Researchers with the UC Davis Center for Watershed Sciences search for clusters of frog eggs as spring snowmelt flows recede in the Rubicon River, a tributary of the Middle Fork American River. Photo by Ryan Peek, April 30, 2013

By Sarah Yarnell and Ryan Peek

In case you hadn’t heard, the annual Sierra “spring snowmelt recession” has begun.

The foothill yellow-legged frog certainly knew.

Adapted to the seasonal patterns of California’s climate, this rare frog and other native amphibians, fishes and bottom-dwelling invertebrates are genetically wired to reproduce during the spring snowmelt when river flows recede.

The river-breeding frogs in the foothills are cued by the decreasing flows and warming water to lay their eggs. They evidently got the word by April 30. Researchers with the UC Davis Center for Watershed Sciences saw their first egg masses of the season that day as they waded the Rubicon River, one of several northern Sierra rivers that the Center monitors for ecological responses to the recession.

Source: UC Davis Center for Watershed Sciences

Source: UC Davis Center for Watershed Sciences

The Center’s Carson Jeffres checks for the response among native fishes in the Cosumnes River. As with the foothill yellow-legged frog, Chinook salmon, rainbow trout, hitch and suckers have evolved to fit the extremes of river flows in California’s Mediterranean climate – flows flush with snowmelt runoff in the spring but anemic through the summer and early fall. For native fishes, however, the spring snowmelt recession sends a different signal: Get off the floodplains. Go back to the river.

Historically, winter and spring floods swept fish onto floodplains – vast stretches of riverside marsh that once dominated the Central Valley. Plentiful food, slow water and few predators made for optimal fish growth. During the recession, as the floodplains drained, fish would move back into the river well-nourished. Young salmon and steelhead trout would be fit for their ocean voyage.

The Cosumnes is one of the few Valley rivers that still have a hydrologic connection to their floodplains. (Flood control projects have eliminated most natural flooding and levees have cut off salmon access to floodplains.) Jeffres’ fish survey during last year’s recession showed the natives moving off the Cosumnes floodplain on cue. This spring, however, is a scratch. Conditions were too dry; the river crested its channel only in early December.

The Center’s studies of the springtime snowmelt recession, of course, are not just for curiosity’s sake. State and federal water managers are currently exploring opportunities to expand native fish species habitat in the Sacramento-San Joaquin Delta to satisfy Endangered Species Act requirements for winter- and spring-run salmon and Central Valley steelhead. Officials need data showing that salmon and steelhead do in fact move off floodplains as waters recede and don’t get stranded, unlike non-native species such as carp.

Eggs masses laid by the foothill yellow-legged frog cling to submerged river rocks in the North Fork Yuba River. Source: Ryan Peek

The frog surveys, meanwhile, are helping to inform decisions in the federal relicensing of Sierra hydroelectric projects. Dam operations have radically changed the spring flow patterns downstream, much to the detriment of the foothill yellow-legged frog and other native aquatic species. Dam-regulated river levels can drop sharply from the peak spring flows spilling over the dam to the low, flat-lined summer flows, as managers fill reservoirs. As a result, frog eggs, which are attached to submerged rocks in the shallows, can quickly get stranded and left to bake in the sun.

Center researchers have helped to develop flow schedules below dams with small daily percentage declines that mirror the natural pattern of the spring snowmelt recession. In the past few years, negotiators have promoted environmentally friendlier springtime flow patterns as part of operating conditions in federal license applications for hydroelectric facilities in the Yuba River and Bear River watersheds.

The rare foothill yellow-legged frog is the only amphibian in California that breeds exclusively in rivers and streams. Photo by Ryan Peek

Stay cued for more developments as we continue our research.

Sarah Yarnell is a hydrologist with the UC Davis Center for Watershed Sciences. Ryan Peek is an aquatic biologist with the Center.

References and further readings
Epke, GA. 2011. Spring Snowmelt Recession in Rivers of the Western Sierra Nevada Mountains. Master’s Thesis. Hydrological Sciences, University of California, Davis.

Jeffres CA, Opperman JJ, Moyle PB. Ephemeral floodplain habitats provide best growth conditions for juvenile Chinook salmon in a California river. 2008. 83(4)

Kupferberg, S., Lind, A., Mount, J., and Yarnell, S. 2009. Pulsed flow effects on the Foothill Yellow-Legged Frog (Rana boylii): Integration of empirical, experimental, and hydrodynamic modeling approaches. California Energy Commission, PIER. CEC-500-2009-002.

Lind, AJ and Yarnell SM. 2011. Frogs that go with the flow. River Management Society Journal 24(4): 10-11.

Yarnell SM. 2012. Sierra frogs breed new insights on river management. California WaterBlog

Yarnell SM, Viers JH, Mount JF. 2010. Ecology and Management of the Spring Snowmelt Recession. Bioscience. 60(2)