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Geologic Publications for Mount Rainier

Environmental and ecological implications of aggradation in braided rivers at Mount Rainier National Park

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Author(s): Scott R. Beason, Paul M. Kennard

Document Type:
Publisher: National Park Service Natural Resource Year in Review
Published Year: 2006
Pages: 52 to 53
DOI Identifier:
ISBN Identifier:

In November 2006 a major storm dropped nearly 18 inches (46 cm) of rain in 36 hours at Mount Rainier National Park, Washington. This event caused severe park-wide damage, but the resulting flood was not entirely to blame for the destruction. The geologic setting, physical characteristics of the rivers, and injudicious placement of park infrastructure made the devastation inevitable.

At Mount Rainier, glacially fed braided rivers radiate outward from the 14,410-foot (4,392 m) volcano. These streams carry materials ranging in size from silt to boulders. As gradients decrease away from the mountain, rivers deposit their sediment loads. The height of the river channels rises while streambanks and floodplains remain at their present elevations. Through this process, called aggradation, rivers at Mount Rainier National Park have been inexorably increasing in height over time.

Exact rates of river aggradation in the park were unknown until a 2006 study, which incorporated survey data from 1997 and 2005. Using 1910 longitudinal profiles and historical topographic maps, the National Park Service and cooperating scientists compared current and earlier rates of aggradation, focusing on river areas near popular visitor destinations and primary park infrastructure. Investigators surveyed and created cross sections of current river channels, which they analyzed using Geographic Information System (GIS) software. Depending on the channel slope and confinement, the background aggradation rate of braided rivers at the park is approximately 6 to 14 inches (15 to 36 cm) per decade. At areas in the park with recent debris flows, however, aggradation is much higher. For example, during a single event, approximately 6 feet (1.8 m) of material was deposited over an area of 107,000 square feet (9,940 m2) in the Nisqually River above Longmire, a primary park visitor and work area.

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Suggested Citations:
In Text Citation:
Beason and Kennard (2006) or (Beason and Kennard, 2006)

References Citation:
Beason, S.R. and P.M. Kennard, 2006, Environmental and ecological implications of aggradation in braided rivers at Mount Rainier National Park: National Park Service Natural Resource Year in Review, pp. 52-53.