Due to the adverse impacts of the the federal Central Valley Project and the State Water Project, those agencies and SDWA developed a barrier program. The program consists of three rock barriers being installed and operated (and removed) each year. One barrier is in Old River just upstream of the CVP export pumps, one is in Grant Line Canal just upstream of the Tracy Road B ridge and one is Middle River just upstream of its intersection with Victoria Canal.
The barriers each include a set of culverts with flap gates. When the flood tide comes into the area, the flap gates allow that flow to move upstream, but when the tide ebbs, the flap gates close and artificially hold the high tide in the southern Delta channels. The main purpose of this is to partially mitigate the lowering of water levels by the export projects.
When originally conceived, it was thought the barriers would result in tidal pumping, in that each time the water filled behind the barriers there would be a periodic push of water back out to the San Joaquin River and a net flow from Old River and Middle River would be established. This was anticipated the help improve local water quality. Subsequent evaluations and modeling indicate that tidal pumping only rarely occurs and so the barriers are mainly used to maintain water levels.
The barriers require various permits, including those issued by the California Department of Fish and Wildlife and the federal Fish and Wildlife Service. Due to fishery concerns, those permits do not allow the operation of the barriers on an as needed basis. The barriers are installed each year beginning in either April (if the Head of Old River Fish Barrier is installed) of May (if the Head of Old River Fish Barrier is not installed). Upon installation, the flap gates of the barriers on Old River and Middle River are tied open (not allowed to operate) and the Grant Line Canal Barrier is not fully closed until July 1.
SDWA's position is that the barriers are necessary (partial) mitigation for the effects of the export projects and so should be operated anytime needed to protect water levels and if not, then exports should be decreased accordingly.
There are also agreement between SDWA and DWR regarding the operations of the barriers which can be found below.
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