Wild & Scenic Mokelumne Update

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Groundwater management plan for Cosumnes basin underway, Plymouth still pursuing river diversions
Cosumnes Subbasin SGMA Working Group
Map of the Cosumnes Subbasin
The Sustainable Groundwater Management Act (SGMA) was passed by the California Legislature in 2014. The law was intended to address the depletion of groundwater basins in much of the state, where overpumping and other issues are leading to saltwater intrusion, sinking ground levels, rivers that lose streamflow to adjacent aquifers, and other serious issues that threaten the sustainability of the state’s groundwater supplies.

One of the affected groundwater basins is that of the Cosumnes River, sections of which go dry every summer downstream of Sloughhouse. The Cosumnes is the only river in the Sierra Nevada without a major dam or diversion between its headwaters and the Central Valley floor. Its groundwater subbasin includes portions of Amador County west of Highway 49 and Sacramento County.

Under the Act, the Cosumnes Subbasin was mandated to form a groundwater sustainability agency or agencies (GSAs) by June 30, 2017, and to have a groundwater sustainability plan in place by 2022. There are now seven GSAs in the district, including one established for Amador County interests. The agencies are working together in the Cosumnes Subbasin SGMA Working Group, which recently held a public workshop at the Amador Water Agency office in Sutter Hill.

According to a December 2018 Cosumnes Subbasin Working Group fact sheet, the subbasin sustainability plan must, “… assess groundwater quantity and determine if there is a balance between water coming in and water leaving the subbasin. If it is determined that groundwater supplies are being depleted over time, the Plan must contain a strategy to reverse this trend and ensure long-term sustainability of the water available to businesses, residences, farms, ranches and ecosystems. The GSP must also assess groundwater quality, surface water depletion, land subsidence and other sustainability indicators and propose measures to correct any undesirable results.’“

While Foothill Conservancy is not actively engaged in developing the sustainability plan, we and our river and fish conservation partners are trying to stay abreast of its progress.

We are also seeking to avoid more upstream diversions from the Cosumnes that could further diminish the water available for spawning salmon and steelhead in the fall. The City of Plymouth asserts that it has pre-1914 water rights to 15,000 acre feet of water from the Cosumnes. But its Cosumnes water delivery system, the Arroyo Ditch, has historically wasted a great deal of water due to leakage and is still pending repairs to damage caused by the Sand Fire and heavy winter rains. Plymouth’s claimed water right has never been validated by the courts, and with groundwater depletion downstream, any effort to withdraw more water upstream could well lead to a legal battle with downstream water agencies.

Our current Cosumnes consultant, Chris Shutes, and Conservancy President Katherine Evatt met with Plymouth’s interim city manager and contract project engineer to discuss our concerns last fall. They confirmed that the city council is still interested in doing what it can to market water from the Cosumnes after repairing the ditch. However, it appears that under the grants the city has obtained to repair the ditch, nothing can be done to eliminate its many leaks. We also learned that the limited liability company set up to work with the city on water marketing has restored its legal status, which was lost in 2017.

We will continue to monitor these issues and keep you informed.

THE FOOTHILL CONSERVANCY  |  35 Court Street, Suite 1   Jackson, CA  95642  |  209-223-3508