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A crumbling rock and concrete jetty largely buried and forgotten beneath the sand at the mouth of the Russian River has become the focus of renewed interest amid studies to determine its impact on the adjacent estuary.

No one knows, for instance, how deep the remnants of the 84-year-old jetty and adjoining seawall extend below the mutable surface of Goat Rock State Beach, nor how they affect the shifting sands and mingling of salt and fresh water where the river meets the sea.

But that's expected to change.

A series of geophysical tests that began Wednesday and are scheduled to extend into next week are aimed at resolving some of the mysteries of the historic structures and how they interact with environmental forces at the river's end in Jenner, the Sonoma County Water Agency said.

Scientists from Lawrence Berkeley National Laboratory and elsewhere plan to use high-tech gadgetry, including ground-penetrating radar, to build a picture of subsurface structures left behind by failed attempts to tame the sea with more than 100,000 tons of stone carved from Goat Rock, as well lumber and steel.

The overall effort is intended to provide information that will help the Water Agency and other entities optimize fish habitat in the estuary, where threatened steelhead trout spend their summers, senior environmental specialist David Cook said.

In particular, biologists want to know how much fresh and saltwater seepage goes on beneath the surface of the sand and gravel spit that encloses most of the estuary and how it might affect the volume and salinity of the water within.

They also want to understand the extent to which the jetty — a failed attempt to keep the mouth open for transit by gravel barges decades ago — still affects the natural dynamics that might otherwise close the outlet all summer long.

"At this point, much of the structure is buried, so we really have to go off of what documentation there was for the construction of the jetty, which there's not a whole lot of," said Chris Delaney, senior engineer with the Water Agency. "Since we can't see much of it, because it's buried in the sand, we have to turn to other methods to really get a better picture of what the composition of the jetty is."

There are no plans currently to destroy or modify the jetty, but it may be that the studies indicate that's appropriate, agency personnel said.

The jetty project was begun in 1924 with the opening of the quarry at Goat Rock and construction of a narrow-gauge rail system to transport stone 3,500 feet up the beach to the point where the eventual stone jetty abruptly angled left and out toward sea.

The jetty itself was completed years later, its foundation built into excavated sand to a depth of perhaps 16 feet and with a height of about 17 feet, according to historical documents.

The money put forward by businesses ran out before the jetty was complete. The state later became involved, in part to keep the passage open for migrating fish. Ensuing periods of construction and reinforcement continued until 1948 but never managed to beat back the forces of nature, Cook said. The manmade structures fell apart, subsided, degraded and were largely buried.

A portion of the concrete covered jetty still remains visible, as do several hillocks of enormous rock chunks, a few wooden posts and two lengths of rusted and pitted railway.

But how much material remains under the surface is largely unknown, Delaney and Cook said.

"I think it's likely that what you see is literally the tip of the iceberg," Cook said, "so there's probably a lot of rock there, and that's one of the reasons we're doing the jetty study, because we really don't know."

The studies are required under the 2008 Russian River Biological Opinion, issued by the National Marine Fisheries Service, which dictates changes in flood control and water supply operations by the Water Agency and the Army Corps of Engineers. The goal is to mitigate effects on steelhead and coho salmon, which are listed under the federal Endangered Species Act.

Biologists already have determined that the goal, as much as possible, should be to maintain a freshwater lagoon behind the sandbar that builds up naturally over time, closing the river off from the ocean for much of the summer.

The idea is to promote a habitat that allows aquatic invertebrates to flourish, as they form the primary diet of young steelhead, permitting the fish to bulk up and improve their chances at survival once they head out to sea, Cook said.

Studies also highlight the benefit for fish of maintaining a high water level and limiting saltwater intrusion into the estuary.

The Water Agency has altered the frequency and method it uses to occasionally breach the natural sandbar that from time-to-time closes the mouth.

But it may be that the jetty, though a failure at maintaining a navigable boat channel, is also keeping the mouth of the river from migrating south and pinching off each year as happens at other estuaries on the coast, Delaney said.

"A part of the study is to look at alternatives, but basically, at this point there's no funding source identified to actually implement any project," Delaney said. "It is just a study."

Visitors are welcome to continue using the beach while the studies are under way, the agency said.

The harbor seal population that uses the beach as a haulout is being monitored while the work goes on under a federal permit, agency personnel said.

(You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com.)