Elwha Dam - Dam Removal

Dam Removal

The Elwha River Ecosystem and Fisheries Restoration Act of 1992 created funding and government support for mitigation of the dams on the Elwha. While fish ladders were proposed as a possibility, they were ultimately ruled out for the reasons above. The major problem raised with regard to removal of the two dams is the approximately 18 million cubic yards of sediment that has built up behind the dams. Of that 18 million, 13 million is behind the Glines Canyon Dam, while 5 million is behind the Elwha Dam. Releasing all of this sediment at once would have devastating effects on the remaining habitats downstream from the dams, as well as along beaches at the coast. A drawdown experiment was conducted on the Glines Canyon Dam to assess the erosion of the sediments from Lake Mills. The lake was drawn down 18 feet (5.5 m) over one week and then allowed to remain stable for one more week. This test showed that the river would rapidly cut into the existing delta as soon as the lake level dropped, creating a deep, narrow channel. A new delta immediately began to form at the new lake level. Once the lake level was held steady, the channel migrated laterally along the existing delta. This test resulted in a sediment erosion model in which gradual drawdown of the lake would result in a greatly decreased sediment load downstream. Using this model, the plan is to drain the lake gradually over a two year time period, allowing much of the sediment to remain in the old lake basin.

After the two year dam removal period, the river will begin to restore itself to natural conditions. Sediment discharge rates will increase, resulting in restored and rejuvenated habitats downstream from the dams. Fish will return to the upper reaches of the Elwha as quickly as a year after dam removal is complete. While the river may never return to historically large runs, after 15–20 years or more, there may be the recovery of salmon to some sort of “pre-dam” condition. In addition to restoring the fish habitats, the draining of Lake Mills and Lake Aldwell will create an additional 715 acres (2.9 km2) of terrestrial vegetation, improving elk habitats as well (Figure 1). Increased sediments loads are also predicted to help restore the retreating delta at the mouth of the Elwha.

Removal of the dams on the Elwha River has been a long, ongoing process. It appears now that the end is relatively near. Removal started on September 27, 2011. Once the removal is complete, there is uncertainty about what exactly happens next. Because so few dam removal projects have been accompanied by scientific study, perhaps fewer than 20, this project will be a model of the effectiveness of dam removal on this scale. If it is successful, there could be a movement towards the restoration of natural stream morphology through the removal of larger dams, although the results may not be apparent for some time. Most fish populations are expected to make a return to the river (Table 1). While fish may return to the river as soon as a year after the removal is complete, it may take 15 to 20 years for the river to fully return to its pre-dam conditions. The ultimate goal of the dam removal is restoring the river to a nearly pristine state.

It has been estimated that the removal of the dams will cost $350 million.