Stuck between a creek and a hot place: Why our rivers are getting warmer

 When I give my ‘elevator-speech’ summary of my research—I study the effects of hot river temperatures on juvenile salmon behavior and the importance of coolwater refugia…—a common question I get is, ‘But why are river temperatures rising?’ Sometimes people are a little abashed about asking, since it seems like a simple question… but really, it’s a great question, and the answer has considerable nuance.

A recent study of 40 rivers across the U.S. found that most showed significant increases in water temperature over the past half-century.¹ A combination of factors affect stream temperatures, including air temperatures, amount of solar input, and land-use (e.g. urbanization, farming, and river management). This makes sense both intuitively and from experience—rivers in tropical climates are on average warmer than in the Arctic, and small high mountain streams are colder than large rivers near their delta (where they are both larger and less shaded, and are therefore open to a lot more solar radiation).

Air temperature is a strong predictor of water temperature, and increases in air temperatures due to global warming are causing a trend of increasing stream temperatures.^1,2 However, this is not the whole story. Urban areas create ‘heat islands’ that can increase water temperatures both through hotter ambient air temperatures, as well as heated water run-off from hot pavement. In addition, land-uses such as irrigation and dams can exacerbate the warming trend further, by storing water in slow-moving or shallower areas (such as irrigation ditches and reservoirs) where it heats up before returning to the river. For example, on the Klamath River where I do my research, the Iron Gate Dam (lowest of 6 on the river) is an old dam that releases water downriver from the top of the reservoir, water that has been sitting in the sometimes 100°F summer heat all day. As a result, summer water temperatures on the Klamath can reach ~80°F (26°C), which feels like bathwater, and is nearly lethal for coldwater-adapted fish like salmon.

There are many problems associated with rising river temperatures, and not just for salmon. Warmer water temperatures can cause increases in primary productivity and lower dissolved oxygen levels, effects that cascade up through the ecosystem, changing aquatic habitat structure and availability, invertebrate community composition, habitat suitability for many fish species, and often making the ecosystem more susceptible to invasive species. On the Klamath River, the summer hot water temperatures combined with eutrophication (excess nutrients, often from farm run-off) cause massive green-algae blooms in the reservoir by late summer, which get released downstream and turn the whole river green.

The Klamath River during an algae bloom

Halting the trend in rising water temperatures is linked to the problem of stopping increasing air temperatures (and rising atmospheric carbon dioxide), and will not be a quick fix. However, there is significant mitigation and restoration that we can do to lessen the impact of elevated water temperatures, solutions ranging from urban greening to dam removal to in-stream habitat restoration. This is where my research fits in—salmon are a coldwater fish, and on rivers like the Klamath, summer water temperatures can reach levels that are sometimes lethal. As a result, the fish seek out colder areas in the river (coolwater refugia), often created by incoming coldwater tributaries. Protecting and restoring these refugia are an important way that we can mitigate the effects of hot summer rivers temperatures on salmon.

Juvenile salmon piling into a thermal refugia on the
Klamath River (photo by Kyle Swann) 

It’s important to note that knowledge of the long-term trends in river water temperatures, as well as the data that produced the now famous graphs of rising C0₂ trends, would not be possible without long-term monitoring projects that were established years ago. These kinds of long-term programs are hard to fund and maintain, yet are essential if we want to understand how our environment is changing over time—this is an interesting problem that my lab is currently researching. Stay tuned!

1) Kauschal, S. et al. (2010). “Rising stream and river temperatures in the United States.” Frontiers in Ecology and the Environment.

2) Webb, B. et al. (2007). “Long-term changes in river temperature and the influence of climatic and hydrological factors.” Hydrological Sciences Journal.