Phoenix reservoir

A $6.3 million project to remove tons of sediment from Phoenix reservoir east of Sonora has been scaled back from removing 235,000 cubic yards of sediment to 175,000 cubic yards, Tuolumne Utilities District communications staff said Thursday.

A $6.3 million project to remove tons of sediment from Phoenix reservoir east of Sonora has been scaled back from removing 235,000 cubic yards of sediment to 175,000 cubic yards, Tuolumne Utilities District communications staff said Thursday.

The reduction means the original goal of restoring 150 acre-feet of capacity at Phoenix is now scaled back to restoring 117 acre-feet of space to the reservoir, which has filled in with sediment over many decades.

Phoenix is the drinking water source for thousands of Tuolumne Utilities District customers, including the City of Sonora.

The Phoenix dredging project is still expected to be completed by October 2021. Steve Manning Construction Inc., of Redding, is still doing the $4.17 construction contract. The current dredging project at Phoenix will restore the reservoir’s capacity to more than 700 acre-feet total.

The original project goal of removing 235,000 cubic yards of sediment from Phoenix, to boost capacity by 150 acre-feet, had to be scaled back because about 60,000 cubic yards of sediment in the reservoir proved inaccessible due to soil conditions that could not support heavy equipment, Lisa Westbrook with TUD said.

In addition to the dredging project at Phoenix, TUD is preparing to repair and upgrade the Phoenix dam and spillway in late spring and early summer, Westbrook said Thursday. Contracted costs and the contractor for the Phoenix dam and spillway project have not been determined yet. The project is currently out for bids. The expected construction timeline is April 8 to May 30.

Phoenix is a reservoir, not a natural lake. A reservoir is a man-made space that is intended to store water behind a dam or dams. A simple fact about reservoirs is they take in water, and over time that water brings in dirt, rock and other water-borne debris called sediment. Sediment has to be removed from a reservoir to keep room in the reservoir for water. If sediment is not removed, eventually the reservoir will fill with sediment.

Planners and elected leaders with TUD identified dredging Phoenix as necessary more than 15 years ago.

Historians say the first dam at Phoenix was built in the 1850s in the immediate aftermath of the Gold Rush. The original dam was destroyed in 1862. The dam that created Phoenix the way it looks today was completed in 1880, according to the California Division of Dams Safety. 

To restore capacity in Phoenix reservoir today, workers removed more than 160,000 cubic yards of sediment from Phoenix reservoir in the summer, TUD communications staff said. Portions of the reservoir’s bottom have been deepened, and new channels have been excavated. Reshaping the reservoir bottom is intended to enhance the way the reservoir works with water in it.

Improvements at Phoenix are intended to reduce growth of prolific, invasive aquatic plants and other vegetation, to enhance water quality, cold water habitat for fish, and use of Phoenix water for domestic supply.

A key new feature in the reservoir bottom is a new sediment capture basin on the north side of the reservoir, TUD communications staff say the sediment capture basin is “complete and operational.”

Without the sediment capture basin, sediment would continue to spread freely through the entire reservoir and accumulate until the reservoir fills up again with sediment. TUD communications staff say the sediment capture basin is critical to ensure the reservoir remains useful in the future.

Most of the digging, excavation, dredging and removal of sediment at Phoenix has been completed, according to TUD. More work remains at Phoenix this summer. Three primary waterways flow into Phoenix: Sullivan Creek, Chicken Creek, and Power Creek, a man made connection that brings water from the Tuolumne Main Canal ditch-flume conveyance downhill to Phoenix.

More sediment needs to be removed from Chicken Creek, which flows into the northwest end of Phoenix, TUD communications staff said.

Over the years, Chicken Creek has filled with sediment and overgrown with brush, TUD communications staff said this week. A 550-foot-long section of the creek will be cleared out and as much as 2,000 cubic yards of sediment will be removed.

On the far west side of Phoenix where the reservoir spillway is located, an additional 400 to 500 cubic yards of sediment will also be removed. That sediment is being removed in an effort to upgrade the dam and spillway, and improve safety and reliability.

Completion of the current Phoenix project is still expected in October. Remaining work this spring and summer includes removal of plants and other vegetation on south and east edges of the reservoir, to improve water quality and improve unspecified aesthetics.

Residents can expect lower than normal water levels at Phoenix until the project is completed.

Some TUD staff have spoken about seeking up to an additional $4 million to restore Phoenix to its maximum capacity, 850 acre-feet. Westbrook said Thursday that TUD has contacted the state Department of Water Resources to inquire about additional funding, and TUD staff will continue to seek additional funding through other agencies and programs.

Contact Guy McCarthy at gmccarthy@uniondemocrat.net or 770-0405. Follow him on Twitter at @GuyMcCarthy.

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