Elwha Watershed Information Resource
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Revegetation

The Elwha and Glines Canyon Dams and their reservoirs (Lake Aldwell and Lake Mills, respectively) cover about 800 acres of land in the Elwha River Watershed that was covered historically with native plants (NPS 1996). Once the dams are removed, 492 of those acres will be exposed and available for re-vegetation with native plants (that is, re-planting the areas with plants that would have grown there before the dams and reservoirs were built in the early 1900s). The remaining areas will either be too steep or unstable to actively revegetate or left open to revegetate naturally. We anticipate natural revegetation to proceed quickly in areas adjacent to undisturbed vegetation, and some areas will be left untreated for comparison to actively restored areas.

People planting native plants in plots
Planting native species in experimental plots with treatment combinations of mycorrhizal and saprophytic fungi.
Kerri Cook, University of California Davis

Along with the ecological role new vegetation will have in a restored Elwha River, it will also play a vital role in stabilizing sediments that will be washed out from behind the dams during the initial drawdown of the existing reservoirs. (Nearly 17 million cubic yards of sediment has built up behind the dams over the last ninety years.)

Native plants

The lower Elwha River is in the western hemlock zone, the dominant vegetation zone in western Washington. The Elwha River is relatively long for the Olympic Peninsula and experiences weather that is relatively cold at its upper end. Rainfall amounts decrease from the headwaters (140 inches annually) to the mouth of the river (30-40 inches annually). The two reservoirs are in the drier part of the watershed, and average 50-70 inches a rain annually. The dominant tree is Douglas-fir, but several other species are common in the lower watershed. 

The following is a list of dominant and common trees found in the lower Elwha watershed:

Areas to be re-vegetated with native plants include riparian areas, terraces, mid-slope depositional landforms, and steep, upland slopes. The selection of native plants for each area will be based on the composition of plants found in similar areas of the watershed, referred to as reference sites. Analysis of data from reference plots measured by park staff has revealed several common plant associations in the lower Elwha watershed:

  • Douglas-fir associations dominate the valley walls in the areas surrounding Lake Mills. Grand fir and big-leaf maple are occasionally prominent. Dominant understory species include dull Oregon-grape, swordfern, and salal. Western hemlock is uncommon on the dry, steep valley walls, but is common on moist sites such as ravines.
  • Riparian areas are more diverse than uplands, containing many different plant associations. Hardwood stands are common, and are usually dominated by red alder and Sitka willow, with cottonwood and bigleaf maple occasionally prominent. The shrub and herbaceous layer in the riparian zone is quite diverse. Common species include salmonberry, thimbleberry, Siberian springbeauty, blue wild-rye, Cooley’s hedgenettle, or Virginia strawberry. Riparian zones are important habitat and migration corridors for wildlife and play an important role in the ecology of the Elwha River.

Exotic Plants

Exotic plants are plants that are not native to the area. Invasive, exotic plants are non-native species that pose a significant threat to native ecosystems by displacing native plants and altering terrestrial and aquatic habitats. It is important to identify potential invasive, exotic plants and prevent their establishment during and after dam removal. If allowed to establish in the exposed reservoirs, invasive, exotic plants may alter successional trajectories, disrupt abiotic ecosystems processes (such as nutrient availability in soils), or reduce biodiversity.

One hundred and fifty species of exotic plants are known to exist in the Elwha watershed. Several of the exotic species are not considered a threat to native ecosystems, but there are many invasive, exotic species that pose threats to successful ecosystem restoration. Olympic National Park natural resource managers have compiled a list of 30 “species of concern” that will be the focus of treatment efforts prior to, during, and after dam removal. These include:

Foxglove
Foxglove (Digitalis purpurea)
© 2002-2005 Steve Baskauf
  • Bohemian knotweed (Polygonum x bohemicum)
  • Canada thistle (Cirsium arvense)
  • Cheatgrass (Bromus tectorum)
  • Common St. John’s wort (Hypericum perforatum)
  • English holly (Ilex aquifolium)
  • English ivy (Hedera helix)
  • Foxglove (Digitalis purpurea)
  • Himalayan blackberry (Rubus discolor)
  • Reed canarygrass (Phalaris arundinacea)
  • Tall fescue (Lolium arundinaceum)
  • Tansy ragwort (Senecio jacobea)
  • Buttercup (Ranunculus repens)
  • Clover (Trifolium repens)
  • Curly dock (Rumex crispus)
  • Dandelion (Taraxacum officinale)
  • Dovefoot geranium (Geranium molle)
  • English plantain (Plantago lanceolata)
  • Foxglove (Digitalis purpurea)
  • Herb Robert (Geranium robertianum)
  • Orchard grass (Dactylis glomerata)
  • Scotch broom (Cytisus scoparius)
  • Sheep sorrel (Rumex acetosella)
  • Stinging nettle (Urtica dioica)
  • Velvet grass (Holcus lanatus)

Revegetation Goals and Methods

The revegetation plan includes planting native plant species in upland and riparian areas to stabilize sediments and speed the initial revegetation process. The river channel area will continue to be naturally revegetated by wind and water-borne seeds. Full revegetation may take as long as 100 years.

Planting native species into the newly exposed reservoirs is the key to achieving revegetation goals. Planting woody species and seeding forbs and grasses will accelerate slope stabilization, help to minimize invasive of exotic species, and jump-start native ecosystem succession, particularly in areas far from the intact, native forests surrounding the reservoirs. A variety of plant materials will be utilized, including:

  • Native grass and forb seeds
  • 2-year old conifer tree seedlings
  • Live-stakes of willows, cottonwoods, red-osier dogwoods, and other woody species.
  • Bare-root stock of many native tree and shrub species• 
  • Containerized plants propagated at the Olympic National Park’s greenhouse
  • Wild seeds collected on-site and immediately hand seeded into the basins

All plant materials will derive from native plants collected in the lower Elwha watershed to preserve genetic integrity. Many areas will be left to revegetate naturally, including steep slopes, the immediate shoreline areas that are expected to readily regenerate from seed in the soil or dispersed from surrounding intact forest, and areas designated as control plots to measure the effects of revegetation efforts. The composition of plants and the type of materials used will be  tailored to local landforms and other site conditions. There are many different types of landforms expected to occur beneath the reservoirs such as terraces, bedrock benches, debris cones, alluvial fans, and steep valley walls. Substrate texture (fine or coarse) will further influence the composition of species chosen for a site. Full development of structurally-complex native vegetation may take as long as 100 years in some portions of the former reservoirs.

Reference

National Park Service. 1996. Elwha River Ecosystem Restoration, Draft Environmental Impact Statement. National Park Service, Olympic National Park, 600 East Park Avenue, Port Angeles, Washington, 98362.

Additional Information

Shafroth, P. B.and others. 2002. "Potential responses of riparian vegetation to dam removal." BioScience. Volume 52. Pages 703 to 712.

Vegetation Restoration after Dam Removal on the Elwha River: Current Projects Eastern Washington University, Rebecca Brown.