by Ann Willis, Ryan Peek, and Andrew L. Rypel

Given the current drought, it’s no surprise that California’s dams are struggling to provide cool water habitats to support native freshwater ecosystems. But what if they were never able to support them under any conditions?

New research shows how current stream management fails to provide the patterns of cool water that California’s native ecosystems need. The challenges stem from two issues: an oversimplification of stream temperature targets and the assumption that dam regulation can replicate desirable cold water patterns.

We analyzed available long-term (> 8 years) daily stream temperature data records for 77 sites throughout California. We then used an algorithm to see whether the sites could be sorted into groups with similar thermal regimes. The sites were sorted based on annual average temperature, annual maximum temperature, and the day when the annual maximum occurred. Aside from these three temperature metrics, no additional information was used to influence how the algorithm grouped the sites.

Results revealed that California’s streams naturally form five distinct classes (Figure 1). Two of the five classes were unique to groundwater springs, whose temperatures are notably constant throughout the year (stable warm and cold). Of the remaining classes, one included only sites not influenced by dam regulation (variable cool), one included sites heavily influenced by dam regulation (stable cool), and the last showed sites whose temperatures were mostly affected by weather (variable warm).

In ecological terms, “cool” and cold” are typically used to describe species that rely on relatively lower temperatures to support optimal growth and survival. These terms are often simplified to mean a specific threshold temperature associated with a species. In the Sacramento River, 56°F is a common target threshold used to sustain salmon. However, our research clearly shows how single temperature metrics like this do not capture the full thermal dynamics of California’s cold-water ecosystems. We must learn to manage towards multiple natural thermal regime endpoints; parallel approaches have greatly improved stream flow management. For example, flow management has transitioned away from single, minimum flow targets to environment flow management that recognizes the importance of diversity in magnitude, frequency, timing, rate of change, and duration of flows. Stream temperature management must make the same change.

When mapped throughout California’s hydrologic regions, the futility of sustaining cold-water ecosystems through regulations is starker (Figure 2). Results show that even when we can heavily manipulate flows or water temperature released from dams, we rarely approach the complexity endemic to natural thermal regimes. Rather than resetting temperature patterns by releasing cooler water than would naturally occur (or warmer, during the winter), dams create an artificial thermal regime that disrupt natural seasonal patterns and create novel thermal habitats – sometimes for tens of miles.

Except for Shasta Dam, no other regulated river showed the kinds of natural temperature patterns that cool- and cold-water ecosystems need. It’s the exception that proves the rule of how poorly dams perform when operated for environmental temperatures. While a few systems may provide opportunities for management of new or novel thermal regimes (which may provide some potential for thermal climate refugia or resilience) these sites are few and far between.

Conservation planning for cold-water species is a risky investment in California. Located at the southern edge of the geographic range of cold-water and anadromous species, California’s freshwater fauna is extraordinarily vulnerable to human-dominated ecosystems. Extinction is likely for most (74%) of California’s native salmonids; though altered or degraded thermal regimes are a major stressor, they are not the only limitations. Bold conservation actions are required to reverse the trend towards extinction. 

Ann Willis is a civil engineer and senior researcher at the UC Davis Center for Watershed Science. This research was part of her dissertation on water temperature management and stream conservation. Ryan Peek is an ecologist and senior researcher at the UC Davis Center for Watershed Science. Andrew Rypel is Co-Director of the UC Davis Center for Watershed Sciences and Peter B. Moyle and California Trout Chair in Coldwater Fish Ecology in the Department of Wildlife, Fish & Conservation Biology.

Further Reading

Willis et al. (2021). Classifying California’s stream thermal regimes for cold-water conservation.

Steel et al. 2017. Envisioning, quantifying, and managing thermal regimes on river networks.

Isaak et al. (2020). Thermal regimes of perennial rivers and streams in the western United States.

Maheu et al. (2016). A classification of stream water temperature regimes in the conterminous USA

Grantham et al. (2014). Systematic screening of dams for environmental flow assessment and implementation

Lusardi et al. (2021). Not all rivers are created equal: the importance of spring-fed rivers under a changing climate.

Moyle et al. (2017). State of the Salmonids II: Fish in Hot Water

Poff et al. (1997). The Natural Flow Regime

Yarnell et al. (2015). Functional flows in modified riverscapes: hydrographs, habitats, and opportunities