Fishing & boating

Water Temperature and Fish: Cold-Water vs Warm-Water

By the Water Level Atlas team · Last reviewed Oct 21, 2026

Water temperature controls which fish can live in a lake or river and how well they do. Each species has a range of temperatures it prefers. Warm water also holds less oxygen than cold water, so hot, low water can stress fish that need cold conditions, such as trout.

Why does water temperature matter so much?

The U.S. Geological Survey puts it plainly: temperature governs the kinds of organisms that can live in rivers and lakes. Fish, insects, zooplankton and other aquatic life each have a preferred temperature range. USGS explains that as water moves further outside a species' range, the number of that species decreases until finally there are none.

Temperature also changes the water itself. USGS notes that warm water holds less dissolved oxygen than cool water, and may not hold enough for some species to survive. Some substances are also more toxic to aquatic life at higher temperatures.

What are cold-water, cool-water and warm-water fish?

Fisheries agencies often group fish by the temperatures they need. Exact ranges vary between agencies and between species, so treat these as general groupings rather than precise limits. The ranges below are from a Maryland Department of Natural Resources fisheries presentation on Deep Creek Lake:

The same presentation explains that rainbow and brown trout in Deep Creek Lake can survive year-round because the lake keeps a cold-water layer in summer. In the river below the dam, the hydropower permit includes releases meant to keep water at the lower boundary of the trout area under 77°F (25°C) in the critical summer period.

What happens to trout when water gets too warm?

Cold-water fish are the most sensitive to heat. The Utah Division of Wildlife Resources explains that the combination of high temperatures and low oxygen can stress many coldwater species, and that fish can die when temperatures are too warm or oxygen too low. Utah also notes that warm-water fish are better adapted to warmer water and slightly less oxygen.

Some states limit fishing when rivers get warm. Montana Fish, Wildlife & Parks can bring in "hoot owl" restrictions, which ban fishing from 2 p.m. to midnight, on most streams once the water temperature reaches 73°F (23°C) for three consecutive days. Montana says fish are more susceptible to stress, disease and mortality when water is low and warm. Its advice to anglers in those conditions includes fishing during the coolest parts of the day, landing fish quickly, keeping them in the water as much as possible, and using single, barbless hooks. Rules differ from state to state, so check your own state's regulations.

Why does low water make temperature worse?

Less water heats up faster. Utah's sportfish coordinator explains that a smaller amount of water heats more quickly and reaches higher temperatures than when there is more water. That is why low lake levels and low river flows in summer often go hand in hand with warm water. See fishing and water levels for how levels and flows affect fishing.

Is surface temperature the same as temperature at depth?

Not always. Some lakes keep a layer of colder water below the warm surface in summer, which is how Deep Creek Lake keeps its trout. A temperature sensor measures the water at one spot and depth. It may not show conditions in deeper parts of the lake or in other parts of a river.

Using this on Water Level Atlas

Water temperature is shown only where the gauge actually measures it; only some U.S. Geological Survey and U.S. Army Corps of Engineers gauges have a temperature sensor. On lake pages it appears in the "Records & details" box in °F with °C alongside. On river pages it appears as "Water temp" in the top data card. Where no sensor exists, the page says "Not measured here" (or "Not measured"), and we never estimate it. See our methodology for data sources.