Last week, while attending the Elwha Science Symposium, I was able to visit the reservoir above Elwha Dam with the scientists who are studying the effects of dam removal on the sediment that has accumulated in the reservoir. There are actually two dams being removed simultaneously from the river, Glines Canyon and Elwha Dam, the latter being the lower dam on the River.
The big question with all dam removal projects is sediment management. The total sediment that has accumulated behind these two dams amounts to over 24 million cubic yards. Since the upper reservoir, Lake Mills, has been trapping the majority of the coarse sand and gravel flowing down the river, the lower reservoir has trapped less material, the majority of it 'fine' clay and silt.
For the removal of the two dams on the Elwha River, the plan is for natural transport, with incremental notching of the dams to allow the river to rework the sediments. This summer, both reservoirs were drawn down, or drained, about 20 feet. This has provided an opportunity to document the initial changes to the delta sediment that has been deposited in the upper end of the reservoirs.
|Reservoir sediments at the delta of Lake Aldwell, upstream of Elwha Dam. |
Puget Sound is visible in the distance.
In order to document the current topography of the sediments, scientists from the USGS are using ground-based LiDAR, which is basically a terrestrial laser scanner. It takes 4400 points per second using a pulsed near-infrared laser. The scanner was set up at different locations around the delta, then all the scans were linked together and to real-world coordinates using a survey grade GPS. The result is this otherworldly image, as well as a set of data that accurately models the surface of the ground and surrounding features.
|LIDAR image of reservoir sediments provides accurate surface topography data|
|LIDAR image of Brian Cluer and Paul Jenkin on reservoir sediments|
|Wider view of Brian and Paul on the reservoir delta sediments|
Of interest to those of us working on Matilija Dam is the impacts to water quality during and following erosion of the reservoir sediments. The fear has been that erosion of fine silt and clay will result in chronic turbidity and downstream impacts. The reality is that this impact is short-lived during periods of active erosion, but water quality improves rapidly thereafter. This was evident while standing on the muddy delta in Lake Aldwell, watching crystal clear water flowing over and through the fine sediments.
|Blair Greiman, USBR hydrology analyst, observes post-drawdown erosion|
of fine sediments in Lake Aldwell above Elwah dam, WA.
other venturariver.org blog posts on Elwha:
Ventura River Ecosystem: Elwah Science Symposium
Last Dam Summer