Elwha Watershed Information Resource

Once dam removal has been completed, areas currently covered by water and sediment (Lake Aldwell) will once again be exposed to the elements. Where once trees and grasses covered soils and sediment before the dams were constructed, a barren landscape will be uncovered as the reservoir is drained and the dams are removed. The weather will begin to impact this landscape causing erosion of soils that were deposited over years behind an artificial barrier.

Although much of this erosion will be allowed to occur with little or no mitigation in order to expidite a return to pre-existing conditions, some attempts to stabilize these soils in place will be beneficial in order to allow normal turbidity levels to occur and lessen sediment loading of fish spawning gravel beds. Planting native species of flora in the exposed reservoir will help to offset continual erosional disturbance of the river system providing slope stabilization, shade, cover from predation and a renewed source of large woody debris. In order to do this, existing and historic drainage pathways needed to be mapped so that re-vegetation efforts could be applied in areas less subject to soil disturbances such as precipitation runoff and landslides from slope failures.

Actual flow path development cannot be 100% predicted however, it is assumed that using existing bathymetry combined with upland LiDaR elevation data in conjunction with ArcHydro modeling tools, a relatively accurate predictive model of potential drainage flow paths could be developed and combined with historical drainage characteristics to delineate the most likely areas to experience soil disturbance. Once these areas are mapped, modest buffers could then be applied and outlying areas could be identified as areas most suitable areas for planting slow growing overstory while areas within the buffered regions could be planted with faster growing slope stablizing vegetation thereby minimizing plant and soil losses.

Once the mitigation for the reservoirs was divied between the National Park Service and the Lower Elwha Klallam Tribe, the Park was tasked with revegetation planning for the upper reservoir (Lake Mills) which resides within the Olympic National Park Boundaries and the Tribe was tasked with revegetation planning for the lower reservoir (Lake Aldwell) which resides on lands slated to be transfered to the Tribe (a.k.a., the project lands). As the GIS Analyst for the Lower Elwha Klallam Tribe, I was tasked with identifying and mapping suitable areas for various types of vegetation.

Maps have been created depicting information useful for determining revegetation suitability, namely:

• A combined elevation/bathymetry map
• An aspect map depicting directions of solar exposure
• A slope map depicting steepness
• The preliminary treatment areas
• A flow direction and drainage flow path map illustrating immediate likely drainage paths for existing and future run-off
• An existing landslide/erosion area map
• An historical overlay map using a scanned and georeferenced/rectified overlay of a 1913 map contributed by Clallam County used to determine pre-existing sediment conditions, stream and river pathways
• A drainage classification map derived from all known existing and potential drainage paths
• Proposed LWD and ELJ treatment map