Last week’s rainfall will help bump the Russian River out of a “critical” dry designation, according to the Sonoma County Water Agency.
Rainfall totals are still running well below normal so far this year, but “We’re definitely out of the critical designation,” said Sonoma County Water Agency spokeswoman Ann DuBay.
The Water Agency makes a monthly assessment of whether conditions warrant a “normal,” “dry” or “critical” rating, which determines how much water is released from Lake Sonoma behind Warm Springs Dam and Lake Mendocino at Coyote Dam to maintain a required minimum River flow.
“Given where we’re at right now we’ll definitely be in a dry year on April 1,” when the next designation is due, said DuBay.
The Russian River water supply was designated “critical” on March 1. Earlier this year it had been given a “dry” designation owing to below normal rainfall.
Now, by May or June the designation “may be close to normal,” depending on how the lake levels look, said DuBay.
Guerneville has gotten about 20 inches of rain so far this year, roughly half its normal annual average of 41 inches, according to the U.S. Geological Survey. That puts the season on track with 2009, the third of three dry years, when reservoir levels reached historic lows and state officials invoked mandatory cuts in water use for the Agency’s 600,000 customers in Marin and Sonoma counties.
To determine whether water supply conditions are normal, dry or critically dry, the Water Agency relies on a “hydrological index” based on the amount of Eel River water flowing into Lake Pillsbury in Lake County.
Water stored in Lake Pillsbury is released into the Russian River through PG&E’s Potter Valley Project, a hydroelectric system utilizing a tunnel into the Russian River watershed. That water flows into Lake Mendocino behind Coyote Dam.
The water supply pool stored in Lake Mendocino was just under 90 percent of capacity this week, according to the Water Agency.
In Lake Sonoma, which dams Dry Creek, the water supply pool this week was just over 87 percent of capacity, according to the Water Agency’s measurements.
The “dry” and “critically dry” designations mean the Water Agency is permitted to reduce normal Russian River flows to preserve water storage in Lake Mendocino.
Lake Mendocino’s supply is monitored so there is enough water in summer and fall for all water users and for release in the fall to support migrating Russian River chinook salmon.
Water Agency officials are still urging domestic customers as well as agricultural users to conserve water as they have been doing for the past several years
“Conservation is now part of the Sonoma County lifestyle,” Water Agency Assistant General Manager of Operations Pam Jeane said this month, when the “critical” designation was issued.
In the last 10 years, local water demands have dropped 20 percent, said Jeane.
“The message right now is to continue using water efficiently — especially for farmers and residents of Healdsburg and communities to the north, which rely on releases from Lake Mendocino,” said Jeane.
The “critical” designation means flows in the upper Russian River (between Lake Mendocino and Healdsburg) could get as low as 25 cubic feet per second (cfs), compared with “dry year” minimum flows of 75 cfs. Normal-year minimum flow is 185 cfs.
In the lower Russian River between Dry Creek and Jenner “critical” allows a 35 cfs minimum flow. “Dry” year minimum flow is 85 cfs; normal minimum is 125 cfs.
In Dry Creek (between Lake Sonoma and the Russian River) there is no minimum flow change because critical, dry and normal minimum flows are all 75 cfs.
In Healdsburg on Monday the Russian River was flowing at the rate of 2,180 cubic feet per second (cfs) according to the U.S. Geological Survey gauge.
Downstream in Guerneville it was flowing at more than double that, 4,320 cfs, the higher volume reflecting the contribution of tributaries, including Dry Creek.