Despite wet year, use of Lake McClure spillways unlikely, MID managers say

There’s hardly a chance that the structural concerns about the Oroville Dam should raise worry at the New Exchequer Dam on the Merced River, Merced Irrigation District officials said.

“Every dam is different,” said Peter Wade, a hydroelectric manager with MID. “The hydrology, geology and watershed all come into play.”

At Lake McClure, the reservoir formed by the dam about 20 miles northeast of Merced, spillways are used only on rare occasions, and officials don’t foresee any need for them. The hydroelectric team checks on the spillways weekly, and the Federal Energy Regulatory Commission does a yearly inspection and additional exercises to ensure that the spillways work correctly, Wade said.

The primary method of releasing water from Lake McClure is through New Exchequer’s powerhouse.

Since last month, when the reservoir topped 67 percent capacity, MID began releasing water down the Merced River per regulations from the U.S. Army Corps of Engineers. About 6,000 cubic feet per second is being released downriver, Wade said. A maximum of 3,000 cfs can be released through the powerhouse before a special bypass valve is used to release additional water down the river.

Typically, the time length of water releases varies depending on the season. But to make room for additional storm runoff in Lake McClure, MID has been making releases 24/7.

Lake McClure on Monday was at 90 percent capacity, up 10 percent just over the weekend, according to data from the state Department of Water Resources (DWR). The water is about 15 vertical feet from reaching the reservoir’s maximum capacity of about 1 million acre-feet. An acre-foot is equivalent to about 326,000 gallons, or enough water to cover an acre of land with a foot of water.

At Lake McSwain, just downstream of Lake McClure, water is being diverted from the dam and through the spillways.

This year has been the wettest year on record for snowpack and precipitation since 1904, MID and DWR records show.

Just two years ago, Lake McClure reached a record low water level of 6 percent.

The last time MID made releases similar to this down the Merced River was in 2011. That year, about 1 million acre feet of water flowed through the reservoir.

So far this year, MID has released 161,000 acre feet of water and anticipates reaching or surpassing the amount released in 2011.

Natural creeks in the area that are not managed by MID also are seeing high water levels just from the amount of rain. For instance, water at the Mariposa Creek recharge reservoir is spilling over the dam. That’s the way the dam was designed, and the dam has not been compromised, county officials said.

Over the weekend, about 25 homes were affected by flood water and given the option of voluntarily evacuating. A larger area was notified prior to the evacuations warning of potential flooding.

The water in the Le Grand area has mostly receded, and county officials are counting on a few dry days to prepare for the next storm forecast for Thursday.

A resource center will be available for Le Grand residents at the Le Grand fire station from 9 a.m. to 2 p.m. Tuesday and Wednesday.

County crews are evaluating the banks and whether repairs to the Mariposa Creek are necessary during the dry period to prevent major flooding in the next round of storms, county spokesman Mike North said. Crews are working to clear debris from the creek such as tree branches.

So far, the biggest impact on county infrastructure from the storms has been to the roads, North said. “But our biggest concern is people and public safety,” he added.

For more information on county resources from flooding, a 24-hour hotline will provide information at 209-385-7379.

This story was originally published February 13, 2017 at 6:24 PM with the headline "Despite wet year, use of Lake McClure spillways unlikely, MID managers say."