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

Riedel, Jon; Dorsch, Stephen; Wenger, Jenna

We present background, methods and results of a surficial geology inventory in the North Cascades National Park Service Complex (NOCA). The study area is the Stehekin River Watershed, which flows southeast from the Pacific Crest into Lake Chelan. We focus on how bedrock geology, glacial history, climate, hydrology, and vegetation have affected landform development in the watershed. Bedrock in the Stehekin River Watershed is part of the crystalline core of the North Cascades. The Skagit Gneiss Complex and the Cascade Pass Family are the dominant rock units exposed in the watershed. Topography reflects multiple glaciations during the past 2 Ma, which have carved deep U-shaped valleys, steep valley walls, jagged horns and arêtes. 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 to 1900 AD), depositing Neoglacial moraines. The watershed climate is continental, but varies substantially by elevation and distance from the Pacific Crest. Yearly temperature mean highs occur in July and August; winter mean low temperatures occur in December and January. The rainy season is November through January. A suite of 29 different landforms is currently being mapped at NOCA at the 1:24,000 scale. Landforms can either be depositional in nature, such as moraines and alluvial fans, or they can be erosional such as bedrock benches and horns. Many depositional features such as moraines and terraces were formed during the last ice age. Other depositional features such as debris cones and landslides are forming today. Approximate ages are assigned to depositional landforms based on available radiocarbon dates, associated process of formation, volcanic tephra, soil development and vegetation type and age. Landforms in the watershed are 68.59% valley wall and 7.27% high elevation cirque; only 3.25% are riparian (floodplain, valley bottom and alluvial fan). High elevation landforms (cirques, Neoglacial moraines, ridges, arêtes, horns, and passes) account for 50 km2, or 9.25% of the watershed. The majority of the area is cirque basin. Steep cliffs on valley walls with north aspects are contrasted by gentler rises on those with south aspects. Landforms in the watershed, such as broad passes, extensive bedrock bench systems, Pleistocene moraines, Neoglacial moraines, and rounded ridges capture the history of ice sheet advances and retreats. The mainstem Stehekin River is a broad U-shaped glacial valley created by the southward excursions of the Cordilleran Ice Sheet and multiple glaciations by alpine glaciers flowing down the major tributaries. Tributary systems were left as hanging valleys with bedrock canyons or narrow, stepped waterfalls at their mouths. Where they join the mainstem Stehekin River, they deposited alluvial fans. At the mouths of some tributaries, fan terraces were deposited at the end of the last ice age when slopes left bare by retreating glaciers fed massive quantities of sediment to streams. There are several large Pleistocene moraines along Stehekin River deposited by the ice sheet across the mouths of tributaries. 382 mass movements have been mapped in the watershed (2% of watershed area) with 13 being large debris avalanches (an area of 1.45 km2). This is a relatively low number for a NOCA watershed, which could be attributed to the competency of the Skagit Gneiss bedrock that is exposed extensively throughout the watershed. Debris torrents are fairly common with 41 mapped in the watershed, which is a relatively high number for a NOCA watershed. This large number may be due to the dryer climate and associated vegetation in the eastern portion of the watershed. Debris torrents are also important contributors to the total amount of sediment delivered to streams.

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

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