Peak spring snowmelt flows will hit the Elwha soon, notes Andy Ritchie, restoration hydrologist for the Olympic National Park. And that means a whole lot of sediment coming out of the Elwha River, where one dam has been removed, and the other, Glines Canyon Dam, about 8.6 miles upriver, is in the bulls eye.
Dam removal on the Elwha is on hold while repairs are made to a water plant needed to manage increase sediment loads. But meanwhile, the Elwha is busily chewing away at the sediment already unleashed by the dam removal so far.
In all scientists estimate 34 million cubic yards of sediment will be mobilized by dam removal on the Elwha. About 40 percent of it is expected to stay behind in the watershed, in stepped down terraces along the valley walls.
But the rest is expected to be eroded out by the river to the nearshore environment and beyond, where it is already building up bars and beaches at the river mouth
Here’s a recent photo from Tom Roorda, pilot at Port Angeles, who took this photo last week:
But just how much is 34 million cubic yards, anyway? I have never been able to imagine it … but Ritchie gave it some thought, and came up with these calculations:
“In terms of football fields, including the end zone, it looks like it would fill an American football field 3 miles high.
Other metrics: It would fill lake Union. And leave a mound about 2 feet high (assuming these dimensions).
10 great pyramids of Giza (assuming a volume of 2.5 million cubic meters)
1/10 of the volume of the great wall of China (approximate)
It could cover the city of Seattle in 2.77 inches of sediment.
It could cover Mercer Island with 2.5 ft of sediment
It could cover Seattle Center with 284 ft of sediment.”
Any way you look it it, that is a lot of mud. Ultimately, it will transform the structure and help restore the natural function of the Elwha and near shore.
But when it comes to sediment, pictures really are worth a thousand words. Here’s one more, from Tom Roorda: