At the confluence of two major rivers, just outside of Walnut Grove, you can find the Delta Cross Channel.
On a normal day, strong currents from the Sacramento River push freshwater through the channel and into the Mokelumne River. At peak summer flows, water rushes by at a rate of up to 9,000 cubic feet per second.
Those flows can confuse migrating salmon. Luckily, there is a set of of massive gates that can control the torrents.
From Oct. 4 to Oct. 14, the U.S. Bureau of Reclamation had the gates shut tight. For salmon and the people invested in their survival, this was a significant win.
Scientists were running a grand and unprecedented experiment. Their hypothesis: If the gates are closed, more salmon will end up back home in the Mokelumne instead of getting lost in the Sacramento River.
The ultimate goal of the experiment is to get as many full-grown, mature salmon as possible back upriver and into the Mokelumne River Fish Hatchery. Here, the slippery, silver-patterned fish go through a moderately brutal process to extract their eggs and milt. Then, fertilized eggs quietly and slowly grow up through the smolt stage at the hatchery before they are released at Jersey Point near Antioch.
"We've got a long list of things that need to be done to improve the salmon run," said Dick Pool, president of Water for Fish, an organization that represents the interests of fishermen and wildlife enthusiasts. "This was number one."
So far, the results are encouraging.
Following a scent home
Salmon are instinctive creatures. They remember the smell of the waters where they were hatched. After swimming, eating and growing out in the ocean, the fish rely on the memory of that smell to lead them home.
When currents from adjoining rivers come along, the smell of the natal stream mixes with a foreign scent. Confused, the salmon rely on current instead, knowing they have to fight the strongest flow to get home.
Right about now, the average flow along the Mokelumne is 80 cfs. Compare that to the 3,000 cfs coming out of the Sacramento River, and it's no wonder the fish get led astray. When the fish stray, they can't complete their instinctive life cycle. They can't find their way back to their natal streams. They can't spawn. And that spells lower numbers for the future.
This was the first time the gates had been closed for any length of time. It took three years of formal requests by several groups to arrange the closure, including the East Bay Municipal Utility District, the California Department of Fish and Game and the U.S. Fish and Wildlife Service.
U.S. Bureau of Reclamation, the group charged with running and maintaining the gates, denied the request in 2009. Closing the gates would have hurt water quality in the Delta, according to Wilbert Loius Moore, a public affairs specialist with the bureau.
But a two-day closure was approved for 2010. Such a short time period wasn't enough for scientific testing, but it opened the door for a longer closure this time around.
Officials say the 10-day closure will prove a success.
"This will put more fish in the ocean than any other short-term action that can be taken," said Pool, who is also secretary for the Golden Gate Salmon Association and serves on the board for the American Sportfishing Association.
Tracking with acoustic tags
It's not enough, however, just to close the gates. To really dig into solving the problem, you need to know how the salmon react to the change. And that means tagging.
About a quarter of hatchery fish are routinely marked with coded wire tags each season. The tags, about the size of a grain of rice, are injected into the fish, which are marked by the removal of the small adipose fin. Technicians at the Mokelumne River Hatchery insert the tags in young fish on site. These are later counted up during the spawning season to see how many salmon actually returned to their natal streams.
Biologists are using something new this year to track fish movements more closely. Acoustic tags about an inch long were inserted down the throat of 30 fish before, during and after the gate closure. They cost about $300 apiece, but they interact with a complex system of transponders around California's waterways. Technicians analyze real-time footage to understand how changes in water conditions affect the fish, and what they can do to help.
Because the transmitters are so costly, river managers do make an attempt to retrieve them after the adult salmon spawn and die.
"If we show we can actually tag and recover a few of them, it will justify spending more money," said Jose Setka, a wildlife biologist with EBMUD who supervises fisheries.
The gate closure experiment isn't the only trick in the bag to help out salmon on their path to spawning. Releasing pulse flows from Camanche Reservoir encourages fish to swim on home. They react best when these pulses mimic storm flows and the water stays around 60 degrees Fahrenheit.
Transporting salmon out to the Delta by truck allows them to avoid predators and the dangerous pumps near Tracy.
"It's a question of balancing the needs of the environment with that of the people," said Richard Sykes, director of water and natural resources for EBMUD.
The Department of Fish and Game is already planning to try this experiment again.
"If we plan it well in advance and we have enough water, we can get fish back up the Mokelumne," said Joe Johnson, senior environmental scientist supervising fisheries.
If all goes well, strong silver Mokelumne Chinook salmon will keep fighting their way upriver to their instinctive spawning grounds through November and into early December.
Is this experiment a success? The numbers show a tentative yes. As of Nov. 1, 13,661 salmon had come into the hatchery. To put it in perspective, the hatchery saw a total return of just over 7,000 fish last season. But the results of the experiment won't be clear until sometime in April, when the data collected from the tags is tallied up.
"People want to know right away, but I just can't say right now," said Setka.
Contact reporter Sara Jane Pohlman at firstname.lastname@example.org.