What Is Lake Turnover? Why Fishing Changes for a Week Every Fall and Spring

The definition, from the agency that manages ponds

Missouri’s Department of Conservation publishes a pond-management guide whose first line is the whole answer:

Turnover is the gentle, natural mixing of pond and lake waters. It is caused by changing temperatures in surface waters brought on by the progression of the seasons.

In a temperate climate, the agency says, most ponds and lakes turn over twice a year — in fall when surface waters cool, and in spring when they warm again. Everything anglers notice about it follows from the physics underneath, so that is where to start.

Why water flips: the 39-degree fact

Cold water is heavier than warm water, but not all the way down. Missouri’s guide explains the exception that makes lakes work: water is most dense at 39°F (4°C). As it cools further toward 32°F its molecules expand into the crystal structure of ice, which is lighter — so ice floats, and a lake freezes from the top rather than solidifying. If water were heaviest at 32°F, the agency notes, ponds and rivers would become solid ice.

That single property drives the whole annual cycle, which the agency lays out in four stages:

Our thermocline page covers the summer stage in depth. This page is about the two mixing events.

Fall turnover: the mechanism and the timing

Missouri’s description of the trigger: as temperatures drop toward 50°F in the fall, the surface layer cools and becomes both more dense and heavier. This cooler, denser water sinks, and fall winds stir the surface layer, gradually eroding the thermocline. Eventually the pond waters become uniform in density and temperature — complete mixing, which the agency notes can happen literally overnight.

Iowa’s DNR gives the calendar for its lakes: the surface water cools to the point where it is colder than the water below it, sinks, and the lake turns over — usually over the course of a few days from late September to mid-October. Fish then use the entire water column.

The date on your lake depends on depth, clarity and the year’s weather. Both agencies give ranges for that reason; a lake that stratified deep and clear takes longer to cool through than a shallow turbid one.

What it looks like from the bank

You can often see turnover happen from the shore. Missouri’s guide notes that the mixing sometimes leads to noticeable changes in the colour, taste and odour of the water, because it circulates decaying organic particles and gases that accumulated in the bottom layer all summer. A lake that was clear a week ago and is now dull, with a faint sulphur smell and no rain to blame, has probably turned.

Then it clears. Over time, the agency says, the freely circulating water becomes thoroughly oxygenated from the surface to the deepest part of the pond.

What it does to the fishing

The agencies describe the physical change; the fishing consequence follows from it, and it is worth being clear which is which.

What the agencies say. Missouri: fall turnover opens the entire pond’s volume to fish, and they may be found throughout the water column. Iowa: after turnover, fish use the entire water column. The EPA’s dissolved-oxygen factsheet states the underlying fact — after turnover, temperature and DO are generally the same throughout the lake.

What follows for the angler. All summer, the thermocline told you where to fish: fish concentrated above it, as close to the cool water as the oxygen allowed. Turnover deletes that rule. A crappie that held at 14 feet in August because the water below was dead can now be at 4 feet or 30, and Wisconsin’s DNR describes crappie as moving in response to temperature, oxygen, season and food — all four of which just changed at once. That is why the week of turnover has a reputation for scattered, hard fishing, and why the weeks after it — cool, evenly oxygenated water top to bottom, fish feeding ahead of winter — have the opposite reputation.

Neither agency puts a number on how much fishing slows during the mixing itself. The murk and the circulating low-oxygen water make a slowdown plausible; treat it as a reasonable inference from what they do say, not a finding.

Spring turnover, briefly

Spring turnover is the same process run in reverse. Missouri’s guide: each spring the ice melts, sun and breeze warm the top layer of cold, light water, and when it reaches 39°F it becomes heavier and sinks. The pond achieves uniform temperature and density, the whole volume has good dissolved oxygen, and fish can be found at all depths. Spring turnover tends to pass with less notice than fall’s because it coincides with the pre-spawn movement toward shallow water — see our fish spawning temperatures page for what happens next.

Winter is not a turnover

This is a common confusion worth clearing up. Missouri’s guide describes winter as a second stratification, run upside down: 39°F water at maximum density sits on the bottom, a layer of colder and lighter water forms above it, and the surface freezes. The agency calls it inverse stratification — colder water on top of warmer. Early in winter fish are in the warmer bottom water; as decomposition uses up the oxygen down there, they move higher. Heavy snow on the ice can block the light that would replenish oxygen, which is the mechanism behind a winterkill. Our ice fishing basics page covers the season that follows.

Reading it in your own conditions

Water temperature is the input, and our water temperature page explains where to get it. Dissolved oxygen covers why the bottom layer dies in summer and why turnover revives it, and wind covers the other half of the fall mechanism — Missouri’s guide credits fall winds with eroding the thermocline, which is why a still, warm October can leave a lake unturned longer than the calendar suggests.