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

Riedel, Jon; Brady, Sharon; 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 Chilliwack River Watershed, which flows north from NOCA then turns west to join the Fraser River in British Columbia. The headwaters of many of its tributaries are on the Skagit or North Cascades Crest, two significant divides affecting the North Cascades hydrology and climate. We focus on how bedrock geology, glacial history, climate, hydrology, and vegetation have affected landform development in the watershed. The watershed is defined by the Straight Creek Fault, the Hannegan Caldera Ring Fault and northeast-southwest compressional faults. The geology consists of Cretaceous to Eocene intrusive granitic plutons with older rocks of the Skagit Gneiss Complex to the northeast. 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 climate is maritime: mild, wet winters and cool, dry summers. Snow pack reaches its 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 to January. Snow, bare rock and ice cover ~ 11.4 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 in nature, such as moraines and alluvial fans, or erosional such as bedrock benches and horns. Many depositional features 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 radiocarbon dates, associated process of formation, volcanic tephra, soil development and vegetation type and age. In the watershed, 50% is valley wall and 23% is high elevation cirque; only 2.8% as riparian (floodplain, valley bottom and alluvial fan). High elevation landforms (cirque, Neoglacial moraines, ridges, arêtes, horns, passes) account for 62.5 km2, 28.7% of watershed area. The mainstem Chilliwack River is a broad U-shaped glacial valley created by southward excursions of the Cordilleran Ice Sheet and multiple alpine glaciations 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 Chilliwack 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. Several large Pleistocene moraines along the Chilliwack River were deposited by the ice sheet across the mouths of tributaries. Lacustrine deposits in terraces far up the mainstem of the Chilliwack River indicate ice-sheet damming at about 17 Ka. 143 landslides have been mapped in the watershed, encompassing ~3.4% of watershed area. Debris slump/creep deposits are uncommon and tend to occur in the valley bottom. Recently active debris torrents are present but are uncommon. Debris avalanches encompass 2.4% of the overall watershed and are important due to large size, potential to block streams and deliver massive amounts of large woody debris and sediment. The largest debris avalanches are typically fault controlled, by either the Hannegan Ring Fault or northeast-southwest trending extensional faults.

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

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