The Folsom Dam Auxiliary Spillway project is an approximately $900-million cooperative effort between the U.S. Army Corps of Engineers and the U.S. Department of the Interior, Bureau of Reclamation that helps the Sacramento region achieve a 200-year level of protection (which means a one-in-200 chance for flooding in any given year).

The project constructed an auxiliary spillway to compliment the functions of the main Folsom Dam, allowing water to be released earlier and more safely from Folsom Lake during high water events.

The spillway includes an 1,100-foot-long approach channel that funnels the water from the lake into the spillway; a control structure with six submerged gates that is controlled in coordination with the main dam gates to control water releases; a 3,027-foot-long spillway chute that transports the water from the control structure to the American River below; and a stilling basin that slows the water to flow levels that the river channel will be able to withstand. The entire spillway structure operates in conjunction with the existing operations of the Folsom Dam and is not intended to replace any of the dam’s functions or increase the authorized capacity of the reservoir.

The level of cooperation between the Corps of Engineers and the Bureau of Reclamation on the auxiliary spillway was extraordinary. The Reclamation has responsibility for dam safety as well as operation and maintenance of the Folsom Dam facility, while the Corps of Engineers is responsible for flood damage reduction. By combining their efforts into a single project, the two Federal agencies were able to complete the project faster and at a lower cost.

The auxiliary spillway project was completed in 2017.