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Healthy Creeks and Flood Protection Projects > Don Castro Reservoir

Don Castro Reservoir

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Silt Buildup, Don Castro Reservoir

Increasing the capacity of this reservoir could reduce the amount of water flowing down to the flood control channel and levees during the 1% annual chance storm by 6,000 cubic feet per second. That, along with some channel improvements, could be enough to lower the water level in the San Lorenzo Creek levee and flood control channel (reducing flow from 16,100cfs to 10,100 cfs). FEMA could then re-accredit the system as providing adequate flood control.

Existing reservoir conditions during
the peak of a heavy rainstorm.
Built in 1963, Don Castro Reservoir was designed to hold about 644 acre feet of water during heavy storms. Water would be slowly released after the heavy rains ended. But the hills above the reservoir are melting like ice cream: the hillsides have severely eroded and soil flows into Eden, Hollis, San Lorenzo and Palomares Creeks. Silt then makes its way to the reservoir and builds up there.

The reservoir is now has so much silt that it contributes little to flood protection downstream. De-silting and raising the dam are possible options, which could increase the holding capacity of the reservoir to 6000 cfs, provide more habitat for fish and aquatic vegetation, and increase recreational opportunities.

Solutions

The overall approach is to raise the dam and install a low-flow culvert (that minimizes silting problems) to provide improved water retention and drainage and to restore aquatic habitat. There are three potential plans.

  • Plan 1: Raise the dam 14', without de-silting, and raise the height of floodwalls downstream (the lowest cost plan).
  • Plan 2: Raise the dam 19', without de-silting, so fewer downstream improvements would be needed (slightly higher cost).
  • Plan 3: Raise the dam 5’, de-silt the dam, and perform some downstream improvements (least visual impact, about 5 times as expensive, with highest ongoing maintenance de-silting costs).

The District is reviewing costs and engineering details and is working with the community to determine preferences. We will post new information as it’s available.