First and foremost, at Don Pedro Reservoir on the Tuolumne River the emergency spillway has been opened for the first time since the floods of 1997. This became necessary when the lake filled nearly to capacity, and a new storm battered the north state over the weekend. The channel downstream floods at around 9,000-10,000 cubic feet per second, but storm runoff threatened to exceed that level. The dam operators had to make a decision to cause some flooding downstream to avoid even greater flooding if runoff was too high.
|Source: USGS (https://waterdata.usgs.gov/ca/nwis/uv?11289650)|
People are understandably concerned about Don Pedro Reservoir as they compare notes with the events at Oroville Dam in the last few weeks. Both dams have an emergency spillway that flows over unlined and unreinforced rock rather than a concrete channel.
There are some important differences, however.
First, the emergency channel below Don Pedro is much less steep than Oroville's, and is therefore less subject to the headward erosion that threatened to undermine the spillway at Oroville. Second, the spillway was tested previously, during the floods of 1997. The runoff amounts in that event were almost astronomically higher than any flows expected in this year's event. A forty-foot channel was carved through the meadow, and the flowing waters are unlikely to do anything worse this time around.