Seasonal and decadal-scale channel evolution on the dammed Elwha River, Washington

To complement ongoing field studies of channel morphology, we evaluate decadal-scale evolution of the dammed lower Elwha River by using historical aerial photographs. Here, we revise an analysis published by Draut et al. (2008), which covered the interval 1939–2006, to include data collected after a major flood on December 3, 2007. That flood, which resulted from substantial rainfall on snow in the upper watershed, had instantaneous peak discharge of 1,016 m3/s (35,900 ft3/s), the second-highest recorded peak on the Elwha River since records began in 1898 and the largest since dam construction. A log-Pearson Type III flood-frequency analysis indicates that this flood would have a return interval of ~30 years (annual exceedence probability of ~0.03; C.S. Magirl, unpublished data). Observing channel change on the Elwha River from that event allows a more thorough characterization of the dammed channel’s response to flow over seasonal as well as annual and decadal time scales.