The Sonoma County water agency received permission Monday to immediately cut stream flows in the lower Russian River by more than half in an effort to conserve water stored in Lake Sonoma.

Instream flows in the upper river, above Dry Creek, which is fed by releases from Lake Sonoma, already are being maintained at a very low threshold to keep as much water as possible in Lake Mendocino, the smaller of the two reservoirs.

The state decision means Sonoma Water, the county agency, and its contractors — the cities of Santa Rosa, Sonoma, Rohnert Park, Windsor, Petaluma and Cotati, and the Valley of the Moon, Marin Municipal and North Marin water districts — will have to use 20% less water from the Russian River, as well.