Flooding is a relatively common occurance along the Elwha River. While the dams do not provide flood protection, they do hold back sediments. Once the dams are removed, sand, gravel and other sediments will again move downstream. As this occurs, sediments will accumulate along the river bed, raising its level and increasing slightly the risk of flood. In accordance with the Elwha Act, flood protection is being provided for downstream landowners.
Reasons for Action:
Although the two dams were never operated for flood protection, the riverbed below the dams will rise as accumulated sediment is released from the reservoirs and the river's natural sediment transport regime is restored. The river has been mapped and areas at risk of aggradation following dam removal have been identified. Depending on location, the river bed may rise from as little as six inches in some areas to as much as 2.5 feet near the river mouth. Providing flood protection to the areas at risk ensures the safety of property and property holders on the lower Elwha River.
Proposed Action for Flood Protection:
A series of other small-scale flood protection measures have been carried out in order to protect private homes. These include raising wellheads, dikes, roads, and property where necessary, based on discussions with each affected home owner. A more detailed description of these mitigations can be found in the Elwha River Ecosystem Restoration Implementation Final SEIS (PDF).