California is at the southern boundary of the distributions of cold-water dependent fish species like salmon and steelhead. In many California rivers, flows and water temperatures challenge the performance and survival of salmonids even in years with average levels of precipitation. Drought and warmer summer air temperatures associated with climate change further exacerbate those stresses. 

At and above the Sierra Nevada foothills, Central Valley rivers are heavily regulated.  The dams there provide a variety of benefits for humans — water supply, flood control, hydropower, and recreation — but have some predictable and well-understood adverse impacts on salmonid populations. Dams block upstream passage of adult salmonids, preventing those anadromous fishes from reaching historically occupied upstream habitats. Dams trap fine and coarse sediments and prevent those materials from being transported downstream. As a result, spawning and rearing habitats downstream of dams tend to degrade over time. Dam operations frequently alter and can reduce seasonal stream-flow patterns. Cumulatively, these dam-related impacts are probably the single largest cause of diminished populations of Central Valley salmon and steelhead relative to pre-Western settlement conditions.


With deleterious impacts of dams acknowledged, we should consider a benefit dams can provide to salmonids. Large reservoirs like Lake Shasta, Lake Oroville, New Bullards Bar Reservoir, and New Melones Lake can release cold (hypolimnetic) waters that support downstream salmon and steelhead populations. The ability of large reservoirs to provide cold waters is recognized as a benefit to salmonids. Regulatory agencies set water temperature criteria to protect below-dam salmonid populations. Daily average water-temperature targets vary among rivers, but typically range between 55oF and 58oF. During multi-year droughts, like the current one that started in 2020, water managers frequently struggle to meet those temperature targets, and that has led to renewed criticism of California dams and their operations. Fair or not, dams are seen by some as failing to provide water temperatures that salmonids need.


To put that criticism in historical ecological context, it seems appropriate to consider water temperatures in California’s few remaining unregulated rivers, and ask the question — are water temperatures in unregulated rivers less impacted by drought conditions and more suitable for salmonids than regulated rivers? Ideally, this question would be addressed by a rigorous and carefully designed study that appropriately pair regulated and unregulated river reaches and assess both thermal regimes and salmonid abundances over time.


A recent post on the UC Davis’s California WaterBlog gave readers the impression that just such an analysis had been completed and that, indeed, California’s regulated rivers and streams are failing to provide cold-waters necessary to support the state’s embattled salmonids. The article in the journal Plos One — Classifying California’s stream thermal regimes for cold-water conservation – was accompanied by a UC Davis press release picked up by more than a dozen media outlets. High Country News published a follow-up with the provocative title Deadbeat dams’ and their impact on cold-water ecosystems, typical of the headlines generated by the Plos One publication.


The California WaterBlog and the peer-reviewed publication, report broad, even grandiose, conclusions about the cold-water failings of California dams and the ostensible thermal superiority of unregulated rivers. As a peer-reviewed publication, resource managers and conservation planners might assume these conclusions were well supported by data and analyses provided in the journal article. Although it pains me to critique research produced by my alma mater (I have a degree in Fisheries Biology from UC Davis in 1994), there are some major problems with the paper that seriously undermine the validity of key reported findings.*


Rather than a rigorous study design adequately representative of regulated and unregulated rivers and accounting for factors known to influence thermal regime — elevation, precipitation, air temperature, stream gradient — the article relied on long-term temperature monitoring sites easily accessible online. In the Central Valley, low elevation sites (<500ft) were over-represented (38 sites out of 50) and intermediate elevations (1,500ft to 5,000ft) — critical habitat for anadromous salmonids like spring-run Chinook and steelhead — were considerably under-represented (just one site out of 50).  Furthermore, Central Valley tributaries notably lacking major impoundments — the Cosumnes River, Mill Creek, Deer Creek, and Butte Creek — were either not included at all or were represented by a single low-elevation site.


A critical step in the publication’s analysis was to classify whether thermal regime observed at each site was influenced by an upstream dam, that is, “regulated,” or not, “unregulated.” The criteria used to make these classifications is not clearly defined in the publication. However, a careful review indicates misclassifications were a major problem. Here are just three examples:


  • The Stanislaus River at Coleville Powerhouse (site CLP) was classified as “unregulated” and credited for providing a “stable cool” thermal regime. In reality the site represents a hydropower discharge outlet that releases waters diverted from Spicer Meadow and McKays Point Reservoirs (Figure 1).

Figure 1 — Google Earth image of the CLP site classified in the Plos One publication as “unregulated.” The yellow pin indicates location of the temperature monitoring site based on coordinates provided in the publication.


  •      The American River near the former Auburn Dam site (site NFA) was classified as “unregulated” and credited for providing a “stable cool” thermal regime. However, temperature data collected by Placer County Water Agency demonstrates the thermal regime at this site is kept cool by hydropower releases from the Middle Fork of American River. Temperatures in the “unregulated” North Fork of the American River are considerably warmer in summer months than in the Middle Fork (Figure 2). 

Figure 2 — Water temperatures in the American River at and immediately upstream of the Auburn Dam site in summer-fall 2005.  NF14.8 shown here is the same as NFA used in the Plos One publication.  The coldest water occurs at MF0.1, the hydropower-regulated Middle Fork American River just upstream of its confluence with the North Fork American River.  The warmest waters occur at NF21.4 which is the unregulated North Fork American River just upstream of its confluence with the Middle Fork.  NF20.8 is at the confluence of the NF and MF.  Source: PCWA 2006. This figure and data from other years are available at

  •      Three sites on Big Spring Creek, a tributary to the Shasta River, were classified as “regulated” even though these sites are fed by groundwater. An agricultural diversion pond is present immediately upstream of one of the Big Spring Creek sites (at the BSC dam), but a previous publication reports minimal influence of this impoundment on thermal regime in Big Springs Creek (Nichols et al. 2014).

Since the Plos One publication did not consider it, it’s pertinent to ask: What do water temperatures in some of the Central Valley’s most important, largely unregulated rivers look like and how are they affected by drought? Mill, Deer, and Butte creeks are home to the Central Valley’s remaining historic populations of spring-run Chinook salmon. On Mill Creek, data collected by the California Department of Fish and Wildlife (2014-2017) to represent temperatures available for holding and spawning spring-run Chinook show a substantial adverse drought effect. In the drought years 2014 and 2015, average daily water temperatures consistently exceeded regulatory targets used for rivers downstream of Central Valley dams (Figure 3a). Thermal conditions improved substantially in non-drought years (2016 and 2017), but temperatures still exceeded regulatory targets during the summer and were insufficiently cold for spawning (<56F) until October. On Deer Creek, at a site also representing spring-run holding and spawning habitat, a similar pattern was apparent (Figure 3b). 

Figure 3 — Average daily water temperatures (red lines) and daily minimum/maximum (grey lines) in Mill Creek, upper panel (a) and Deer Creek, lower panel (b).  Data collected by California Department of Fish and Wildlife to represent thermal conditions for spring-run Chinook holding and spawning habitats (2014-2017). The shaded area depicts a range of regulatory temperature targets (55oF to 58oF) that are applied to Central Valley rivers downstream of large reservoirs.  Sacramento River basin water year classifications shown for each year. “Critical” water years were considered to represent drought conditions. “Below Normal” and “Wet” years were considered to represent non-drought conditions.

Unlike for spawning areas downstream of Central Valley dams, monitoring data are not available to assess the impact of drought-elevated temperatures on spawning success of salmon in these three creeks. However, abundances of adult spring-run Chinook salmon in Mill and Deer creeks crashed after the 2013-2015 drought and still have not recovered. A combined total of just 170 spawning adults were estimated in 2020.  Drought-year water temperatures on Butte Creek also appear to be problematic — more than 10,000 adult spring-run salmon reportedly died before spawning this year. 

With its Mediterranean climate, California has always been prone to hot summers and periods of drought that are stressful for salmonid populations. Despite some claims to the contrary, unregulated rivers and rivers upstream of Central Valley rim dams are not a panacea of optimal thermal regimes. California’s salmonid populations have evolved to persist in challenging environmental conditions. One of the Chinook salmon’s most important strategies for persisting despite poor freshwater spawning conditions is diversity in age-at-maturity. Older Chinook salmon are important because they buffer populations, allowing them to persist through consecutive years of poor freshwater conditions that would drive a population composed of younger fish to extinction. In the early 20th century, most Central Valley Chinook returned to spawn at age-4 and age-5, and a small, consistent fraction of the salmon even returned at age-6. Presently, the vast majority of Central Valley Chinook return to spawn at age-3, thus can be expected to have poor resilience to multi-year stressors like droughts.


Cold waters provided to salmonids and their habitats from large reservoirs are unnatural, and sometimes imperfect; however, those regulated rivers under a wide range of conditions can provide thermal regimes suitable for sustaining Central Valley salmonid populations under a wide range of conditions – seemingly more favorable than thermal conditions in unregulated rivers in some circumstances.


Scientific investigations that objectively contrast thermal regimes of regulated and unregulated rivers and evaluate the related performance of cold-water fishes are needed and welcome, particularly when considering passing the merits of fish passage above dams and their reservoirs. However, researchers have an obligation to identify appropriate sites for those comparisons and to confront hypotheses about the superiority of “natural” thermal regimes — rivers colder in winter, warmer in summer — with actual fish data. Reporting poorly supported conclusions critical of regulated river thermal regimes can certainly generate media attention, but if that work is not based on rigorous study design and on a data-based evaluation of the hypothesized phenomena, it distracts us from the substantive real-world challenge of conserving and recovering California’s salmonid populations. 



* The problems with the published paper were shared with the lead author. Concerns were acknowledged, but the suggestion of providing corrections to the published article was rejected.  The journal Plos One provides a formal process for concerns pertaining to the validity or reliability of published articles to be evaluated.  A letter detailing numerous concerns with the published article has been submitted to Plos One and is currently under consideration by the journal.  




Nichols AL, Willis AD, Jeffres CA, Deas ML. 2014. Water temperature patterns below large groundwater springs: management implications for coho salmon in the Shasta River, California. River Research and Applications 30:442–455


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