Downstream-Propagating Channel Responses to Decadal-Scale Climate Variability in a Glaciated River Basin

Regional climate is an important control on the rate of coarse sediment mobilization and transport in alpine river systems. Changes in climate are then expected to cause a cascade of geomorphic responses, including adjustments in downstream channel morphology. However, the mechanics and sensitivity of channel response to short-term climate variability remain poorly documented. In the Nooksack River, which drains a glaciated stratovolcano in Washington State, bed elevation changes were inferred from shifting stage-discharge relations at seven U.S Geological Survey stream gages. Decadal-scale elevation trends can be explained as a downstream-propagating channel response to regional climate variability, where periods of persistent warm, dry (cool and wet) conditions corresponded to periods of aggradation (incision). The channel elevation response propagated downstream at a rate of 1 to 4 km per year; propagation rate scaled closely with channel slope. Historical trends in glacier extent and flood intensity both show some potential to explain climate-sediment linkages, though assessing causation is complicated by the shared climate signal in both records. Results show the influence of the Pacific Decadal Oscillation, with relatively high coarse sediment yields prior to 1950 and since 1980, and notably lower sediment yields from 1950 to 1980. Measured sediment yields from nearby glaciated basins corroborate this history, suggesting a regional consistency to these climate-sediment linkages. These results document consistent relations between climate, sediment supply, and downstream channel response at the basin scale, with channel responses propagating downstream over periods of decades with little apparent attenuation. Plain Language Summary The shape and form of a gravel-bedded river is a function of the amount of water and sediment supplied from upstream. Climate-driven changes in the amount of sediment supplied from higher-elevation source areas may then cause changes in downstream channel form, resulting in changes in habitat suitability or flood conveyance. However, the linkages between climate, sediment production, and downstream channel response are complicated and often difficult to monitor, leaving it unclear if or how rivers might respond to climate variability over societally important timescales of years to decades. In the glaciated Nooksack River in Washington State, we observe that changes in climate over the past century have resulted in distinct and consistent changes in channel bed elevation. Those changes appear first in the headwaters and then propagate downstream over a period of decades. Channel change in the upper river is then a response to climate about 20years prior, while channel change in the lower river is a response to climate about 70years ago. These results provide evidence that, at least in certain settings, short-term climate signals influence downstream river systems and help define the timing and magnitude of those adjustments.