Tackling Cold-water Droughts in a Changing Climate

Drought comes in many forms. In California and the West, we focus on the scarcity of precipitation (meteorological drought) and the availability of water in rivers and reservoirs (hydrologic drought). These two translate into drought that impacts social and economic uses of water as well as the environment.

But recently, the warming climate has revealed different dimensions of drought. The emergence of "snow droughts"-which are unusually low snowpack conditions-has become a major issue. And now the "cold-water drought"-which has always been part of California's hydrology-is becoming more common throughout the West. We believe it's time for regulators and researchers to invest in understanding cold-water droughts and to incorporate the concept into resource management and climate change adaptation strategies.

What is a cold-water drought? First, cold water in our lakes and streams comes from natural sources including springs, melting snow, and rain that falls when temperatures are low, typically during winter.

But in highly-managed water systems like California's, reservoirs are also a critical source of cold water-often called the "cold water pool." Cold water from reservoirs is released over the course of the summer to meet the downstream temperature needs of salmon and steelhead. The amount of cold water in reservoirs depends on the amounts and timing of cold-water inflows, and also on when and how much water is released into downstream rivers and diversions.

While there is no formal definition of a cold-water drought yet, we define it as a period when there's not enough cold water to maintain historical river and reservoir temperatures.

Why does river temperature matter? Warm rivers bring a host of consequences. Warmer water absorbs more nutrients and toxic metals, and it also creates physiological stress on many native fishes. (It can be lethal for cold-water-dependent fishes like salmon and steelhead.) Many of the most destructive invasive species, including both aquatic weeds and non-native fish, are well adapted to warmer water, causing them to proliferate in many parts of the state, clogging waterways and crowding out native species.

Finally, warmer water can lead to the rapid growth of toxic algae. Reflecting recent warming trends and increased cold-water droughts, harmful algal blooms (HABs) are occurring with far greater frequency and intensity in rivers and reservoirs throughout the state, with consequences for human health, fish and wildlife, and our dogs. The state is actively monitoring this, and the pace of change is worrisome. Already there may be insufficient cold water to meet current, much less future, ecological needs.

And as cold-water droughts increase, they bring a growing cascade of ecological and health concerns. Increasing air temperatures and declining streamflows have contributed to increased cold-water droughts in recent years and are expected to intensify with climate change.

However, climate change is not the only culprit. The way California manages its water and land also has magnified cold-water droughts and their consequences. In far northern California, there has been long-running controversy over how to manage the spring-fed Scott and Shasta Rivers, tributaries of the Klamath River. The rivers are warming because of a combination of the removal of river-shading riparian vegetation, water diversions, groundwater pumping (which diminishes the flow of cold springs), and agricultural return flows. ("Agricultural return flows" refers water that's applied for irrigation but isn't used by crops and returns to rivers, streams, or lakes and can be reused. It's often warmer and can contain fertilizers and pesticides not absorbed by crops.) In fact, the management problem is less about the volume of flows needed for salmon than about rising water temperatures.

In the Central Valley, stark trade-offs have emerged between managing river temperatures below reservoirs and supplying water to cities and farms. New regulations from the State Water Resources Control Board to try to conserve Shasta Reservoir's cold-water pool to protect winter-run Chinook salmon will have major water supply impacts for the Central Valley Project, particularly in average and dry years when reservoir operators have to hold water back in the reservoir to maintain the cold-water pool.

Historically, some years yielded abundant cold water, while others-most often during meteorological and hydrological droughts-fell short. Now, though, cold-water droughts, exacerbated by climate change and water and land management practices, are not just limited to years of low precipitation. Operators of Folsom Reservoir have been unable to meet water temperature goals for spawning and rearing steelhead for decades. Over the past ten years, the Lower American River has exceeded its temperature thresholds for an average of more than 100 days each year. Even in years of abundant water-like 2023-temperature standards were not met on more than 50 days.

The decline in cold water is also hitting California's Sacramento-San Joaquin Delta hard. The Delta is warming fast, principally due to increasing air temperatures. This is amplifying ecological changes in the Delta, making it increasingly inhospitable to native species and contributing to the proliferation of non-native plants and animals as well as HABs. All climate change projections for the Delta indicate that it will continue to warm and experience more cold-water droughts, with ever-increasing consequences.

Water temperature is extremely important to both water supply and ecosystem management in California. As the climate continues to warm, cold-water droughts will grow, exacerbating trade-offs between water supply and ecosystem health. As highlighted in PPIC's Climate-Smart Tools to Protect California's Freshwater Biodiversity, change is happening fast and the window of opportunity to prepare and respond is closing.

The conservation of California's cold water will be an essential climate change adaptation strategy. And there is reason to be optimistic that reservoir and river management can be improved to safeguard this vital but threatened resource (while reducing impacts to water users). But to improve management, we need to understand cold-water droughts much better. These droughts merit more attention statewide as they concern both human and ecosystem health-and the time to study and prepare for them is now.

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