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 sediment delivery
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/km2)/yr] on the upper Nisqually River
to 350 (tonnes/km2)/yr on the lower Puyallup River, notably larger than sediment yields of 50–200
(tonnes/km2)/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 bedload transport
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 in the Puyallup
River and an order of magnitude greater than bedload in the Carbon River. Analyses indicate a general
increase in the sediment loads in Mount Rainier rivers in the 1990s and 2000s relative to the time period
from the 1960s to 1980s. Data are insufficient, however, to determine definitively if post-1990 increases
in sediment production and transport from Mount Rainier represent a statistically significant increase
relative to sediment-load values typical from Mount Rainier during the entire 20th century.
One-dimensional river-hydraulic and sediment-transport models simulated the entrainment,
transport, attrition, and deposition of bed material. Simulations showed that bed-material loads were
largest for the Nisqually River and smallest for the Carbon River. The models were used to simulate
how increases in sediment supply to rivers transport through the river systems and affect lowland
reaches. For each simulation, the input sediment pulse evolved through a combination of translation,
dispersion, and attrition as it moved downstream. The characteristic transport times for the median
sediment-size pulse to arrive downstream for the Nisqually, Carbon, Puyallup, and White Rivers were
approximately 70, 300, 80, and 60 years, respectively.