Water Temperature Cooling from Tidal Marshes May Provide Refuge for Fish

Restoring connectivity between tidal marshes and tidal sloughs may provide a way to keep some of the water in the Delta and Suisun Marsh cooler-which could have long-standing restoration implications for native fishes.

In research conducted for the Department of Water Resources (DWR) by researchers Chris Enright, Jon Burau and Steve Culberson* in Suisun Marsh, pilot study results suggest that having tidal marsh landscapes that flood on a biweekly, spring/neap timescale can contribute to significant temperature variation in the sloughs that drain these marshes.

These temperature differences in water that drains from tidal marshes may provide refuge from high temperatures that could be beneficial to some creatures, such as delta smelt. “One way to find relief is to live in and around a marsh where you have inputs of cooler water from time to time on the spring/neap cycle,” Culberson said. “Smelt and other aquatic organisms may be able to find them and use them as part of life history.”

“Natural tidal marshes adjust their marsh plain elevation to balance tidal inundation, plant productivity and sediment deposition and erosion processes,” Enright said. “Mature tidal marshes often find this balance near local mean high-high tide elevation.” Culberson explained that when this occurs at night during the late spring and summer, the water that drains back into the surrounding sloughs overnight can be significantly cooler than it was during the warm afternoon. “The water sitting in very shallow pools on the marsh plain cools by evaporation during nighttime and this cooler water slowly drains out into the tidal sloughs,” Culberson said.

The researchers wondered whether the cooler water draining from the marsh plain was sufficient to change the temperature in the adjacent tidal sloughs. They measured temperature responses to particular combinations of tidal and seasonal tide phase and time of day on a natural marsh and discovered that the current summertime high-high spring tide in the Suisun Marsh region is near midnight, one of the coolest times of the day.

When the researchers measured temperatures at various monitoring stations after a high-high nighttime tide event, they observed decreases up to 5 degrees Centrigrade in the tidal sloughs within approximately six hours-a significant decrease. “This process is a direct result of the landscape morphology (form and structure),” Enright said. “There is reasonable certainty that this is a real phenomena in mature tidal marshes and can generate significant cooling that may have regional effect.” He added that they found no such response from diked sloughs, where slough water is blocked from overflowing onto the marsh plain even at high-high tide.

“Temperature is a critical issue for most native fishes,” Enright explained. “Summertime water temperatures can exceed survival thresholds, especially in sloughs that otherwise would provide food and refugia. Sloughs that are diked from one end to the other have no mechanism for cooling water. In those sloughs, water temperature is highly correlated with average air temperature. In natural sloughs with high tide connectivity to marsh plain, water temperature correlation with air temperature is reduced by the geomorphic cooling process.”

These results are important because they suggest a management tool to reduce slough water temperatures and thus improve habitat for native fish such as delta smelt. These concepts could play an important role in tidal marsh restoration design in the San Francisco Estuary and Sacramento-San Joaquin Delta in the future. Fieldwork is ongoing and in time there will be a publication developed.

*Chris Enright is a Senior Water Resources Engineer with the California Department of Water Resources, Jon Burau is a Research Hydrologist with the U.S. Geological Survey and Steve Culberson is a former Staff Environmental Scientist with the CALFED Science Program.