Differentiating the effects of logging, river engineering, and hydropower dams on flooding in the Skokomish River, Washington, USA

Abstract

A decades-long, progressive loss of channel capacity in the Skokomish River, a 622 km2 basin draining the southeast Olympic Mountains of Washington State, has caused increasing flooding with severe consequences to endangered salmon runs, infrastructure, and private property. To differentiate among multiple potential drivers of the capacity loss, we analyze the geomorphic evidence for the potential effects of: flow regulation by two dams constituting the Cushman Hydroelectric Project, which began regulating flow in the river's North Fork in 1925 and diverting water out of the basin in 1930; sediment production from mid-twentieth century logging in the river's South Fork basin; and twentieth century river engineering in the mainstem. Bankfull channel capacity in the mainstem has steadily declined since about 1940 from 370 m3 s−1 to <100 m3 s−1 due partly to the narrowing of the Skokomish River, which in 2015 was only 45% as wide as it was in 1938. The capacity loss is also due to sediment filling the channel, with nearly 2 m of aggradation measured at a stream gauge since 1965. Comparison of channel cross sections surveyed in 1994, 2007, and 2016 show that about 20,000 m3 yr−1 (34,000 Mg yr−1) of sediment is accumulating in the Skokomish River. The nature, timing, and spatial pattern of this channel narrowing and shallowing are consistent with the response expected from the Cushman Project, which exports water out of basin and thus substantially reduces downstream flows, but, because the dams were built below a natural lake, does not reduce the sediment supply. While sediment yield from the South Fork is high, accounting for about three-fourths of the total sediment supplied to the Skokomish River, it is dominated by the progressive widening of the channel and recruitment by lateral fluvial erosion of glacial sediments in alluvial terraces; landslides associated with logging in the South Fork basin produced a small amount of sediment relative both to the sediment produced by channel widening in the upper South Fork and to the rate of aggradation in the mainstem Skokomish River. The naturally-high sediment load from the South Fork and the flow reduction in the North Fork result in the unusual effect of flooding having increased downstream of the dams despite substantial reductions to peak flows. This case study illustrates how a watershed-scale analysis of multiple land uses and flow management and their interaction with the basin's geology and geomorphology can make use of geomorphic evidence to differentiate among the possible drivers of channel change and associated flooding.