Geomorphic analysis of the river response to sedimentation downstream of Mount Rainier, Washington

Abstract

A study of the geomorphology of rivers draining Mount Rainier, Washington, was completed to identify sources of sediment to the river network; to identify important processes in the sedimentdelivery system; to assess current sediment loads in rivers draining Mount Rainier; to evaluate if there were trends in streamflow or sediment load since the early 20th century; and to assess how rates of sedimentation might continue into the future using published climate-change scenarios. Rivers draining Mount Rainier carry heavy sediment loads sourced primarily from the volcano that cause acute aggradation in deposition reaches as far away as the Puget Lowland. Calculated yields ranged from 2,000 tonnes per square kilometer per year [(tonnes/km 2)/yr] on the upper Nisqually River to 350 (tonnes/km 2)/yr on the lower Puyallup River, notably larger than sediment yields of 50-200 (tonnes/km 2)/yr typical for other Cascade Range rivers. These rivers can be assumed to be in a general state of sediment surplus. As a result, future aggradation rates will be largely influenced by the underlying hydrology carrying sediment downstream. The active-channel width of rivers directly draining Mount Rainier in 2009, used as a proxy for sediment released from Mount Rainier, changed little between 1965 and 1994 reflecting a climatic period that was relatively quiet hydrogeomorphically. From 1994 to 2009, a marked increase in geomorphic disturbance caused the active channels in many river reaches to widen. Comparing active-channel widths of glacier-draining rivers in 2009 to the distance of glacier retreat between 1913 and 1994 showed no correlation, suggesting that geomorphic disturbance in river reaches directly downstream of glaciers is not strongly governed by the degree of glacial retreat. In contrast, there was a correlation between active-channel width and the percentage of superglacier debris mantling the glacier, as measured in 1971. A conceptual model of sediment delivery processes from the mountain indicates that rockfalls, glaciers, debris flows, and main-stem flooding act sequentially to deliver sediment from Mount Rainier to river reaches in the Puget Lowland over decadal time scales. Greater-than-normal runoff was associated with cool phases of the Pacific Decadal Oscillation. Streamflow-gaging station data from four unregulated rivers directly draining Mount Rainier indicated no statistically significant trends of increasing peak flows over the course of the 20th century. The total sediment load of the upper Nisqually River from 1945 to 2011 was determined to be 1,200,000±180,000 tonnes/yr. The suspended-sediment load in the lower Puyallup River at Puyallup, Washington, was 860,000±300,000 tonnes/yr between 1978 and 1994, but the long-term load for the Puyallup River likely is about 1,000,000±400,000 tonnes/yr. Using a coarse-resolution bedloadtransport relation, the long-term average bedload was estimated to be about 30,000 tonnes/yr in the lower White River near Auburn, Washington, which was four times greater than bedload i...

Channel‐planform evolution in four rivers of Olympic National Park, Washington, USA: the roles of physical drivers and trophic cascades

“…Our analysis of temporal changes in flood magnitude showed that Q2 (two‐year‐flood) values were relatively low in the late 1960s and early 1970s, but increased thereafter (Figures , A6), consistent with previous findings of greater flood magnitude and frequency in recent decades in this region (Piety et al , ; Bountry et al , ; Beschta and Ripple, ; Czuba et al ., ; Tohver et al ., ). The Q2 magnitudes over the most recent ~4 decades have been 10–35% greater than over the entire length of each stream‐gage record.…”

“…The lowest annual flows occur in September. Flood magnitude typically fluctuates over decadal scales, due in part to Pacific Decadal Oscillation (PDO) cycles (Mantua et al , ; Czuba et al , ). Flood magnitude and frequency generally have been greater in Pacific Northwest rivers since the late 1970s than over the period of record as a whole (Piety et al , ; Bountry et al , ; Beschta and Ripple, ; Czuba et al , ; Tohver et al , ).…”

Large-scale dam removal on the Elwha River, Washington, USA: River channel and floodplain geomorphic change

“…For example, alluvial sections of gravel-bed rivers subject to increased bedload and aggradation often respond with channel widening and increased braiding (Leopold and Maddock, 1953;Lyons and Beschta, 1983;Knighton, 1989;Madej and Ozaki, 1996;Miller and Benda, 2000;Lauer et al, 2008;Pierson et al, 2011;Czuba et al, 2012). For natural river systems where new sediment is sourced from the upper watershed or from upstream channel migration, increased bedload is usually associated with large flood events.…”

Carbon accumulation and vertical accretion in a restored versus historic salt marsh in southern Puget Sound, Washington, United States

“…The dominant source of the inorganic sediment at SG is likely tidal and not riverine material as there is limited sediment available from the Nisqually River, due to the upstream La Grande and Alder dams. Such restriction of river flow results in only about 10% of the total sediment load of the Nisqually River reaching Puget Sound on any given year (Czuba et al ; Curran et al ), resulting in a shortage of sediment available for marsh‐building at the river mouth.…”