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

Riedel, Jon; Larrabee, Michael; Wenger, Jenna

This report describes the background information, methods and results of a surficial geology inventory within the North Cascades National Park Service Complex (NOCA). A suite of 29 landforms types were used to map NOCA at the 1:24,000 scale. The study area for this report is the Bacon Creek Watershed, which includes Bacon Creek, East Fork and their tributaries Triumph, Falls, Jumbo, and Oakes Creek. The Bacon Creek Watershed is located on the western slopes of the Skagit Crest and flows south into the Skagit River. This report focuses on the relationship between bedrock geology, glacial history, climate, hydrology and vegetation and landform development in the Bacon Creek Watershed. The geology of Bacon Creek Watershed is largely controlled by the presence of the Straight Creek Fault, a 400 km long strike-slip extensional fault, and other smaller faults. The Straight Creek Fault separates low grade metamorphic greenschists and phyllites from higher grade schists with intrusive granitic plutons of the Eocene and Cretaceous. The topography of the Bacon Creek Watershed reflects multiple glaciations during the past 2 Ma, which carved deep U-shaped valleys, steep valley walls and jagged horns and arêtes. The North Cascades were inundated by the south flowing Cordilleran Ice Sheet during the Fraser Glaciation (Armstrong et al. 1965). Advance of alpine glaciers during the Little Ice Age deposited Neoglacial moraines. The watershed climate is classified as maritime with relatively mild, wet winters and cool, dry summers. However, climatic conditions vary widely with large gradient in temperature and precipitation. Subalpine lowland and montane forests cover approximately 76 percent of the watershed and bare snow, rock and ice cover the remainder. Rivers and creeks typically reach flood stage during the spring and late fall. The main stem of Bacon Creek is a narrow, glacially scoured valley with a straight profile and moderate gradient. The East Fork is a hanging valley above the main stem, and has a steep gradient in its lower 1 km. Within the watershed, 66% is valley wall and 13% is high elevation cirque; with only 2% as riparian (floodplain, valley bottom, and alluvial fan). High elevation landforms (cirque, Neoglacial moraines, ridges, arêtes, other mountains, horns and passes) account for 20 km2, which is 15% of the total area in the watershed. Aspect is a strong control on the development of cirque basins and valley walls, with north and east facing cirques deeper, broader and with lower elevation Neoglacial moraines than those on southerly aspects. Valley asymmetry, reflected by steep northeast-facing valleys walls and rock fall and more gentle forested southwest facing slopes, is a product of differential freeze-thaw cycles. The transition between floodplain and valley bottom in the watershed occurs at a notably lower elevation than elsewhere in NOCA, likely a result of CIS erosion along the Straight Creek Fault. Debris aprons are composed of till, colluvium and a significant component of volcanic ash. Several Pleistocene moraines are located in the watershed, the largest is an end moraine of the Sumas Stade Bacon Glacier (Riedel 2007), located at the confluence with the East Fork. Terrace heights range from four to 36.5 meters. The higher terraces are likely late ice age outwash from valley glaciers. The lower terraces are likely alluvium deposited over the last few hundred years. Seven large debris avalanches have occurred in the watershed, most in relatively weak Darrington Phyllite oversteepened by glaciation. Two of the debris avalanches have blocked the Bacon Creek entirely, delivering large amounts of sediment to the creek.

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

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