Geomorphology of a Cordilleran Ice Sheet drainage network through breached divides in the North Cascades Mountains of Washington and British Columbia

Abstract

The Skagit River valley in northern Washington and southern British Columbia contains long interconnected valleys, barbed tributaries, underfit streams, and low-elevation mid-valley divides. Quantitative measures of connectivity between valleys draining through breached mountain divides, aerial photographs, and a field-verified, process-form model were used to assess the impact of continental glaciation on the drainage pattern and geomorphology of the watershed. Structural control on drainage patterns and intense, repeated continental glaciation rendered the Skagit an extremely interconnected valley, with meltwater routes opening it to the Fraser and Okanagan watersheds. Divide elimination by drainage of large proglacial lakes led to several hundred meters of erosion at 10 breached divides. Other geomorphic effects of recurrent continental glaciation include the bisection of a major mountain valley, divide migration of as much as 50 km, erosion of canyons 15–30 km downstream of breached divides, and deposition of exceptionally large alluvial fans at canyon mouths. Drainage patterns established in the Tertiary were reorganized to accommodate southern drainage of glacial meltwater trapped in mountain valleys at the southern margin of the Cordilleran Ice Sheet. The Skagit basin is now a regional focus for drainage to Puget Sound from the northernmost North Cascades and parts of southern interior British Columbia. The key to establishing this system was the elimination of a regional divide in what is now Skagit Gorge, which resulted in the capture of the upper Skagit valley by the lower Skagit River. Patterns of drainage modification caused by continental glaciation in the Skagit basin are similar to those in other mountain areas. The Skagit's interconnected valleys are similar to the great through-valleys of Scotland, transmountain valleys of British Columbia, and ice portals of Scandinavia.