Hoover Dam has two giant spillways built to carry floodwater around the dam, and they have been used only twice. Both times, the rushing water badly damaged the tunnels. Today, with Lake Mead near record lows, the spillway channels stand dry.

Two Tunnels Built for Niagara-Sized Floods

A spillway sits behind each end of the dam, and each one is a safety valve. The entrances are 27 feet below the top of the dam, so water only goes in if the lake climbs almost to the crest.

Each spillway has four steel drum gates, each 100 feet long and 16 feet high. Each gate weighs about 5 million pounds.

Behind the gates, the water drops into a steep tunnel about 600 feet long and 50 feet across. The Bureau of Reclamation says each spillway can handle 200,000 cubic feet per second, about the flow of Niagara Falls.

The Gate That Dropped

The Arizona spillway ran for the first time on August 6, 1941. After a hurried inspection on August 14, engineers found the tunnel lining intact.

Operations continued, and on October 28, 1941, one of the drum gates dropped without warning. The flow through the Arizona spillway peaked at 38,000 cubic feet per second that day.

That much water falling 600 feet down a steep tunnel eroded the lining. The eroded area was about 115 feet long and 30 feet wide, with a maximum depth of about 45 feet, a cavity of roughly 1,070 cubic yards.

Engineers later attributed the damage to cavitation, where vapor bubbles in fast water collapse with explosive force. Engineers who inspected the tunnels found major damage to the concrete linings and the rock underneath.

Patched, Polished, Tested Again

Repair work started almost immediately. Crews used a pack-and-inject concrete process, and then polished the tunnel smooth to help prevent more erosion. In 1947 the spillways were modified with flip buckets, which slow the water and reduce the spillway's effective capacity.

In 1983, record flows into Lake Mead sent water through the spillways for the second time. The record flows again eroded the concrete base of the tunnels, and the damage had to be repaired. That damage was also cavitation, and air slots called aerators were added afterward.

The dry Arizona spillway tunnel mouth at Hoover Dam, a dark concrete opening with no water, seen from near the dam — The dry mouth of the Arizona spillway tunnel at Hoover Dam. Photo: Jarek Tuszynski, CC BY-SA 4.0.

Bone Dry Today

The spillway entrances sit at roughly 1,205 feet above sea level. Lake Mead set a record low of about 1,040.5 feet in August 2026 and has stayed below 1,040 feet since, so the water is roughly 165 feet below the spillway entrances.

Both channels are empty, and the tunnels that water once chewed through are waiting for a flood that the lake is nowhere near.

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