The low summer flows of the Russian River have exposed the massive rock-and-concrete jetty built at the river's mouth at Jenner in the 1930s in an attempt to keep the river open for barge traffic.

"It is definitely more exposed than it has been in the last 10 or 12 years," said Don McEnhill, executive director of Russian Riverkeeper, a Healdsburg conservation program. "It has become the topic of conversation; a lot of people are saying, &‘wow, I had no idea it was even there.'"

The jetty is 1,600 feet long, 10 feet high and 40 feet across at its base and within the boundaries of Sonoma Coast State Beach.

The jetty near Goat Rock is usually buried under a sand bar. Typically, only the jumble of rocks at its northern-most end are visible.

Now, the historic jetty, its affect on the Russian River and whether it should be removed will be part of a $450,000 study being undertaken by the Sonoma County Water Agency.

The analysis was ordered by the federal government to improve conditions to protect chinook salmon, which are on the threatened species list.

Before heading out to sea, juvenile chinook will spend a year in the fresh-water estuary that forms at Jenner when a sand bar builds up across the mouth of the river.

The issue is the effect of the jetty on how the mouth closes, and how often, to form that estuary.

"We will try to get a better understanding of how the beach interacts with the waves and how the outlet channel interacts with the waves and the beach," said Chris Delaney, a water agency engineer.

The closing of the river mouth is critical for the chinook, which need that time to acclimate to a mixture of river and sea water before going to the ocean to feed for two years.

Under ideal conditions, the mouth will be closed by a naturally forming sand bar, which lets water from the Russian River water seep out to the ocean, but keeps ocean water from coming in.

"So much of the jetty we can't see," Delaney said.

Water agency consultants "will be doing a number of tests to try to evaluate what the beach is comprised of and what the jetty is comprised of," Delaney said. "One of the theories is that the jetty itself impedes the seepage of water and that by removing or modifying the jetty, we can enhance the seepage."

Work on the jetty began in 1929 with $35,000 by the Russian River Improvement Co. and $35,000 by the state of California.

The purpose was to keep the river mouth open so barges could haul out gravel from the mining operations just inside the river mouth, according to a study presented at an International Conference of Coastal Engineering in Hamburg, Germany, in 2008.

The rock used as the base and for the fill of the jetty was blasted out of the distinctive Goat Rock nearby and hauled by a specially built narrow-gauge rail line to the jetty.

The jetty was originally rock and timber, but much of it was destroyed in the winter of 1929-1930 and replaced by steel, rock and concrete in 1932.

You can reach Staff Writer Bob Norberg at 521-5206 or bob.norberg@