Adjacent to the well-known Ballard (aka Hiram M. Chittenden) Locks, is the spillway dam. The Army Corps of Engineers uses the spillway to maintain Lake Washington, Lake Union, and Salmon Bay at the congressionally authorized level.
The Lockmaster, in addition to managing some 75,000 commercial and recreational vessel passings through the locks each year, must also maintain the freshwater level within about 2 feet, all while monitoring and controlling the intrusion of tidal saltwater into the freshwater ecosystem.
Sen. Patty Murray's efforts to restore full funding for around the clock operations of the Locks are dependent on U.S. Senate and House budget negotiations to take place this fall, which will hopefully override the 40% cuts proposed by the White House.
Check out the satellite view of the locks and spillway.