Invasive golden mussels halt Three Valleys MWD recharge despite record regional storage

Invasive golden mussels halt Three Valleys MWD recharge despite record regional storageThe California wholesaler has been unable to deliver water to the Main Basin since September due to biological contamination, complicating storage efforts even as Metropolitan Water District reserves hit record highs.

Deliveries to the Main Basin were suspended in mid-September 2025 due to the detection of invasive golden mussels.

The district’s cyclic storage account is projected to end the calendar year at approximately 2,900 acre-feet, well below capacity.

Metropolitan Water District (MWD) currently holds a record 3.8 million acre-feet in storage, but local contamination issues are limiting downstream banking.

MWD is expected to set an initial allocation between 10% and 45% for the coming year.

Staff anticipate MWD may make a call on the Chino Basin Conjunctive Use Program before it expires in March 2028.

Three Valleys Municipal Water District (TVMWD) is facing significant operational hurdles in its groundwater recharge programs due to the presence of invasive golden mussels, despite robust regional water supplies. During a December 17, 2025, board meeting, staff reported that water deliveries to the Main Basin have been suspended since mid-September due to the infestation. Sylvie Lee, presenting the water supply update, described the golden mussels as the district's "newest annoyance," confirming that the biological issue has halted the ability to bank imported water locally. "We will not have any water until that gets resolved," Lee told the board. Consequently, the district’s cyclic storage account in the Main Basin is expected to end the calendar year with a balance of approximately 2,900 acre-feet. While 12,700 acre-feet had been delivered under the ten-year agreement, the district has been selling out as much as it puts in, preventing the accumulation of significant reserves. This bottleneck comes at a time of high water availability at the regional level. The Metropolitan Water District of Southern California (MWD) currently holds a record 3.8 million acre-feet in storage. Lee noted that MWD’s initial allocation is expected to be 10%, though forecasts suggest it could range between 10% and 45%. There is also a possibility that MWD may classify some supply as Article 21 water due to the high storage levels. However, the district faces pressure regarding the Chino Basin Conjunctive Use Program (CUP). Lee indicated that MWD is likely to make a call on the stored water regardless of high allocation percentages, as the program in the Chino Basin is set to expire in March 2028. MWD currently has about 65,000 acre-feet stored in the basin and must extract it before the agreement sunsets. The expiration of this program carries broader economic implications, potentially leading to increased water acquisition costs, reduced groundwater storage capacity, and a higher reliance on expensive imported water. TVMWD is currently facilitating discussions with member agencies, including the City of Pomona, to manage these obligations and mitigate the associated risks.