Geomorphology of Mount Rainier: Landform mapping at Mount Rainier National Park, Washington

Riedel, Jon; Dorsch, Stephen

The Geomorphology of Mount Rainier National Park (MORA) was completed as one of the 12 basic inventories desired for each park in the 1998 Natural Resources Challenge Inventory Program. It is closely linked to ongoing mapping of soils by the Natural Resource Conservation Service and to the multi-scaled USFS National Hierarchical Framework for Ecological Units to provide opportunities for ecosystem management with adjacent national forest lands. The geology of MORA is a testament to the power of tectonic forces near a subduction zone that created the largest stratovolcano in the lower 48 states. Mount Rainier volcano is surrounded by mountains dominated by older volcanic rocks, setting it apart from Glacier Peak and Mount Baker to the north. The geomorphology of the park is also strongly shaped by surficial Earth processes that are controlled largely by climate, such as glaciation, mass wasting, and river erosion and deposition that are controlled largely by climate. These processes created the shape of the landscape adjacent to the volcanic cone that dominates the skyline for hundreds of miles and accounts for about 8% of the park. Erosion by glaciers created the dominant landforms of the park, the cirque and valley wall. Slightly more than half of MORA is mapped as valley wall, while the heads of all major valleys are mapped as glacial cirques and account for 7% of the park. Glacial moraines are common landforms and are scattered at various elevations. The youngest of these moraines tower above the termini of modern glaciers and attest to the sensitivity of the glaciers to climate change. Only 100 years ago, glaciers filled the valleys above the moraines to cover about 30% more of the park than they do today. Five major watersheds radiate from the volcanic cone, each with a unique geologic history. Because they head on the heavily glaciated active volcano, Cowlitz, Carbon, Nisqually, Puyallup and White rivers have broad glacial valleys with wide, terraced floodplains and braided rivers. They all have carried large lahars from the volcanic cone to surrounding lowlands. In contrast, the Ohanapecosh valley is the largest in the park that does not head on the volcano and, as a result, has a different geomorphology than the others. This is evidence by a higher proportion of valley walls and a narrow floodplain that accounts for less than 1% of the watershed. Several unusual landforms were identified in this inventory. A sackung was identified along the west side of Iron Mountain in the Kautz Creek watershed. Sackungs are typically identified as depressions running near the crest of a ridge and are deep-seated slope failures driven by gravitational forces acting on valley-walls over-steepened by glacial erosion. Parklands are gently sloping former lava flows that are favored destinations for visitors because of their accessibility and beautiful open subalpine meadows. This inventory identified 55 individual parklands that cover about 29 km2 of MORA. Another noteworthy landform at MORA is patterned ground, which consists of stone lines or rock polygons created by frost action on fine-grained deposits at elevations high above treeline. We identified nine sites in the park with these features; all are found along high ridges where winter temperatures are extremely cold and high winds remove insulating snow cover.

Geomorphology of Mount Rainier: Landform mapping at Mount Rainier National Park, Washington

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