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Tahoe Nugget #227:

Prosser Reservoir Saves Reno
March 11, 2012

The NWS forecast for the week ahead looks very wet indeed with between 3 to 7 inches of water set to impact the Sierra Crest! The series of storms are juicy but warm with snow levels rising above 7,000 feet at times. This translates into a rain event at lake level with heavy Sierra Cement snow in the higher elevations at the resorts. By next weekend, however, Tahoe ski areas will have picked up another three feet or more of snow.

The past two winters have been influenced by cooler sea surface temperatures in the Pacific Ocean (La Niña). It's a pattern which often generates colder than normal temps due to a north-south orientation in the storm track, but subtropical systems are not uncommon during a Sierra winter and that's what in store for this week. Depending on how much rain falls and how high the freezing level rises, hydrologic impacts on regional river systems can be dramatic.

An extreme rain event during the winter of 1963 tested the Truckee River flood control system and confirmed the importance of the Prosser Creek and Stampede reservoirs that were under construction at the time. Similar floods had swamped Reno in prior years leading engineers to build a series of reservoirs upstream to protect "The Biggest Little City in the World."

Unlike most eastern Sierra flood events, it wasn't warm rain melting the snowpack that caused the trouble in 1963. At the Mt. Rose Highway Station survey site there was only six inches of snow at the end of January when the rain arrived. The action started on Jan. 29 when a powerful Pacific storm loaded with sub-tropical moisture overran the Sierra and Western Nevada. The saturated atmosphere pounded the region with intense rainfall. This was a very warm storm with snow levels ranging between 8,000 and 11,000 feet. Mt. Rose was inundated with more than 13.5 inches of water in 72 hours, setting a new Nevada precipitation record.

On the Sierra west slope, the Big Bend Ranger Station near the Yuba River measured more than 21.5 inches of rain in three days. (Hydrologists estimated that the frequency of return for such a storm is 88 years.) The torrential rain coupled with snow levels exceeding 8 ,000 feet generated intense runoff out of the mountains. For the third time in 12 years, the normally peaceful Truckee River turned its fury on Reno. Flood debris in the muddy torrent pummeled bridge supports in Reno, and authorities were forced to close 10 of the 12 bridges in the Truckee Meadows. Water on the runway shut down the Reno airport and incoming flights were redirected.

All major highways in the Lake Tahoe region were closed due to flooding, rocks, and debris. The California Highway Patrol in Truckee closed U.S. Highway 40 over Donner Pass due to nearly 20 washouts. Route 89 along the Truckee River between Tahoe City and Truckee was underwater, and Squaw Valley was cut-off when Squaw Creek flooded the road. Even the trains-Sierra routes run by Southern Pacific and Western Pacific railroads were blockaded by rock and mudslides.

In the early morning hours of Feb. 1, a volunteer army of men and boys tried to protect Reno by filling sandbags and building barricades. National Guard troops were deployed, and Reno police and fire units were split on both sides of the river in case all the bridges washed out. Despite their best efforts flood waters ultimately inundated the downtown district. Fortunately there were no fatalities or injuries, and overall damage was much less than prior floods of similar magnitude.

The Truckee River channel had been dredged after a disastrous flood event in 1955 and the newly completed Prosser Creek Reservoir added more storage capacity to the existing Boca Reservoir's 40,000 acre feet. The Washoe Flood Control Project manager, H. Smith Richards, said, "Usually we don't like to fill a new dam this fast, but in this case there was no choice…and without a doubt Prosser Dam kept [Reno] flood damage from soaring any higher."  

The 1963 rain event tested the Truckee River flood control system and confirmed the importance of expanded upstream storage. Within seven years Stampede Reservoir would be completed, adding another weapon in the battle to tame the volatile Truckee River.