Coho salmon are trapped in the Russian River and urgently need a boost from Mother Nature.

Cut off by lack of rain from most of the small streams where they habitually spawn, the endangered coho face a ticking biological clock that could decimate this year's reproduction.

"We know their time is running out," said Nick Bauer, a biologist with UC Cooperative Extension's coho monitoring program.

Bauer was one of seven fish specialists who donned wetsuits and drysuits Thursday to check on the condition of the coho forced to congregate in pools along the river where they are exposed to foraging harbor seals and sea lions as well as human anglers.

Upstream from Monte Rio, the divers spotted coho, steelhead and primarily chinook salmon -#8212; the river's three spawning species -#8212; all apparently waiting for more water.

Flow in the lower river was 100 cubic feet per second on Thursday, a trickle compared with the average January flow of 8,000 cubic feet a second. At this time in ultra-dry 1977, the flow was 96 cfs.

Access to the major coho spawning grounds, such as Green Valley, Mill and Dutch Bill creeks, is blocked by low or no flow of water in the tributaries.

Without significant rain -#8212; and none is forecast through the end of the month -#8212; the coho are likely to die without spawning or lay their eggs in the main stem of the river where the eggs survival rate would be low.

"We might lose a year's class of fish," said Mariska Obedzinski, coordinator of the coho monitoring program.

Coho salmon have a three-year life cycle that begins in freshwater creeks, includes a year or two of growth in the Pacific Ocean and ends with a one-way spawning run to the creeks of their birth.

Adult coho exhaust their strength and die after spawning, with their carcasses adding ocean-based nutrients to an inland food web that sustains plants, animals and even insect larvae that are the young coho fry's primary food.

"It's a pretty elegant system," said David Manning, environmental resources coordinator for the Sonoma County Water Agency.

But the burgeoning drought threatens to interrupt the cycle, at least for a year.

Biologists estimate there are 150 coho in the river, and with the river's mouth closed for now by nature at Jenner, no more salmon or steelhead can swim in from the ocean.

How the spawning season plays out is a question mark, Obedzinski said, given the coho's tenuous recovery from near extinction in the past 20 years.

Up to 500,000 coho spawned in Northern California rivers in the 1940s, dwindling to about 5,000 by the mid-1990s. Millions of dollars have been spent since restoring their spawning habitat in Sonoma County creeks and maintaining a hatchery breeding program at Warm Springs Dam.

About 500 coho returned to spawn in Russian River tributaries last year, a step up from the early 2000s, when there were no natural, wild coho in the river system, Manning said.

Two years ago, following a December rainfall that measured in the 10ths of inches, coho and the more abundant steelhead were bunched in river pools for two months, awaiting the normal winter storm-fed rush of water.

"This year is even worse," Obedzinski said, recalling that the fish got relief from a downpour in the third week of January, 2012.

Now, following another December with less than an inch of rain, the wait is two months and counting. A stubborn high-pressure ridge still shunts storms far to the north and officials are issuing fire warnings over the county's mostly brown hills.

The hatchery breeding program guarantees that coho will continue be planted in the river system, regardless of what nature brings, Manning said.

But it's unknown whether more coho will enter the river or if the 150 already in it will reach spawning grounds in the gravel beds of small creeks.

In yet another twist, most of those 150 coho are 2-year-old fish, known as jacks, which are predominantly males, implying a shortage of females, Obedzinski said.

"Until we have rain, we won't know the answers," she said.