1997 was the flood of record in most of Northern California. It was much greater than a 100-yr event in some places, but not everywhere. By comparison, high hazard dams are designed for a statistical recurrence interval of tens of thousands, or hundreds of thousands of years. Smaller low-hazard dams are designed for nothing less than a 1,000-yr storm.
Here's the 1997 data for the Walker River Basin
USGS Report on Flood of 1997
