Lake Ilsanjo, a reservoir in the hills above Santa Rosa at Annadel State Park, needs rain after park officials allowed the water level there to drop well below normal this winter.
"The valve apparently was not closed early enough and so inadvertently it drained down a little bit farther than was anticipated," said Mary Pass, a parks official.
The water level has dropped all the way down to the release valve at the base of the dam. State officials said Friday that they were unsure if the lake had ever been this low before.
Now, without an unusual amount of spring rainfall, the lake likely will dry up this summer or autumn, said Mike Drake, a Rohnert Park resident who has fished Ilsanjo for 10 years.
"There go the fish," Drake lamented.
Parks officials said they need to better monitor the operation of the release valve that was installed at the lake's earthen dam five years ago.
With the ability to release water through the valve, parks officials try to reduce flows over the spillway while still holding back enough water to keep the lake at a level for fishermen and the sightseers to enjoy in spring and summer.
"We do the best we can to balance" those priorities, said parks ecologist Marla Hastings.
"We were expecting more rain," Pass said. "We still think that there will be additional rain that will assist in filling that a little bit more than it is right now."
The reservoir was built on Spring Creek in the 1950s, before the land became a popular state park on the edge of Sonoma County's largest city.