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

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

We present background information, methods and results of a surficial geology inventory in the North Cascades National Park Service Complex (NOCA). The study area is Thunder Creek Watershed, which flows north from Thunder Basin and is NOCA's most heavily glaciated watershed. Headwaters of many of its tributaries are on the Skagit or Pacific Crest, two significant divides strongly affecting hydrology and climate in the North Cascades. We focus on how bedrock geology, glacial history, climate, hydrology, and vegetation have affected landform development in the watershed. The watershed lies between two major fault systems: Straight Creek Fault to the west and Ross Lake Fault Zone to the northeast. It also contains the Thunder Lake Fault, which runs north-south through the western part of the watershed. Other structural features include northeast/southwest trending compressional faults. Most of the watershed is orthogneiss and gneiss. 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 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), depositing Neoglacial moraines. The watershed climate is maritime, although drier conditions prevail in Fisher Creek valley. Climatic conditions vary widely with large temperature and precipitation gradients from west to east and with elevation. Snow pack reaches maximum depth at high elevation sites in April to May. Yearly temperature mean highs occur in July and August; winter mean low temperatures occur in January. The rainy season is November through January. Snow, bare rock and ice cover ~40 km2 of the watershed. The remainder is dominated by conifer species; deciduous species dominate along river corridors and avalanche chutes. 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 the watershed, 53% is valley wall and 20% is high elevation cirque; only 2.6% is riparian (floodplain, valley bottom, alluvial fan). High elevation landforms (cirques, Neoglacial moraines, ridges, arêtes, horns, passes) account for 72.5 km2, 24% of the watershed. Broad passes, bedrock bench systems, Pleistocene moraines, Neoglacial moraines, and rounded ridges capture the history of ice sheet advance/retreat and subsequent drainage reversals. The mainstem Thunder Creek is a broad U-shaped glacial valley created by southward excursions of the Cordilleran Ice Sheet and alpine glaciations flowing down major tributaries. Tributary systems were left as hanging valleys with bedrock canyons or narrow, stepped waterfalls at their mouths. Where they join the mainstem Thunder Creek, they deposited alluvial fans. At the mouths of some tributaries, fan terraces were deposited. Several large Pleistocene moraines along Thunder Creek were deposited by the ice sheet across the mouths of tributaries. 114 landslides were mapped in the watershed (4.8% of the area). Debris avalanches comprise 3.4% of the watershed and displaced ~0.675 km3 of debris. Debris slump/creep deposits are uncommon. Debris torrents are fairly common, indicating the active nature of the smaller tributaries. South-facing river canyons produce flashy floods and debris torrents in spring. The mass movements tend to be on north or north-east to east facing slopes.

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

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