Ask MidCurrent: Where Do Trout Go When Rivers Become Too Warm in Summer?

Two things that trout need—cool water and oxygen—are in short supply in midsummer. On this stream, the plunge pool below the riffle (right) provides highly oxygenated water, while the pool’s depth and shade (left) offer thermal refuge. Photo: Charles Hildick-Smith

Question: I know that I should stop fishing for trout when the water hits 65 degrees Fahrenheit, but the water temperatures in my local rivers get much higher this time of year. I hardly see any fish at all when I spend time near the water. Where do they go, and how do they survive?
—Joe H., Chittenango, NY (via the Ask MidCurrent form)

Answer: Summer heat can turn a trout stream into a challenging environment for coldwater fish. Trout thrive in cold, oxygen-rich water, and they begin to struggle when water temperatures rise into the upper 60s. Trout and other salmonids are extremely sensitive to water temperature because it directly affects dissolved-oxygen content and their metabolism. Cold water can hold much more oxygen than warm water, and trout require high oxygen levels to survive. As water warms, its oxygen content drops. For example, when water temperatures climb past roughly 68°F, the dissolved oxygen level can become suboptimal for trout, putting them under stress.

At the same time, the trout’s metabolic rate increases with temperature. Because they are cold-blooded animals, their bodily functions speed up in warmer water, meaning they need more oxygen just as oxygen availability is falling. This double bind is why 68°F is often cited as a critical threshold. Beyond this point, a trout’s respiration and heart rate go up while oxygen intake goes down, leading to severe physiological stress. If temperatures continue to rise, trout can literally suffocate in warm water that would be harmless to bass or other warmwater fish.

Please note that the information below is not meant to help you target fish, as trout struggling to survive should be left alone. Instead, it’s a way to help you understand the trout’s world and how it affects behavior.

Behavior and Physiology in Warm Water

When river temperatures climb above the mid-60s, trout undergo noticeable changes in behavior and physiology. One of the first signs is that trout stop feeding aggressively once water gets too warm. In fact, by the time the water hits about 67°F, trout feed much less, or even stop feeding altogether. Oftentimes, trout that may have been rising in the morning suddenly “shut off” as the water approaches 68°F. The fish aren’t eating because they are starting to experience thermal stress, and feeding becomes a low priority as they conserve energy. Additionally, the aquatic insects that trout eat may also reduce their activity in extreme heat, leading to sparse hatches. So, warm water not only makes trout lethargic, but it can also shrink their food supply, creating a double stressor.

Trout behavior also shifts towards seeking comfort and survival rather than expending energy. As stream temperatures rise, trout migrate to cooler, deeper, more oxygen-rich spots. Anglers may find that fish have seemingly “disappeared” from a stretch of river, when in reality, the trout have moved to whatever cold-water refuge they can find. They often hug the bottom of deep pools or crowd into shaded spring-creek inlets. In extreme cases, they might even migrate upstream to higher elevations in search of cooler inflows.

Here are some of the common refuges trout use when main river temperatures exceed their comfort zone:

Cold Springs and Seeps

Groundwater springs (either visible springs or invisible seepages up through the riverbed) provide nature’s air conditioning for trout. Groundwater often emerges at around 50°F—far cooler than a sun-baked river in summer. Trout will huddle in the plume of a spring or seep where the water might be 10–15°F colder than the surrounding river. These spring flows are typically found along banks, in side channels, or even mid-stream bubbling up through gravel. Studies of trout streams have observed fish congregating near groundwater inflows. For example, if there’s a tiny spring trickling in, a dozen trout might cram into that chilly trickle to cool off.

Deep Pools and Drop-offs

Deeper water tends to stay cooler, especially in rivers that don’t completely mix top-to-bottom. A deep pool can have layers of cooler water near the bottom (sometimes fed by springs or simply insulated from the hot sun at the surface). Trout will dive into the deepest holes they can find when the shallows get too warm. These areas also often have slower currents, allowing trout to conserve energy while they wait out the heat of the day. In summer, a normally empty deep pool might suddenly be full of trout that moved in from nearby shallow runs. The importance of deep water is one reason habitat restoration projects sometimes create deeper holes for fish, giving them shelter during both winter and summer extremes.

Undercut banks (top) and shaded parts of the river (bottom) lessen the effects of sunlight and offer water just a little cooler than elsewhere in the stretch of river. Photos: Phil Monahan

Shaded Areas and Undercut Banks

Shade from streamside vegetation or canyon walls can significantly reduce water temperature locally. A stretch of river that flows under heavy tree cover will be cooler than an exposed stretch downstream. Trout know this and often linger in shaded areas during the hottest parts of day. Undercut banks (where the stream has carved out a hollow under the bank, often protected by overhanging grass or trees) provide both shade and cooler micro-habitats. The water under an undercut stays cool and dark, plus such spots are usually out of the sun’s direct heating. A trout packed tight against an undercut bank or log in summer is likely there for the cooler water and refuge from the sun. While shade alone can’t offset a broad warming of the river, it can make a marginal difference that trout instinctively seek.

Fast Riffles and Oxygenated Runs

Although riffles are shallow (and shallow water can warm quickly), the turbulence of fast riffles and cascades oxygenates the water, which can help trout cope with heat. In high temperatures, trout will often move into choppy, aerated water like riffles, rapids, or the plunge pools below waterfalls. The dissolved oxygen in these areas is higher due to mixing with air, and even if the temperature is the same, the extra oxygen can be the difference between life and death for a trout. Additionally, the rapid flow can mix the water column allowing colder water from the bottoms of pools upstream to lower the overall temperature. Anglers on some rivers notice trout podding up at the head of riffles in late summer – those fish are essentially trying to breathe. Riffles also often have coarse substrate that can harbor cooler micro-currents (from groundwater), so trout position themselves there for a combination of cooler temperature and higher oxygen.

Tributary Inflows and Confluences

Small feeder streams are frequently colder than the main river, particularly if they originate from springs, snowmelt, or shaded forests. The junction of a cold tributary with a warmer river creates a pocket of cooler water that trout flock to. It’s common in midsummer to find trout piled up at the mouth of a tiny creek flowing into a larger river. Think of it like a cool-water bubble where the tributary fans out into the river. Fish will align themselves right at the seam where the two waters mix, often nose pointing into the cooler inflow. These confluence zones can be critical for trout survival during a heat wave.

Groundwater Upwelling Zones

Even without a visible spring or tributary, certain spots in a riverbed have subtle upwellings of cold groundwater. These might be areas with lots of porous gravel where spring water percolates up from the aquifer below. Trout are very adept at finding these micro-refuges. You might notice trout consistently holding in one oddly specific spot midriver on a hot day; likely there’s a cold spring trickle there that’s undetectable to us but very apparent to the fish. Upwelling zones often occur at the tail ends of pools or in side channels.

Let Them Be

Again, it’s important to recognize these refuges not to target trout there, but to avoid stressing those fish further. If you encounter trout packed tightly in a shady spring creek mouth or hugging a cold inflow, consider that a red flag – those fish are at their thermal limit. Ethical anglers will let them be and maybe even dampen their presence (avoiding wading through the refuge or spooking the fish) to not disturb the trout’s last sanctuary.

As fly fishers, we all cherish the thrill of catching wild trout, but with that comes a responsibility to ensure their survival for the future. When river temperatures climb above 68°F (or perhaps even lower), trout experience serious physiological stress and seek out the few refuges that can sustain them. By fishing only in cooler conditions (or switching locations/species), using proper fish-handling techniques, and advocating for healthy river flows and riparian shade, we can greatly reduce trout mortality in summer.

The bottom line: if the water is too warm for trout, let them be. As hard as it is to walk away from a river, knowing that doing so will help preserve the trout population is its own reward. After all, a true fly fisher’s pride is not just in the fish we catch, but in the fish we respectfully leave uncaught. When the heat breaks and the water cools, the trout will be back in action, none the worse for wear.

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