Recent publicity about full reservoirs and overflowing dams piqued my interest in how the dams in our state are holding up How many dams do we have and where are they? What safety risks do they pose. As it turns out, the answer to these and many other dam questions in the state are now are now available thanks to an updated report prepared last fall by the state Department of Ecology. Would you believe that our state as 1,189 dams with the biggest share located right here in King County?
Under state law, DOE is responsible for regulating dams that capture and store at least 10 acre feet (about 3.2 million gallons) of water or watery materials such as mine tailings, sewage and manure waste. Most are privately owned and not used for hydroelectric power generation.
I guess it should be no surprise that recreation and irrigation top the list of dam uses. The number of dams being used for environment protection such as water quality maintenance, flood control, stormwater management and fish and wildlife is for me a surprise totaling nearly 30% of all dams.
As we can see in Figure 6, the vast majority of dams do not pose a high risk to our state population, but we do have a few monsters with the Howard Hanson Dam and reservoir on the upper Green River being closest to us here in Black Diamond. Though the reservoir has a surface area of over 20,000 acres, the dam height is just 235 feet, nothing like the scenes we see of the overflowing Oroville dam in CA. Reservoir storage capacity at the Howard Hanson dam is 136,700 ace feet of water. For comparison, the Hiram Chittenden Locks, Dam and Lake Washington Ship Canal is much larger with a capacity of over 458,000 acre feet right in the middle of a dense population center.
Do we have dams in Black Diamond? Yes and I'm not counting the City Council. We have two regulated dams in the city and one nearby in Ravensdale.
Name of Dam Owner Crest Length Surface Area Dam Height Max. Storage
Black Diamond Aerated Lagoon (Rock Creek) City of Black Diamond 400 ft 12.0 acres 14 ft 15 acre ft.
Lake Sawyer Outlet Weir Private 94 ft 697.0 acres 10 ft 1,116 acre ft.
Ravensdale Pit King County DNR 2,950 ft 17.0 acres 18 ft 165 acre ft.
Looking at a google map of the Ravensdale area, I was unable to locate the Ravensdale Pit dam. Is it related to an old mining pit along Rock Creek or perhaps the Reserve Silica mine?
Two years ago, we were able to have the Lake Sawyer Outlet Weir inspected for structural integrity and the weir got a clean bill of health. For more info about the weir, ownership and maintenance, follow this link to one of my earlier posts Lake Sawyer Weir - Structural Analysis.