change in flow direction since the Last Glacial Maximum. Elevation data measured in the field and remotely using a 1m LIDAR DEM give incision estimates of ~100m since that time in some areas, or incision rates of 10 mm/yr for the last 10,000 years. In southwest Norway, domal isostatic uplift patterns appear to be perturbed by reactivated faults (Helle, 2007). Our data provides insight into this discussion, as fluvial systems are sensitive to changes in uplift patterns and their predicted responses to topographic changes allow landscape features to serve as a record of regional evolution.