Geomorphology of the Upper Skagit watershed: Landform mapping at North Cascades National Park Service Complex, Washington

Riedel, Jon; Brady, Sharon; Dorsch, Stephen; Boweman, Nicole; Wenger, Jenna

We present background, methods and results of a surficial geology inventory within the North Cascades National Park Service Complex (NOCA). The study area is the Upper Skagit Watershed, which includes all creeks that flow into Ross Lake. The headwaters of many of the tributaries are on the Skagit or North Cascades Crest, two divides strongly affecting hydrology and climate in the North Cascades. We focus on how bedrock geology, glacial history, climate, hydrology, and vegetation affected landform development in the watershed. The watershed is defined by the Straight Creek Fault, Ross Lake Fault Zone, Hozomeen Fault and northeast-southwest compressional faults. Between the Straight Creek Fault and Ross Lake Fault Zone is the Chelan Mountain terrane, composed mainly of Skagit Gneiss Complex orthogneiss. West of Ross Lake Fault Zone are intrusive plutons emplaced during crustal extension. East of Ross Lake Fault Zone are older, mostly sedimentary rocks of the Hozomeen and Methow terranes. Topography reflects multiple glaciations during the past 2 Ma. West side valleys are U-shaped and extend all the way to Ross Lake and enter Skagit Valley as hanging valleys. East side valleys, with alpine glaciation restricted to upper valleys, enter the Skagit Valley as narrow river canyons. The North Cascades were inundated by the south flowing Cordilleran Ice Sheet during the Fraser Glaciation. The most extensive advances of alpine glaciers since the last ice age occurred in the Little Ice Age (1350-1900 AD). Climatic conditions vary widely with large temperature and precipitation gradients. The most pronounced difference is between the wet, cool west side and dry, hot east side of Ross Lake. Snow pack reaches maximum depth at the higher elevation sites by late April; lower elevation sites reach the maximum in March. Most precipitation occurs November through January. Landform mapping (1:24,000 scale) is an input to a NOCA soils map. Landforms are depositional, such as moraines and alluvial fans, or erosional such as bedrock benches and horns. Many depositional features such as moraines and terraces were formed during the last ice age; others, such as debris cones and landslides are forming today. Approximate ages are assigned to depositional landforms based on radiocarbon dates, associated process of formation, volcanic tephra, soil development and vegetation type and age. In sub-watersheds west of Ross Lake, 56.7% is valley wall and 12.7% is high elevation cirque; only 0.91% is riparian (floodplain, valley bottom and alluvial fan). West of Ross Lake, Big and Little Beaver Creeks are classic U-shaped glacial valleys. Other glacial characteristics include oversteepened valley walls, hanging tributary valleys and truncated valley spurs. In sub-watersheds east of Ross Lake, 58.2% is valley wall and 2.3% is river canyon, reflecting the steep, narrow V-shaped east side tributaries. Drainage of glacially dammed lakes caused divide lowering and migration northwards as well as modification of dendritic, trellis and other drainage patterns. High elevation landforms are underrepresented on east of Ross Lake, as the headwaters of these creeks are outside NOCA. West of Ross Lake, mass movements constitute 2.8% of the landforms. 42 debris avalanches total 7,387,600 m2. Of these, 31 delivered ~178,283,400 m3 of sediment to streams. East of Ross Lake, mass movements constitute 3.2% of the landforms. 15 of 17 mapped debris avalanches delivered ~57,450,000 m3 of sediment to streams. Glacially over-steepened valley walls, erosion on north-facing slopes and structural weakness associated with faults and hydrothermal alternation are likely responsible for the majority mass movement avalanches and rock fall in the sub-watersheds.

Geomorphology of the Upper Skagit watershed: Landform mapping at North Cascades National Park Service Complex, Washington

Series

Collections