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IEP Cache Slough Studies

Cache Slough Map

The IEP is currently examining the hydrodynamic "footprint" of Liberty Island. Below are the preliminary (still unpublished) results of the ongoing IEP studies, provided by Ted Sommer of the Department of Water Resources (DWR):

Turbidity (Effort led by Dave Schoelhammer and Tara Morgan-King, U.S. Geological Survey)

  • Turbidity in this area seems to be controlled by two things: big inflow events and wind.
  • Seasonally, big inflow events such as the flooding of Yolo Bypass bring big sediment pulses into the region.
  • Overall, the region is remarkably turbid because of windy conditions that stir up lots of sediment.

Why This is Important:

  • Turbidity has been dropping a lot in the Delta due to reduced sediment inputs from upstream and the "filtering" effect of expanding beds of aquatic plants.
  • Pelagic native fish like delta smelt need turbid water.
  • A working hypothesis is that the resulting high turbidity in Liberty Island (and the broader region) is a key reason why the area has become a "hot spot" for delta smelt.

Food (Effort led by Gina Benigno and Ted Sommer, DWR)

Cache Slough Sample Sites
  • Food availability (phytoplankton, zooplankton) varies a lot with tide in the Cache Slough region.
  • Zooplankton densities are relatively high in the Cache Slough region compared to many other parts of the Delta.
  • Phytoplankton levels aren′t remarkably high in Liberty Island itself, but are at impressive levels in the smaller sloughs that surround the island.

Why This is Important:

  • Phytoplankton and zooplankton have declined in the Delta, likely reducing food availability for imperiled species like delta smelt.
  • There is some evidence that some of the variability in delta smelt populations is due to zooplankton availability.
  • Higher zooplankton abundance in Liberty Island likely makes the region attractive to delta smelt.
  • A hypothesis is that phytoplankton transported from the smaller channels around Liberty Island act as a "food bank" for zooplankton in the region.

"The Cache Slough complex (including Liberty Island and the Deep Water Ship Channel) seems to provide decent habitat for delta smelt and other fishes because it is turbid and has more food," Sommer said, adding that they would continue to flesh out this story as the data are analyzed.