It will serve as a second dam, with six gates located 50 feet below those at the main dam.
The new gates will allow water to be released earlier if the lake reaches dangerous levels. Colonel Bill Leady with the Army Corps of Engineers says the second spillway will provide greater flood protection for Sacramento.
LEADY: "This new structure will be able to release a little more than 300,000 cubic feet per second. So, that's enough to fill about two Olympic swimming pools every second."
It took workers four years to remove three million cubic yards of rock and dirt to create the space for the spillway.
The 915 million dollar project is a collaboration of the Bureau of Reclamation and the Army Corps. Federal funds are paying two-thirds of the cost.
Construction is due to be completed in 2017.