By Sarah Yarnell, hydrologist, UC Davis Center for Watershed Sciences
When dam operators schedule outflows to satisfy their downstream environmental obligations, they typically want to know, “How much?”
How much cold mountain water must Shasta Dam release to preserve the Sacramento River’s imperiled winter run of salmon? How much fresh Sierra water must flow out of New Melones Dam to dilute environmentally harmful salinity levels in the Sacramento-San Joaquin Delta?
The “just add water” recipe, however, is not a cure-all for endangered aquatic species. Increasingly, studies have shown that the pattern of flows from dams can matter as much as the volume.
Consider the foothill yellow-legged frog. It’s a rare frog, the only one in California that breeds exclusively in streams and rivers – not ponds or lakes. Its life strategy evolved to fit the seasonal pattern of river flows in our Mediterranean climate: flush with snowmelt runoff in the spring and anemic through the summer and early fall.
In the Sierra Nevada, early spring is no time for these fist-sized amphibians to breed. Erratic torrents of snowmelt in river channels would wipe out egg masses, which are attached to submerged rocks in the shallows. Mid-spring is no better. Fast-dwindling flows would leave the eggs and tadpoles high and dry. The frog’s sweet spot for reproduction then is usually late spring – late April to June – a period of moderate, gradually declining flows known to hydrologists as the annual “spring snowmelt recession.”
This recession has ecological benefits beyond reproductive cues for the river-breeding frogs. As flows gradually decline, rivers interact with their floodplains, delivering nutrients and providing habitat for aquatic bugs, fish and amphibians. Recession rates also can influence the shape of river bars and how riparian shrubs and trees take root. The magnitude, timing and rate of change of the springtime flows all affect the diversity of native habitat and species in mountain rivers across the western United States.
The importance of the spring snowmelt recession, though, has only recently gained the attention of water managers – mostly in the federal relicensing of northern Sierra hydroelectric projects. In the past two years, negotiators have preliminarily agreed to modify springtime flows to be more like natural rivers.
River flows below dams are unnatural. The typical seasonal hydrograph shows a sharp drop from the peak spring flow to the low, flat-lined summer flows. This abrupt change is the ecological equivalent of pulling the rug out from under the frogs, which see the season suddenly jump from winter to summer. Submerged egg masses and tadpoles are now stranded in the sun. They’re history.
The foothill yellow-legged frog has disappeared from much of its range in California. Scientists attribute the decline to the unnatural river flows below dams, among other factors. Few knew of the frog’s plight in the northern Sierra until biologists started monitoring their populations on several dammed rivers.
On the north fork of the Feather River, frog surveys grew out of a 2000 legal settlement dictating future operations of Pacific Gas and Electric Co.’s Rock Creek and Cresta hydropower dams in Plumas National Forest. PG&E also agreed to intermittently release high flows for whitewater enthusiasts once a month in the summer, beginning in 2002.
These “recreational” pulses were surreal. Boaters paddled roller-coaster rapids and raft-flipping drops along a stretch that only hours earlier could be waded.
Meanwhile, the frog populations were plummeting.
Scientists monitoring the river found that the whitewater pulses released from the Cresta Dam had flushed tadpoles and other small aquatic species downstream, wiping out a year’s reproduction. At the same time, tadpole production remained stable below the Poe Dam, just downstream of Cresta Dam, without pulsed flows.
Although the cause of the frog’s decline was not definitive, the Forest Service in 2005 suspended the whitewater releases as a precaution. The foothill yellow-legged frog was already under the agency’s watch as a federal “species of special concern” because of population declines in the 1990s.
Parties in the relicensing settlement labored to devise a new flow regime that offered boating opportunities without harming frogs, fish and aquatic life.
Templates for environmental flows from dams are limited. A century-old California Fish & Game law simply requires “sufficient water at all times” to the keep fish below dams “in good condition.” About 15 years ago, operators of some dams began scheduling “peak flushing flows” to clear salmon spawning gravels of sediment. More recently, federal operators of the Trinity Dam added a plateau of consecutive high-flow days in the spring to give young salmon a helpful push on their ocean-bound journey down the Sacramento River.
None of these flow strategies mimicked the pattern of the spring snowmelt recession so crucial to the frog and other riverine species.
To help inform decisions on the Feather River, the UC Davis Center for Watershed Sciences in 2006 launched a series of California Energy Commission-funded studies to investigate the environmental effects of pulsed releases from mountain dams. Field and laboratory experiments and computer modeling all showed such high velocity flows lethal to tadpoles.
Discussion among interests in the Rock Creek-Cresta settlement – resource agencies, environmental and boating advocates and PG&E – eventually sparked a winning idea: Reschedule the water allotted for summertime boating for release in the spring, to benefit both frogs and boaters.
PG&E implemented the innovative flow schedule in 2009 for the Cresta dam. The arrangement allows the utility to support both environmental and recreational benefits without sacrificing any additional water for power generation. The boaters lost the luxury of scheduled rapids in the heat of summer, but gained a longer, albeit less predictable springtime whitewater season.
“While unnatural and unpredictable flows are inconvenient and frustrating for whitewater enthusiasts, it can mean life for our fellow river dwelling species,” said Megan Hooker, a representative of the American Whitewater advocacy group, which has been one of the most influential players at the table. “It’s a clear example of what’s good for the river is good for recreation.”
The modified springtime flows may have come too late for the frogs, however. Their population in the Cresta reach of the Feather has not recovered, perhaps because it was so dismally low in 2005. Scientists who continue to monitor the river are nonetheless confident the new flow regime will significantly improve habitat elsewhere for riverine species.
Just this past year, negotiators preliminarily adopted similar springtime flow patterns as environmental safeguards in the federal relicensing of powerhouses on the Poe reach of the north fork Feather, the middle fork of the American River, the Bear River and the larger reaches of the Yuba River. Researchers at the UC Davis Center for Watershed Sciences have helped to develop the flow schedules with daily percentage declines that mirror the natural pattern of the spring snowmelt recession
The window of opportunity for California water managers to restore some facsimile of this ecologically important flow pattern looks promising: About 50 hydropower projects are scheduled for relicensing in the next 15 years.
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.
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, A. J. (2005). Reintroduction of a declining amphibian: determining an ecologically feasible approach for the foothill yellow-legged frog (Rana boylii) through analysis of decline factors, genetic structure, and habitat associations. PhD Dissertation. Ecology, University of California, Davis.
Lind, AJ and Yarnell SM. 2011. Frogs that go with the flow. River Management Society Journal 24(4): 10-11.
Yarnell SM, Viers JH, Mount JF. 2010. Ecology and Management of the Spring Snowmelt Recession. Bioscience. 60(2)