SACRAMENTO COUNTY, Calif. — California is known for its big droughts, but the most recent storm has reminded us flooding is also part of the Golden State's weather history.
The New Year's Eve storm actually started Friday with a moderately wet but warm storm. Initial snow levels were lower — around 5,000 feet — but would move up the Sierra to above 8,000 feet by Friday night. This created a "rain on snow" scenario with some lower elevation snow melting and adding additional water to the runoff.
Friday's storm was wet but nothing compared to what came Saturday on New Year's Eve. This much wetter band of moisture produced very heavy and steady rain for the valley and even more so for the higher elevations. The Sierra snow level dropped during the day but the damage was done and some watersheds were awash with days of runoff flowing into creeks and streams. These eventually merge with bigger waterways and the Cosumnes River was hit hard during the event.
Early forecasts from the California-Nevada River Forecast Center pegged the Cosumnes as a river that would reach high levels with potential for flooding. The storm behaved as predicted and in some cases produced more since it was fairly stationary. The Cosumnes recorded the 2nd highest flow with more than 60,000 cubic feet per second and is now just below the record 1997 flood on the same river. That storm was also a "rain on snow" event in early January.
The Cosumnes is unlike most in California because it remains free of any large dam. There are a few diversions and one higher, smaller dam in the watershed but it does not have one for flood protection. Levees act as flood protection and the levee was breeched in at least one location during this event.
The un-dammed nature of the Cosumnes River is unique since it offers a free flowing river for flora and fauna. The Cosumnes flows into the Mokelumne and then into the Sacramento River.
Future storms will also see rises on the river but are not forecast to be as high as New Year's Eve in the next few days.
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