Big Springs Creek near Mount Shasta (background) hosts an abundance of aquatic plants that lower water temperatures when salmon and trout need it most, during the dogs days of summer. Photo by Carson Jeffres/UC Davis

By Ann Willis and Andrew Nichols

Summer has just begun and conditions on many of California’s drought-stricken rivers and streams are already looking grim for cold-water fish.

Endangered winter-run salmon may not survive a repeat of last summer’s nearly total loss of eggs and fry from an over-heated Sacramento River. Low and warm flows in the Russian River watershed are threatening coho salmon and steelhead, prompting emergency water restrictions. And, last week, the state began evacuating rainbow and brown trout at the American River and Nimbus hatcheries to prevent die-offs over the summer.

However, not every California stream will turn perilous. In fact, some spring-fed streams are likely to become more hospitable during the dog days of summer.

Our on-going investigation of Big Springs Creek near Mount Shasta found that from May to August – when California streams generally warm up – maximum water temperatures cool by almost 3 degrees Fahrenheit. The cooling is all the more remarkable considering the creek is practically devoid of shade trees.

How could this be? The answer lies just below (and above) the water surface: aquatic plants.

A young rainbow trout in Big Springs Creek. Photo by Carson Jeffres/UC Davis

Generally, the best way to maintain cold-water fish habitat is to keep cold water cool – because once water warms, it’s difficult to reverse the trend. In Big Springs Creek, plants known as macrophytes help to provide that benefit in the absence of shade trees. Just as streamside trees form a shady canopy over a creek, mature stands of these rooted vascular plants create a sun-blocking umbrella within the creek channel.

The plants typically grow in spring-fed streams with stable flows, open canopy and low gradient. Big Springs Creek nourishes an abundance of macrophytes because its waters are enriched with nitrogen and phosphorous from volcanic and sedimentary rock.  

Snowmelt from Mount Shasta runs underground through porous volcanic and sedimentary rock before eventually bubbling up in Big Springs Creek (center). Source: UC Davis Center for Watershed Sciences

In 2011, scientists with the UC Davis Center for Watershed Sciences and Watercourse Engineering Inc. had an opportunity to quantify just how much influence these plants have in regulating water velocities, depths and temperatures in Big Springs. The predominant shady macrophyte species in the creek are water pepper, cutleaf water parsnip and seep monkey flower.

Not surprisingly, our study found that plant growth improved the stream’s physical habitat by slowing the flow, which deepened the creek and better protected fish from predators. However, we did not expect the plants to have such a pronounced effect on seasonal water temperatures.

The plants start their seasonal growth in the spring when the creek is shallow and widely exposed to the elements. But as the plants grow and emerge above the creek surface, their influence on water depth and temperature increases. 

Aquatic plants grow spring through summer. Early in the growth season, they have little influence on water temperature. But as they grow and emerge above the water surface, the plants deepen the creek and provide extensive shade, causing water temperatures to cool. Source: UC Davis Center for Watershed Sciences.

In May 2011, maximum water temperatures in the creek reached 68.5 degrees. By August, beds of these plants covered almost half the creek, water depths nearly doubled and 84 percent to 93 percent of solar radiation was blocked. Maximum water temperatures fell 3 degrees, to 65.5 degrees.

That’s right, summer temperatures in the creek cooled during the hottest time of the season. While maximum water temperatures have varied from year to year, the summer cooling pattern has held throughout the current drought.

The cooling effect of aquatic plant growth has important implications for restoration and management of certain spring-fed rivers and streams where these plants grow. For example, it’s easier to manipulate water temperatures by allowing these plants to flourish than by reshaping the stream channel or changing the flows from groundwater springs. Also, the plants’ rapid growth provides considerable short-term cooling compared with the time and cost of establishing canopies of shade trees.

The findings suggest that spring-fed streams have an important role to play as refuges for cold-water fish in a warming climate. Giving high priority to the stewardship of these streams will help sustain these important ecosystems in an uncertain future.

Ann Willis and Andrew Nichols are research scientists with the UC Davis Center for Watershed Sciences.

Further reading

Willis et al. 2012. Executive analysis of restoration actions in Big Springs Creek, March 2008-September 2011. Report prepared for National Fish and Wildlife Foundation

Willis et al. 2015. A salmon success story during the California drought. California Waterblog

Lusardi and Willis. 2014. Aquatic plants: unsung but prime salmon habitat. California Waterblog