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.

What fish want

What do fish want? I thought I would try to focus on a seasonally-appropriate question, but also one that I spend a lot of time these days pondering. You might think that it would be simple to figure out what motivates a fish—after all, they probably aren’t influenced by the complex emotions that drive people. But unfortunately, we can’t just ask them (I have often wished for just one hour as a fish—I would learn… and publish… so much!).

In general, animal behavior is driven by a balance between the drive to maintain/gain energy stores, survive, and ultimately reproduce. So when asking why a fish chooses to inhabit a particular area of the river, we start by looking at the environmental factors (such as water temperature and shelter from potential predators) that might be influencing its behavior. In my research, I’m interested in what environmental factors are influencing juvenile steelhead behavior surrounding coolwater refugia (areas that are cooler than the main river, often because of an incoming cold creek). What causes fish to use coolwater refuges? And when the river is really hot, what causes fish to leave the refuge? In asking these questions, I’m hoping to gain an understanding of when these refugia areas are most important to fish.

Juvenile steelhead being measured

A heat image of a coolwater refugia area on the Klamath River (Image from U.S. Bureau of Rec)

Taking a step back… many rivers are getting hotter, both because of climate change (influence of higher air temperatures), and land use practices such as logging and agriculture. Logging the riparian area next to a stream reduces shading,  causing the water to heat up. Irrigation can lead to hotter water temperatures if water is diverted into shallow, slow-flowing irrigation ditches, then returned to the river. The Klamath River in northern California, where I do my research, can reach 80°F in the summer, which feels like bathwater. On cool mornings I would sometimes even get into the water to warm up.

Salmon are a coldwater fish, and prefer a temperature range of about 10-18°C (50-65°F). This means that during the summer, when the river temperatures reach 80°F, they are under serious thermal stress—the high temperatures increase their metabolism, so they struggle to maintain weight. As a result, fish will seek out areas in the river that are cooler, such as areas where tributaries flow into the river (i.e. thermal refugia). You can sometimes see hundreds of juvenile salmon packed into small coolwater refuges. However, fish sometimes leave the coolwater areas, even when the river is hot. Why? This question forms the root of my research.

Juvenile steelhead in a coolwater refugia area

I’m hypothesizing that it’s a trade-off between water temperature and the need to find sufficient food. While the fish gain a thermal benefit by hanging out in the cooler water, the high densities of fish suggest that there may be competition for food, forcing fish to leave for brief periods in search of prey.

To put this dilemma in a human perspective, imagine that it's 120°F outside, and you are in a nice air-conditioned house… with no food. You have the choice to go out and pick some food from the garden, or just stay inside and feel hungry. It’s a trade-off! At what point do you decide to leave? Now imagine that your body temperature changes to that of the outside temperature—so as soon as you walk out the door, your temperature begins to rise. This is what happens for fish, since they are poikilotherms—cold-blooded animals that do not stay in a temperature-constant environment, so their internal temperature varies over a wide range. While they are adapted to function over a certain range of temperatures, water temperatures may be rising more quickly than fish can adapt, leaving them with a shrinking amount of available habitat.

In order to understand when and where coolwater refugia are most important, we need to figure out what specific environmental factors are driving fish’s decisions to enter and leave refugia. To do this, I conducted behavioral field studies on juvenile steelhead at coolwater refugia sites on the Klamath River for the past 3 summers. I put radio tags into steelhead (see video below), and was able to track their movement and body temperature (pretty cool!) over time. So far, I’ve found some interesting effects of water temperature dynamics on the likelihood of fish using thermal refugia… stay tuned! (And to learn more about how I’m researching this, visit my website!)

Put in a larger (non-fishy) context, refugia habitats such as this will become increasingly important as the climate continues to change. Whether it’s a range shift upwards onto mountaintops for a high altitude terrestrial species, or a fish seeking out cooler water, understanding how animals use refugia and when they are crucial to their survival will be an important aspect of preserving some species.

And now, if you’ve made it this far, enjoy the holidays! I, for one, will be enjoying a taste of the fruits of my labor.