Dungeness River near Sequim, Washington
Upland Areas Supply Coarse Sediment
The Dungeness River is a gravel and cobble-bed stream located on the Olympic Peninsula of northwestern Washington State. The river flows northward about 30 miles from the base of Mount Deception in the Olympic Mountains to the Strait of Juan De Fuca near the town of Sequim. The watershed has a drainage area of more than 156 mi^2, a portion of which is located within the boundaries of Olympic National Park. The river is steep, falling about 3,300 feet in 28 miles for an average slope of 0.022. The annual mean flow of the Dungeness River is 380 ft^3/s with a mean annual runoff of 33 inches. The highest peak flow recorded was 7,120 ft^3/s on November 24, 1990 and the lowest mean-daily flow recorded was 65 ft^3/s.
Road Construction Can Cause Land Slides
Dikes and Levees Constrict the Channel
The river slope is steepest in the upper watershed canyons, but decreases downstream. The downstream most 10 miles of river flows across a piedmont surface, composed of glacially-derived sediment and outwash. The channel in these lower 10 miles has incised into this material since the Pleistocene (about 10,000 years ago) and meandered some, but within a relatively narrow corridor.
There has been a sharp decline in the numbers of salmon returning from the ocean to spawn in the Dungeness River. Two species of salmon are now listed as endangered. The upstream watershed has been affected by logging and the associated road building. The lower ten miles of river have been affected by the construction of levees, dikes, bridges, bank armoring, and by irrigation withdrawals and the removal of large woody debris. The physical processes of the Dungeness River are being studied to determine the degree to which natural processes have been impacted by human development. A comprehensive study approach has been initiated because the processes are complex and interdependent. The following types of data have been collected and analyzed during the course of the study (Piety and others, written communication, 2000):
- A network of more than 60 river cross sections was established with monuments along the lower 10 miles of the river in 1997. Data from this cross-section network define the channel geometry and slope and will serve as a baseline for future monitoring. The data were also used in a hydraulic model to identify reaches of low sediment transport capacity and to document the hydraulic impacts of levees, dikes, and bridges.
- The sediment grain-size distribution of the river-bed material was measured at various locations along the lower 10 miles of the river. Both the surface and sub-surface layers of the bed material were measured in the field. These data were collected to determine if the coarse gravel and cobble-sized sediments were being transported all the way to the estuary.
- Suspended sediment load and bed load rates were measured to determine sediment yield rates from the upstream watershed and to verify predictive sediment transport equations. Sediment modeling can then be applied to estimate rates of channel degradation, aggradation, or avulsions.
- Current and historic aerial photographs and maps were examined to determine the historical boundaries of the channel, rate of channel migration, and if the channel was subject to avulsions. Soil samples were collected from river terraces for radiocarbon 14C dating to determine how many years it had been since the channel had abandoned the terraces.
- Time lapsed photography was also taken at three locations along the river channel to determine if large woody debris caused the formation of gravel bars and to determine the rates at which the bars are formed.
Download
- Lai, Y.G. and Bountry, J.A. (2007). Numerical Modeling Study of Levee Setback Alternatives for Lower Dungeness River, WA. U.S. Department of the Interior, Bureau of Reclamation, Technical Service Center, Sedimentation and River Hydraulics Group, Denver, CO. May. (pdf 30.9MB)
References
Piety, L.A.; Randle, T.J.; Bountry, J.A.; Link, R.A., January 2000, written communication: Geomorphology of the Lower Dungeness River, Draft Progress Report for Fiscal Years 1998 and 1999, Dungeness River, Washington, U.S. Department of the Interior, Bureau of Reclamation, Denver, Colorado, 63 pages.