CHANNEL AND FLOODPLAIN DYNAMICS, QUINAULT AND QUEETS RIVERS, WASHINGTON

The distal alluvial segments of the Quinault and Queets Rivers of the western Olympic Peninsula, Washington, have measurable differences in channel slope, active channel width, and channel migration rates that are attributable to differences in dynamic interactions involving channel migration processes, instream wood and sediment fluxes, and floodplain forest conditions operating over time scales of decades to centuries. The Queets and upper Quinault Rivers have gravel bed channels with abundant sand, gravel, and wood supplied from the Olympic Mountain headwaters. The lower Quinault River is also gravel bedded, but it is downstream of Lake Quinault and receives much less sediment and wood from upstream. Analysis of historic photographs and maps dating from circa 1900 show that channel migration rates vary spatially and temporally, but range from 5 to 10 m/yr, with generally faster rates on the steeper and sediment?rich Queets and upper Quinault Rivers than on the lower Quinault River. Likewise, active channel width is significantly greater for the upper Quinault and Queets Rivers. Floodplain turnover rates are similar for all three reaches, with channels eroding the floodplain at the rate of about 0.2% of the floodplain area per year, and with corresponding floodplain half-lives of 300 to 500 yr. On the upper Quinault and Queets Rivers, log jams promote bar growth and consequent channel shifting, short avulsions, and meander cutoffs, resulting in mobile and wide active channels and abundant patches of young riparian vegetation. On the relatively sediment-poor lower Quinault River, long channel reaches are stable and the river flows within floodplains covered with mature forests. Historically, on the lower Quinault River, channel-spanning log jams instigated short reaches of instability that have persisted for several decades through local feedback mechanisms of bank erosion and sediment and wood transport. For all three reaches, log jams keyed by large conifer stems become nurseries for conifer germination and growth. The resulting conifer groves influence future channel and floodplain conditions by forming floodplain areas resistant to channel migration, and they eventually provide key members of future log jams.