Elwha Dam: Keeping an eye on sediment flowsApril 5th, 2012 by cdunagan
Removal of the Elwha Dam and drawdown of Lake Aldwell behind it have gone faster than originally planned, and now the story of the Elwha River restoration becomes a story of erosion. Experts are watching the sediment movement very closely.
The Elwha Dam has been entirely removed down to the river bed (see photos below), and the river is now flowing in its original channel, where it will remain. The river is being held back mainly by a “check dam” of boulders. At the moment, the drawdown has been halted at 133 feet elevation for a scheduled two-week holding period.
Andy Ritchie, restoration project hydrologist with Olympic National Park, says the pause in drawdown will allow the river to snake around to redistribute the sediment more evenly across the valley. The final target elevation for the river bed is 100 feet.
Drawdown of Lake Mills, behind the upper Glines Canyon Dam, also is on hold at the moment. Even more sediment is trapped behind that dam. While project managers have largely lost control over the movement of sediment behind the lower dam, the upper dam remains intact enough to control migration of sediment from farther up the canyon.
As the weather improves this spring (or at least we can hope), it may be time for many of us to visit the former lake beds at the two dams. We can walk out onto the deltas and see the new vegetation starting to grow. Lake Aldwell’s delta can be reached from the old boat launch. For Lake Mills, take Whiskey Bend Road, which has been reopened, and you will come to Humes Ranch trailhead with access from there.
It appears the sediments are stable enough for people to walk out onto the former lake beds, though visitors are cautioned about steep banks, overhangs and mud, and people should watch to avoid new plantings. Before visiting, you may wish to check out Olympic National Park’s “Access Updates” or stop in at the Olympic National Park Visitor Center.
Computer modeling shows that sediment loads may be high at times, but suspended solids should not exceed 40 grams per liter, a tolerable level for the downstream water-treatment plant, Andy told me. If needed, the treatment plant can be turned on to clean up the drinking water for Port Angeles and for raceways at a fish hatchery operated by the Lower Elwha Klallam Tribe. So far, the turbidity has rarely exceeded 12 grams per liter.
Andy said fine sediments are flushing downstream, but a “wedge” of coarser sediment has not yet moved past the lower dam site. Those coarser sediments are more likely to be deposited where the flow of the river declines — especially where the river widens near the entrance to the Strait of Juan de Fuca.
“In a lot of ways, what the sediment does from Elwha Dam downstream will give us a warning about what will be happening with Glines,” he told me. “The mean grain size there will be larger.”
As Lake Aldwell has been drawn down, Ritchie said the researchers realized that the pre-dam ground level was higher in some places than they had originally estimated. The result is that some areas of the former reservoir contained less sediment than originally calculated.
Also, because of the mild winter just past, lower river flows did not transport as much sediment as originally hoped. The effect is that the delta is further from pre-dam conditions than it would have been with higher flows — though everyone realizes that it could take a decade or more before conditions really settle down.
Andy said those working on the project would like to see larger flows just to measure the effects of a good flood on sediment movement while intensive monitoring equipment is still in place. The equipment is scheduled for removal no earlier than next winter.
Seattle Times reporter Lynda Mapes recently wrote a couple of excellent stories about changing conditions along the Elwha River. The first, called “Rush of freedom for Elwha as dam comes down,” describes the landscape around the two dams. The second, called “Man giving nature a helping hand in laying a new Elwha carpet,” focuses on the struggles to get plants to grow in areas long submerged.