Brad Cavallo, a principal scientist at Cramer Fish Sciences, is challenging assumptions that hydropower facilities have an inevitable negative effect on water temperatures and therefore on salmon. He advocates the idea that effects and benefits depend on how dams are operated—and calls for researchers to rely on data and analyses that can accurately account for the thermal effects of river regulation. 

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Hydro Leader: Please tell us about your background and how you came to be in your current position. 

Brad Cavallo: I’m a fisheries biologist and principal scientist with Cramer Fish Sciences, an independent scientific consulting company working in California, Idaho, Oregon, and Washington. I lead one of the company’s California-based offices. I started my career as a biologist at the California Department of Fish and Wildlife. Later, I transferred to the California Department of Water Resources, which is responsible for managing and providing water for urban and agricultural uses. In 2006, I made the transition to independent scientific consulting by joining Cramer Fish Sciences, and today, I do a little bit of everything related to water and fisheries throughout the western United States, but I mainly focus on California’s Central Valley. 

Hydro Leader: Would you tell us more about the work of Cramer Fish Sciences? 

Brad Cavallo: Cramer Fish Sciences is a private-sector scientific consulting firm that focuses on fish issues and water resource management. Many environmental consultants do environmental compliance work, which involves producing analyses that are needed to comply with environmental laws. Our firm does some of that, but we are more focused on research that helps resolve water management conflicts. We develop methods and conduct studies to try to improve our understanding of how these systems work. Frequently, our work culminates in peer-reviewed publications, thus improving the understanding and acceptance of our findings. While our work is fairly West Coast focused, much of it is relevant to other regions of the country and the world— anywhere there is a need for applied science to conserve aquatic ecosystems while still taking into account human demands for water and land. 

Hydro Leader: Please tell us about Delta Currents. 

Brad Cavallo: Delta Currents is a blog that was launched a year ago. I was invited to be one of the regular contributors. I have always had a lot of thoughts about controversial fishery management issues that aren’t typically addressed in peer-reviewed journal publications or by environmental journalists’ reporting. Delta Currents provides a forum to reach a wider audience and share insights about how aquatic ecosystems function and how they are affected by human activities and facilities, such as water diversions, hydropower facilities, hatcheries, and harvesting. 

Hydro Leader: Who publishes Delta Currents? 

Brad Cavallo: It is affiliated with the Center for California Water Resources Policy and Management, an entity that is committed to collaborating to resolve water and fish resource conflicts in the San Francisco Bay, its rivers, and its estuary. We hope to be read by biologists, agency managers, and policymakers from Sacramento to Washington, DC. The objective of the center is to identify, generate, and interpret scientific information that can contribute to innovative and beneficial resource management. The organization emerged to respond to a situation in which many people felt that the policies that were being developed for water management and fish species conservation were not always informed by the best available scientific information. We try to help provide and disseminate that information. The center’s website has an archive of publications on topics like delta smelt, adaptive management, and Chinook salmon in the Sacramento Delta. 

Hydro Leader: Recently, you wrote a blogpost on Delta Currents titled “Misunderstanding Influence of Dams and Droughts on the Availability of Cold Water to Support Central Valley Salmon and Steelhead.” What led you to look into this particular issue? 

Brad Cavallo: This is a topic that I’ve been working on in various contexts for quite a while. Recently, I ran across an article in the online journal PLOS One that purported to have examined water temperatures in California rivers and found that rivers downstream of dams performed poorly for cold-water fish species. Press releases associated with the PLOS One publication led to news articles with inflammatory and misleading titles like “Deadbeat Dams and Their Impact on Cold Water Ecosystems.” The publication was being used to promote the idea that dams are really terrible for temperature management, but the publication’s analysis really did not support such a conclusion, and I felt a professional obligation to respond. After reviewing the publication carefully, I submitted a lengthy letter to the editors of PLOS One explaining in considerable detail serious problems with the paper. However, scientific journals are typically reluctant to publish corrections or retractions to papers they have already decided to publish. I wrote a blog post so that others could see for themselves the serious problems I found in my review of the PLOS One article. 

Hydro Leader: What were some of your concerns about the journal article? 

Brad Cavallo: The article seemed to address Chinook salmon and steelhead, species that people are quite concerned about in California. Many populations of these species are listed under the Endangered Species Act. In California, considering its relatively hot and dry climate, there are many challenges associated with managing water temperatures and flows to allow these species to persist and thrive. Naturally, there are many variables playing into such temperatures and flows. One of the most significant is providing cold enough water for the fish to spawn successfully, particularly Chinook salmon. If the temperatures aren’t cold enough for eggs to survive, there can be damaging consequences for the reproductive success of the salmon. There are many ways that water temperatures can affect the fish, but spawning and egg incubation are particularly important. 

In the blog post, I acknowledge and discuss the negative effects dams can have on migratory fish, such as salmon and steelhead. The thing I took issue with in the PLOS One article was how the authors portrayed the effect of dams on flow and temperature management. Managing flows and temperatures can be challenging, but a lot depends on how the dam is being operated and what our basis for comparison is. The premise of the article was that dams just can’t provide the kind of temperatures salmon need to be successful, and furthermore, that temperatures in natural river systems—or unregulated systems, meaning those without dams—are always better. The paper portrayed dams as failures in terms in providing suitable thermal regimes, but the analyses described in the publication simply did not support those conclusions. 

Hydro Leader: What was your response to that claim? 

Brad Cavallo: First, I considered what analyses would be needed to objectively evaluate how dams perform with regard to thermal regime relative to unregulated rivers and outlined the approach. It includes identifying and then contrasting thermal regimes observed in rivers that represent regulated and unregulated situations while accounting for or controlling for factors such as elevation, shading, stream gradient, and local weather. There’s actually some great work on these types of studies in the published literature. I described the sort of analysis that would be necessary to test the hypotheses that had been reported in the article as firm conclusions, as though an appropriate analysis had already been done.

Then, I took a look at a few systems that I’m familiar with in California, just to demonstrate that when you do look at the data more closely, you see patterns quite different from the ones reported in the article. For example, the North Fork of the American River was described as having cold temperatures because it is an unregulated system. The authors apparently didn’t really understand that water temperatures at the site they evaluated are heavily and beneficially influenced by hydropower operations in the Middle Fork of American River. Without hydropower operations on the Middle Fork American River, water temperatures at the authors’ reference point would be warmer, not colder. So the authors had misclassified the North Fork American River as unregulated. I found many other examples of rivers that had been misclassified. Another big problem with the article was that it considered few sites at intermediate elevations of California rivers, where Chinook salmon and steelhead were most abundant historically. A newer publication did look at such sites in two California rivers, finding that hydropower-regulated segments of these rivers provided more suitable water temperatures for the reintroduction of spring-run Chinook salmon than unregulated rivers. It was gratifying to see that, as it is exactly the kind of analysis the PLOS One article had failed to provide. 

Hydro Leader: Is the bottom line here that you can’t just make a blanket statement that dams have a negative effect on migratory species and that they might even have a positive influence? 

Brad Cavallo: Yes; dams can indeed have a positive influence on water temperature, and I think that’s the central theme behind many of the topics we write about in Delta Currents. We make a genuine effort to point out the importance of being scientifically rigorous when approaching such questions. We also strive to showcase the importance of letting the data guide us to the truth. Unfortunately, today there seems to be a pattern emerging in publications that suggests that the authors may have preconceived ideas about the right answer and then gravitate toward the interpretation of their data that supports that conclusion. It’s important for us all to be willing to critically evaluate our work and the work of others and to challenge findings that don’t line up with the evidence. 

Hydro Leader: With those thoughts in mind, is it possible that your findings with regard to salmonids in California might be applicable to other species in this or other river systems? 

Brad Cavallo: The key is to consider how you approach these kinds of problems and to maintain a degree of skepticism when making comparisons and judgments. Every system has its own issues, species, and management challenges. But yes, I think these concepts are broadly applicable as a starting point for addressing problems with water resource management. 

The Middle Fork American River is hydro regulated but provides water temperatures more suitable for cold-water fish species.

Hydro Leader: What can dam owners and operators take away from this conversation? Are there practices they can embrace or improve upon that will contribute to ensuring that cooler waters flow through their systems during run times? 

Brad Cavallo: I think so. My core advice to operators and waters users is to be attentive to these kinds of issues, to look into them, and not just to let them slide by. The regulatory community wants to help fish, but too often, there is an overreaction that leads it to quickly accept a proposed solution without carefully evaluating it. Sometimes, momentum builds, and then policies begin to be implemented that may not only impede your ability to carry out your essential operations but may not even help fish in the way the policy is intended to. We all need to be willing to check the science, to engage in the process, and to better inform the debate so that we ultimately find the best solution. Once flawed findings become embedded in the regulatory framework or the scientific literature, they are really hard to dislodge. 

Hydro Leader: What role do you see Delta Currents playing going forward? 

Brad Cavallo: The idea behind Delta Currents is for it to provide a place to present scientific information to inform policy decisions about how we manage California water resources. My writing and research tend to focus on how we achieve better results for salmon conservation. We want to help the salmon, yet we still need to be able to use water, so there are tradeoffs, and it’s our responsibility as scientists to guide policymakers to strike an effective balance. Delta Currents serves as a platform for that dialogue. We’d like to have a positive effect in the domain of California water resource management and ensure that such decisions are being informed by rigorous science. That will allow us to achieve conservation outcomes for the species while also allowing for reliable consumptive or hydropower use of water. 

Brad Cavallo is a fisheries biologist and principal scientist at Cramer Fish Sciences. He can be reached at The Delta Currents blog can be found at