The tip of the thermometer needs to be at least 3-5 inches below the surface water. Warm water streams are very different from mountain or spring-fed streams that are generally cooler. Warm water streams should not exceed temperatures of 89 degrees F, and cold water streams should not exceed 68 degrees F. Summer heat can cause fish kills in ponds due to high temperatures reducing available oxygen in the water. Warming of waters comes from the sun. Dark muddy bottoms also absorb heat.
Deep waters are usually cooler than shallow waters simply because they require more time to warm up.
The more water there is, the longer it takes to heat or cool. Shade prevents water temperatures from rising too fast on bright sunny days. Temperature of waterways varies with the seasons. When high water temperatures are combined with low DO levels, toxicity is increased. Agricultural diversions may reduce flow and flow rate so drastically that water temperature rises to harmful levels.