February 21, 2017
In the mid-1990s, a scientist named Ted Sommer commuted routinely by bicycle between Davis and Sacramento, on the causeway that passes over the vast floodplain called the Yolo Bypass. At the time, state biologists had observed that numbers of Sacramento River fish increased almost every year following a wet winter.
“We wondered what it was about those wet years that was so good for fish,” Sommer said.
The answer occurred to him during one of his bike rides. It was a rainy winter, and the Yolo Bypass, sprawling away to the north, was covered with river water—as it often was after unusually heavy rains, and as it is this week. Perhaps, he speculated, those many acres of ephemeral wetland had something to do with the observed spikes in fish numbers.
Sommer, a scientist with the California Department of Water Resources, organized a novel two-year experiment in which he released hundreds of thousands of tagged salmon smolts into the wild. Half went directly into the Sacramento River, the rest into the inundated Yolo Bypass during high-water periods. The finger-sized fish were recaptured many miles downstream—and what Sommer saw would help change the way scientists think about water, rivers, and fish: Those that had spent time on the floodplains had grown dramatically faster and larger than the salmon released directly into the river — presumably making them more fit to survive.
These results, published in 2001 in the Canadian Journal of Fisheries and Aquatic Sciences, provided the first explanation for the correlation between wet winters and fish population jumps.
Almost 20 years later, aquatic biologists understand very well why wet years are so good for Central Valley salmon.
“Those floodplains are like a food bank for fish,” Sommer says.
Those floodplains are back, too — at least for the moment. While weak to moderate rainfall rarely causes flooding anymore in the heavily leveed Central Valley, this winter’s gushing deluges have spilled the Sacramento River’s banks and laid a thin blanket of water across tens of thousands of acres of the valley’s historic floodplains. The heavy precipitation has caused some degree of damage, too. In early February, high waters nearly tore the rim off of Lake Oroville and opened a jagged hole in the reservoir’s concrete spillway. Abrupt flooding elsewhere has also washed fish and their fertilized eggs out of some streams, especially in coastal watersheds where coho salmon spawn.
But overall, the wet, wet winter should be a big boon to salmon, precisely because of the widespread flooding.
“This is a great time to be a fish in the Central Valley,” says Jacob Katz, a researcher with the conservation group California Trout who has been studying salmon and floodplains for years.
Katz explains that the particular way in which water behaves as it flows onto and across a flat piece of land is significantly different from how water flows through the main channel of a river.
“The water [on a floodplain] slows down and spreads out, and it creates a huge solar panel for the sun’s energy,” Katz says. “You get incredible productivity from a simple food chain of light striking the water and generating blooms of algae, algae feeding bugs, and bugs feeding fish, and that is the engine of productivity that once supported Central Valley wildlife.”
Now, Katz, Sommer and other scientists want to restore the valley’s annual flooding cycles by carving a few notches into key levees along the Sacramento River. These earthen walls — thousands of miles of them — were built a century or more ago to control annual flooding. They did their job well, confining the river most of the time to narrow, fast-moving channels of water.
But levees also eliminated that critical solar panel effect described by Katz, turning them into relatively sterile environments low in natural food production. Such altered rivers, with swift currents and few places to hide from predators, are hostile places for young salmon trying to fatten up for their seaward migration.
Not surprisingly, the widespread channelization of California’s rivers, usually associated with intensive water removals for agricultural irrigation, has correlated with drastic drops in salmon and steelhead numbers. In California, a population of Chinook millions strong has declined to just a few hundred thousand, and they still exist in fishable numbers thanks mostly to fish hatcheries, which produce tens of millions of juveniles each year, rear them for a few months in tanks, and release them into the wild.
In fact, aquatic biologists believe that without access to floodplains and other off-channel habitats, like oxbow lakes and river-bend eddies, California’s wild Chinook salmon will face an impossible upstream battle to survive.
“If we want to have salmon in the future in California, we have to start reconnecting rivers to these pieces of off-channel habitat,” says Carson Jeffres, a biologist with the UC Davis Center for Watershed Sciences.
Under current conditions, most of the millions of salmon that are born in the river system, or born in hatcheries and released into the river, die before reaching San Francisco Bay. By some estimates, the mortality rate exceeds 95 percent. The little salmon starve to death, get hopelessly lost in remote backwaters, and are eaten by predatory fish and birds.
Abundant research, beginning with Sommer’s work in the 1990s, has shown that restoring floodplains could turn this devastating mortality rate upside down and make the Central Valley capable again of supporting wild, self-sustaining runs of salmon. In 2013, Jeffres and Katz collaborated on a project in which they fitted dozens of small Chinook salmon with acoustic transmitters before releasing them into the river system to migrate to the sea. They found that fish that had spent six weeks on a flooded rice paddy in the Yolo Bypass were seven times more likely to be detected swimming past underwater sensors at the base of the Golden Gate Bridge than the salmon in the control group. The theory is that the floodplains produced both ample food for the fish as well as a safe alternative route to the sea. That is, the floodplains allowed the fish to swim 30 or 40 miles downstream without entering the relatively perilous waters of the river, where predators and diversion pumps pose considerable hazards to migrating salmon.
This winter, the ribbons of water that usually flow through the Central Valley have swollen into a giant complex of wetlands. John McManus, executive director of the Golden Gate Salmon Association, an advocacy group geared toward conservation and fishing, says it is probably too early to tell how the valley’s Chinook population will respond to the wet winter.
“It’s hard to know yet how many of the eggs hatched and how many young fish got out of the gravel beds before the floods came and washed everything away,” he says. High flows can, in fact, destroy nests of salmon eggs—a process biologists refer to as “spawning gravel scour.” If the scour is bad enough, it can abort entire generations of fish. For better and for worse, the season’s rains came very early—in October.
“That means there were probably a lot of eggs still in the gravel when the heavy flows came tearing through,” McManus says. “But for those fish that successfully hatched, it will be a great year because of all the rearing habitat out on the floodplains.”
So, while many of the fish that would have become adults in two to three years were likely lost to scour, there may still be elevated survival this year — meaning more fish to catch in 2019. Worth noting is that floodplains don’t only provide food and shelter for newly born fish. They can also indirectly benefit nests of fertilized salmon eggs by allowing a swollen river to spread out. This disperses the water’s energy and reduces the lethal effects of gravel scour.
Jeffres says that until about 20 years ago, nearly all salmon management and restoration work focused on producing fish in hatcheries and improving spawning habitat for wild fish. Only more recently have scientists come to understand that the middle reaches of a river play an almost equally important role in supporting fish.
“Addressing the spawning habitat after the dams went in was just the most obvious thing to address, the lowest-hanging fruit,” he says.
Katz, Jeffres and Sommer now hope to go the next step in rebooting the valley’s flagging salmon numbers.
“We need to translate these experiments into actual management practices,” Sommer says.
This will require cooperation with local landowners, approval from water and fishery agencies, and plenty of patience as the institutional wheels of bureaucracy slowly turn.
With the benefit of hindsight and the understanding of why flooding is so important to a river system’s ecology, Sommer looks back at outdated management protocol with some amusement.
“For a fairly long time, people thought of the river as a leaking pipe,” he says. “You plugged up the leaks along the edges and meanwhile pumped as many hatchery fish into the river as you could. Now we know that fish really need those leaks.”