New regulations designed to protect spawning steelhead and salmon during exceptionally low stream-flow conditions already are putting a crimp in the fishing season, prompting closures of most coastal freshwater fisheries in Marin, Sonoma and Mendocino counties effective Thursday, according to the California Department of Fish and Wildlife.

The main stream of the Russian River and the Napa River, between Napa and Yountville, are still open to anglers.

But most all other coastal tributaries where fishing normally would be allowed from the Golden Gate to the Humboldt County Line will be closed until rain or some other circumstance raises water levels enough to warrant reopening, Fish and Wildlife officials said.

Under the revised regulations, adopted in December but only in effect since Monday, access to creeks and rivers in Marin and Sonoma counties for fishing is tied to the flow rate on the South Fork Gualala River at the Mendocino County border.

Any time the river flow falls to or below 150 cubic feet per second, or cfs, it triggers closure of Salmon Creek, Russian Gulch Creek and the Gualala River, in Sonoma County, and Walker Creek, in Marin County, Fish and Wildlife personnel said.

The Gualala is already well below the threshold, but Thursday is the first day it could trigger official closures.

The Navarro River flow rate governs the streams and rivers in Mendocino County and was expected late Wednesday or today to fall below the applicable minimum flow rate of 200 cfs, Fish and Wildlife fisheries biologist Allan Renger said.

As of Thursday, the following Mendocino County waterways are closed, listed from north to south: Usal Creek, Cotaneva Creek, Ten Mile River, Noyo River, Big River, Albion River, Navarro River, Greenwood Creek, Elk Creek, Alder Creek, Brush Creek and the Garcia River, he said.

The new regulations were first proposed last summer after extreme drought conditions prompted a flurry of emergency policy decisions. The regulations were adopted Dec. 3 by the California Fish and Game Commission following several months of public input.

The minimum flow rates for various rivers proved the most controversial aspect of the rules, according to fisheries biologist Ryan Watanabe, with many anglers arguing for lower thresholds that would have permitted them to fish longer into dry weather.

Bodega Bay fly fisherman Charlie Beck said Wednesday that he and many of his friends don’t feel there is enough scientific study to support what they believe are unnecessarily high minimum flows that hurt coastal economies and restrict fishing at peak steelhead season.

But biologists say that low flows subject already threatened and endangered fish to warmer water, predation and physiological stress even without the intrusion of human beings.

The new regulations obligate Fish and Wildlife personnel to evaluate flow rates three times a week and update closure information lines by 1 p.m. Mondays, Wednesdays and Fridays. The number for Sonoma, Marin and Napa counties is 707-944-5533. For Mendocino County, it is 707-822-3164.

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.

EDITOR’S NOTE: This story has been updated to remove incorrect information about fishing on the Laguna de Santa Rosa and Santa Rosa Creek.