Thermocline

Sun warms the top of a lake through spring and summer, and the warm layer floats on the colder water underneath. Between them is a narrow zone where temperature drops sharply. Below that zone, water stops mixing with the surface, oxygen gets used up and is not replaced, and very little can live there.

So the thermocline sets a hard floor on where fish can be. In a lake with a thermocline at eighteen feet, there is no point fishing a crankbait at twenty-five, no matter what the sonar shows on the bottom.

You can often see it on a fish finder as a faint horizontal band, especially with the sensitivity turned up. It shows because the density change between the layers reflects a little sonar back.

Not every water has one. Shallow ponds, rivers and windy lakes mix enough that no stable layer forms, and a lake that never stratifies has no thermocline to work around. Deep, calm reservoirs stratify most strongly.