Influence of climate and land cover on river discharge in the North Fork Stillaguamish River

A 2014 report prepared by the Stillaguamish Tribe analyzes potential causes of changes in peak and low flows in the Stillaguamish River basin.

Abstract

We analyzed trends in a wide range of flow and climate metrics using the Indicators of Hydrologic Alteration software. We then examined correlations between the flow metrics and potential drivers to ascertain whether climate trends, climate oscillations, or logging were the most likely drivers of temporal patterns in stream flows. We found increasing trends in peak flows (1-day, 3-day and 7-day average high flows), and that those trends are most likely driven by long-term climate trends, specifically increasing rainfall and decreasing snowfall. High flows increased steadily throughout the time period of analysis (1954-2011), whereas the proportion of immature forest land cover increased from 1954 to 1991 and declined from 1991 to 2011. Hence, land cover was not correlated with trends in high flow. Low flows (1-day, 3-day and 7-day average low flows) did not show a significant linear trend during the 1954-2011 time period. Rather, low flows increased during the first part of the study period and decreased during the second part. Low flows were correlated with total annual rainfall and snowfall, as well as land use and the climate oscillation metrics, suggesting that both decadal oscillations in climate and the timing of clearcutting may have influenced low flows, but it is not possible to discern which drivers are most important with our analysis. These results indicate that climate trends and oscillations are a likely cause of changes in both flood flows and low flows, although low flows may also have been influenced to some degree by land use.

Citation

Hall, J. E., Beechie, T. J., & Pess, G. R. (2014). Influence of climate and land cover on river discharge in the North Fork Stillaguamish River. Contract report by NOAA Fisheries, Northwest Fisheries Science Center, Seattle, Washington for the Stillaguamish Tribe of Indians.

Download the full report (PDF)