The Battle of Arizona Spillway was fought between the Caesar's Legion and the New California Republic. It took place after the Battle of Willow Beach and was part of the attempt to force the NCR presence off of the eastern side of the Colorado River.
After the fall of Camp Willow at Willow Beach, Caesar's Legion attacked the New California Republic military camp at Arizona Spillway, one of two water overflow diversion tunnels underneath the highway surrounding the Hoover Dam. As a result of the battle, the camp was destroyed and the NCR lost its bridgehead across the dam, giving the Legion complete control of all territory east of the Colorado River.
- Fallout: New Vegas Official Game Guide pp. 372-376:
"Recently, the Legion pushed the NCR off of the east side of the river at the Battle of Willow Beach (which destroyed an NCR military camp) and the Battle of Arizona Spillway. Day to day military operations at the dam are under the command of Colonel Moore. While troopers are active here, there are also a large number of civilian contractors who are trying to keep the dam running. They are of a secondary concern to Moore, who is preparing for an impending attack by Caesar and a visit by President Kimball."
(Fallout: New Vegas Official Game Guide Collector's Edition Tour of the Mojave Wasteland)