USGS Science to Support the Elwha River Restoration Project
The Elwha River basin covers about 833 square kilometers and contributes more than 1.2 cubic kilometers per year of freshwater to the Strait of Juan de Fuca. The river also discharges 220,000 to 510,00 tonnes of sediment each year (Curran and others, 2009), although until dam removal much of this sediment was trapped within the reservoirs created on this river. Following dam removal much of this sediment will be restored to the river and this will cause elevated water turbidity, changes to the channel shape including filling of river pools with sediment, and modifications to river habitats. The USGS is providing fundamental monitoring and sampling to track the effects of the dam removal project on the river.
Most flow from the Elwha River comes from large winter storms in November through February and snowmelt and precipitation in the late spring. Most of the sediment carried by the river is sand, silt, and clay and is transported out to the coast during high flows. The river also carries larger gravel, cobble, woody debris, and even whole trees and boulders during storms.
The USGS is monitoring water and sediment discharge in the Elwha River at several stations to track the baseline conditions before dam removal and to track changes with time. This work includes the use of both traditional river sampling and gaging and the use of state-of-the-art acoustic and optical sensors. Data from these stations can be obtained from the Products page of this website.
Results of the river and sediment monitoring include:
These and other results can be found on the USGS Washington Water Science Center’s project websites:
The channels of the Elwha River are important flow pathways for sediment and organic materials from the mountains to the sea. These materials are also deposited in and along the river channel where they form important habitats for fish, aquatic invertebrates, and riparian vegetation. The shape and form (i.e., the morphology) of the river channel also controls the flow of water down the channel, which influences the extent and duration of flooding throughout the floodplain.
The USGS has worked with partners at the Lower Elwha Klallam Tribe, Bureau of Reclamation, and the National Park Service to measure the current and historic morphology of the Elwha River channel. These studies use detailed topographic surveys and measurements from historical aerial photographs to track changes with time. These studies will form the basis for detecting changes caused by the renewal of sediment supplies to the lower river. Scientists have also tracked changes to the deltas of sediment with the river’s reservoirs during the 1994 drawdown experiment, and USGS scientists are working to track erosion and changes to these deltas during dam removal.
Studies of Elwha River morphology have found that:
Floodplain vegetation communities along the Elwha River are high in species diversity and also influence salmonid, aquatic invertebrate, and wildlife populations through their effects on in-stream and terrestrial habitat, and organic matter and nutrient inputs to the river. Floodplain vegetation dynamics and diversity in the Pacific Northwest are linked to stream flow regimes, and fluxes of sediment and large woody debris. Dam removal on the Elwha River will alter these system drivers, inducing changes to floodplain vegetation.
The USGS has worked with partners at the Lower Elwha Klallam Tribe, Eastern and Western Washington Universities, and the National Park Service to characterize baseline conditions of floodplain vegetation, including structure, composition, and diversity, including exotic and invasive species. These studies have involved detailed surveys of vegetation in > 150 permanent plots between 2003 and 2010 along five cross-valley transects in each of four reaches of the Elwha: above both dams (reference), between the dams, downstream of both dams, and in the estuary.
Studies of Elwha River floodplain vegetation have found that:
The flow of nutrients and other chemicals within a river is directly related to the physical state and conditions of the river’s watershed. Many waters in the Pacific Northwest are naturally nutrient poor, or oligotrophic. Salmon directly influence the nutrient status of rivers by adding nutrients derived from the ocean (“marine-derived nutrients”) to the river waters as they migrate and spawn. The decomposition of their carcasses following spawning also releases nutrients, some of which enter freshwater foodwebs. These marine-derived nutrients provide an important pulse of nutrients that can influence the growth rates of floodplain vegetation and riverine plants and animals, including the following generations of juvenile salmon.
Because the two dams on the Elwha River watershed, were restricted to the lower 7.9 km of river below the Elwha dam. The river sections between the two dams and above the Glines Canyon dam did not have spawning salmon, or their marine-derived nutrients, for nearly 100 years. As reported by Munn and others (1998), and Duda and others (2010, 2011), the nutrient status of the Elwha River, its estuary, and tributaries was oligotrophic, or nutrient poor. The areas below the dams, where salmon still have access, had elevated levels of marine-derived nitrogen in the tissues of stream invertebrates and fish when compared to areas between and upstream of the two dams. This eleveated marined-derived nutrient signature was revealed by stable isotope analysis.
For more information about these studies, see: