CLIMATE CHANGE AND INCREASING AGGRADATION RATES IN GLACIALLY-FED BRAIDED RIVERS AT MOUNT RAINIER NATIONAL PARK, WASHINGTON

Mount Rainier (4392 m) is the tallest of the Cascades Mountains and one of most hazardous volcanoes in the United States. Previous research (Beason et al., 2006; Beason, 2007) has quantified aggradation rates in braided rivers at Mount Rainier. In November 2006, an intense rainstorm dropped 46 cm of rain in 36 hours inside Mount Rainier National Park leading to a historic 6-month park closure. Researchers resurveyed many locations in the park and found few locations where the river channel degraded. This unexpected finding indicates that sediment production overwhelmed erosion in an extreme flood. In areas that had no debris flow activity, aggradation was measured ranging from 12 cm to over 1.5 m. Overall, aggradation rates observed following the floods represent approximately 2 decades of historic, background aggradation in 1 event.

Aggradation is a serious management and safety concern for Mount Rainier National Park, as a great deal of park infrastructure is located in valley bottoms near – or in – major river channels. Many locations in the park are tens of meters below rapidly aggrading rivers. As aggradation fills up braided river channels, the conveyance capacity of the rivers during flood events is decreased – meaning, similar sized floods will become increasingly more destructive over time.

Changing climates in the Pacific Northwest are leading to receding glaciers, decreased snowpacks, and increased air temperatures (Mote, 2007). At Mount Rainier, glacial recession and thinning has resulted in the disappearance of approximately 25% of the mountain's glacial volume between 1913 and 1994 (Nylen, 2001). When glaciers retreat, metastable, oversteepened lateral moraines can be undercut by river flow and fail, adding acres of material to rivers. During high energy events, traditional bank erosion also supplies material to channels, and together with moraine failure, leads to “hyper aggradation,” or the rapid accumulation of material. The relationship of air temperature and suspended sediment, and the temperature trend of increasing air temperature in the park, are well-known. The result is exponentially more sediment is introduced to the system, a relationship noted in other glacially-fed braided river systems world-wide. Increased aggradation rates are tied into the changing climate at Mount Rainier and are a good indicator of climate change at the park.