Federal officials have discovered damage inside Glen Canyon Dam that could force limits on how much Colorado River water is released at low reservoir levels, raising risks the Southwest could face shortages that were previously unforeseen.

The damage was recently detected in four 8-foot-wide steel tubes — called the river outlet works — that allow water to pass through the dam in northern Arizona when Lake Powell reaches low levels. Dam managers spotted deterioration in the tubes after conducting an exercise last year that sent large flows from the dam into the Grand Canyon.