STREAMBANKS AND RIVER DYNAMICS ON THE NORTH FORK NOOKSACK RIVER, WASHINGTON

The North Fork Nooksack River (NFNR) provides an interesting and instructive example of the coupling of streambank and floodplain processes. The NFNR originates on glaciated mountains of the North Cascades. There is a naturally high sediment load, and many reaches of the river are braided. The Washington Department of Transportation (WSDOT) initiated an analysis of the river and streambanks along a reach where portions of a state highway are threatened by erosion. The project reach included a braided channel with a 20 m high bluff (Warnick Bluff) on the right bank. Warnick Bluff is cut into alluvial fan and floodplain deposits, and extends for approximately 600 m. Three “subreaches” were delineated, including the upper subreach, which is very strongly influenced by the alluvial fan and the landslide, the middle reach, a short and narrow section between the tributary fan and the landslide; and the lower subreach, which is long, wide, and very shallow.

The river in this reach is affected not only by a major late Holocene landslide, but also by the high sediment load of tributary streams. The complex interplay between streambank and alluvial fan influence the morphology of the river itself, which in turn affects the streambanks. In addition, loss of root strength along the streambanks, due to timber harvest, may also be a contributing factor in channel instability. Efforts to stabilize portions of the streambanks are underway. A series of structures has been installed along the base of Warnick Bluff in order to deflect the thalweg away from the toe of the slope. Monitoring of the logs and the channel cross-section is ongoing. Given the numerous sediment sources, several questions concern management of the highway corridor. Have thresholds of streambank stability been crossed? If so, what structures might be used to stabilize the channel and banks?