Lake Turnover: How Fall Mixing Works and Where the Fish Go

Eagle Lake in Acadia National Park, Maine, in early October: a red maple on the shore, fallen leaves floating in the shallows, reeds, and calm water reflecting the hills — Photo: National Park Service, via Wikimedia Commons (public domain)

Key takeaways

Turnover is the fall mixing of a layered lake: the surface cools to the temperature of the deep water, sinks, and wind finishes the job; the thermocline disappears overnight.
Water is densest at 39.2 °F (4 °C). A deep northern lake finishes mixing near that point; a southern reservoir turns in the upper 50s because its deep water never got colder.
Signs: murky water, a sulfur smell, lines of debris on the surface, no thermocline band on the sonar, and a bottom temperature within a few degrees of the surface.
It lasts about a week or two on most lakes. Lakes shallower than about 20 ft (6 m), rivers and windswept ponds rarely stay layered, so they have no turnover to wait out.
During turnover fish water under 6 ft (2 m), creek arms with inflow or the end of the lake that has not turned; afterward the whole lake is open and walleye and crappie go deep.

Lake turnover is the fall mixing of a lake: the surface cools until it is as heavy as the cold water below, sinks, and the lake ends up one temperature from top to bottom. It happens on lakes that layer in summer, most of them deeper than about 20 ft (6 m), and takes a week or two on most of them. For those days the fish scatter and the bite slows; afterward the whole lake is theirs again.

Water is heaviest at 39.2 °F (4 °C), not at freezing. That single quirk is why a lake turns over in fall, why ice floats, and why a lake freezes from the top down instead of into a solid block.

A lake drawn in cross-section in three seasons: summer with a warm upper layer, a thermocline and cold deep water; fall turnover with the whole lake mixing; winter with the coldest water just under the ice

Why does a lake turn over?

Because cooled water is heavier, and it sinks. From late spring the sun warms the top of a lake faster than wind can mix the heat down. By early summer a deep lake has split into three layers:

The epilimnion, on top: warm and light, stirred by wind and fed oxygen by the air and by plants.
The thermocline, under it: a band where the temperature falls fast with depth. Most of the drop happens within about 3 ft (1 m).
The hypolimnion, at the bottom: the coldest, densest water in the lake, sealed off from the air by the two layers above.

The thermocline sits about 6–8 ft (2–2.5 m) down in a dingy lake and 30 ft (9 m) or more in a clear one. In Michigan’s Lost Lake, in the Upper Peninsula, it sat at 20–21 ft (6–6.4 m) in two surveys a quarter century apart.

No light reaches the bottom layer, so no plant makes oxygen there. Everything that dies sinks into it, and its decay burns oxygen. By mid-July the bottom of a fertile pond may be completely without oxygen; Missouri’s biologists see little point fishing it in summer. Michigan DNR’s lake reports put the limit for cool- and warmwater fish below 3.0 mg/L of oxygen and the lethal line below 0.5 mg/L. In Lost Lake the August 1975 profile turned hypoxic at about 24 ft (7.3 m) and had no oxygen at all by 27 ft (8.2 m), just below the thermocline.

Clear, infertile lakes are the exception: light reaches deep enough for plants to make oxygen below the thermocline, so a clear trout lake keeps breathable water there all summer.

In September the surface loses more heat at night than it gains by day. The cooled water sinks and warmer water rises. The upper layer cools and deepens, eating the thermocline from above, with the fall wind stirring it along. When the surface matches the bottom temperature, nothing holds the layers apart, and the final mix can happen literally overnight. Wind gets more credit than it earns: on a small Canadian Shield lake monitored for ten years, wind shear mixed only the top 35% of the surface layer. Sinking cold water did the rest.

Has your lake turned?

Yes, if the surface and the bottom read within a few degrees of each other; that is Minnesota DNR’s test. The other signs back it up.

Take two temperatures. Surface, then bottom, with a thermometer on a line or a fish finder that reads temperature at depth. If they are still more than a few degrees apart, the lake has not turned.
The water went murky. Bottom sediment and nutrients hang in the whole column for days. The Ausable River Association calls that the telltale sign.
It smells. Rotting matter on the bottom made sulfur gas all summer, and turnover brings it up: hydrogen sulfide, the rotten-egg smell.
Lines of debris on the surface. Decayed weed and leaf fragments float up, and the wind rafts them into streaks that look like grass clippings. Tournament angler Michael Iaconelli first spotted a turnover from the launch ramp by the debris alone.
The thermocline band on the sonar is gone. The clutter line that sat at one depth all summer disappears, because there is no longer a density change to reflect sound.
The fish moved. Summer spots are empty, arches show at odd depths, catches crash. At one event of a youth league that typically draws 30 to 40 boats, Iaconelli saw only one team catch a limit.
In a small pond, fish at the surface at sunrise. Mississippi State Extension lists gasping fish at dawn as an early sign of an oxygen crash. In a big lake it is rare; Gilliland ties the fatal cases to confined areas.

Where do the fish go?

During turnover they scatter, with no layer left to hold them. Afterward they spread out to their fall water.

Phase	What the water does	What the fish do	Where to fish
Stratified summer (July to September)	Warm top layer, thermocline at 6–30 ft (2–9 m), bottom layer cold and nearly without oxygen	Crowded into the top layer and the first feet above the thermocline; cold-water fish in the coolest water that still has oxygen	The thermocline edge: humps, points and weedlines that cut through it; shallow at dawn and dusk
Turnover (a week or two, September to November)	Surface cools to the bottom’s temperature, the column mixes, water goes murky and smells, one temperature top to bottom	Scatter, with no layer to hold them; stressed in fertile water	Water under 6 ft (2 m), creek arms with inflow, windy banks, a single dock or laydown that runs shallow to deep; the end of the lake that has not turned
Mixed fall (after turnover to ice)	Same temperature and oxygen at every depth, cooling week by week through the 50s and 40s (10–4 °C), clearing	The whole lake is open; walleye, pike and crappie go deeper; bass follow bait into creeks; trout and salmon roam the shallows	Deep structure for walleye and crappie, creek mouths and coves for bass, shoreline rock and inlets for trout at first light

Bass. Iaconelli’s answer to turnover is water 5 ft (1.5 m) or shallower, where there was never a layer to mix; Virginia’s biologists draw the same line at 6 ft (2 m). He looks for feeder creeks and causeways with current, wind-hit banks, and isolated “ladder” cover: one dock or fallen tree running from 2 ft at the bank to 20 ft at the end. Afterward bass follow the bait into adjacent coves and creeks, and the bass temperature bands take it from there.

Walleye. After turnover, Minnesota DNR’s depth-sensor tagging studies show walleye, northern pike, muskellunge and cisco all using deeper water than they could in summer. As the mixed lake cools through the 50s, the bite comes back on deep structure after dark. The walleye page has the fall bands and speeds.

Crappie. Once the lake mixes, crappie pro Dan Dannenmueller says, the fish seek the depth closest to 68 °F (20 °C). Late in fall that is deep basin water that cools slowest, typically 28 ft (8.5 m) or more, where barotrauma becomes a risk for released fish. Fish the deep weed edge beside the basin first, then the basin; the crappie page has the depths by water clarity.

Trout and salmon. Lake trout, rainbows, browns and salmon spend the open-water season confined to the cold layers. Turnover opens the shallows to them just as the fall spawners head for shallow rock and tributaries. Fish shoreline rock and inlet mouths at dawn; the trout page carries the temperature lines.

Catfish. Channel catfish grow best at 75–85 °F (24–29 °C), and Mississippi State Extension says their feeding essentially stops below about 50 °F (10 °C). Turnover reopens the deep holes and channels that had no oxygen in August. Through the 60s and 50s cats work flats and creek channels near the bait, then settle into the deepest holes near 50 °F. The catfish page separates channel, blue and flathead.

Bluegill. The deep weed edge and dock shade pattern breaks with the thermocline. After turnover bluegill group on the deepest weedline and the basins beside it, where they will winter; the bluegill page has the bands.

At what temperature does a lake turn over?

There is no single turnover temperature. A lake turns when its surface reaches the temperature of its bottom water, and the bottom water is whatever the lake went into summer with.

A deep northern lake keeps the cold of its spring mix, near 39 °F (4 °C), on the bottom, so the surface has to drop into the 40s. Gord Pyzer puts complete turnover at about 40 °F (4 °C) on Ontario lakes. In a Missouri pond or a Virginia reservoir the deep water warmed into the 50s and 60s. Missouri expects turnover “as temperatures drop toward 50 °F”. Virginia’s Department of Wildlife Resources expects it “as waters fall into the low 60s and upper 50s.”

The floor is 39.2 °F (4 °C), fresh water’s maximum density.

Warm spells stall it. Lake Mendota in Madison, Wisconsin, usually turns in late October. In September 2017 it had begun mixing and then re-warmed under a heat wave: 71 °F (22 °C) on top, 64 °F (18 °C) in the middle, 57 °F (14 °C) at the bottom, three layers instead of two.

How long does turnover last?

About a week or two on most lakes, by the count of Virginia’s fisheries chief and of Iaconelli. Missouri’s rule of thumb runs longer: a few days for a small pond, several weeks or more for a lake. On the Canadian Shield lake the measured fall mix took about 13 days, from the day the column came within 0.4 °C of uniform to the start of winter layering. Minnesota DNR research scientist Andy Carlson gives the honest range: days to months, depending on shape, depth and weather.

A deeper lake has colder bottom water, so it turns later than the shallow lake next door. A big reservoir turns in sections, the shallow upper end first and the deep water by the dam last, which is why Iaconelli runs to whichever end has not turned. Lakes shallower than about 20 ft (6 m) and rivers rarely build a lasting thermocline, and windswept ponds can turn over any day and layer again when the wind drops (Missouri). No fall turnover to wait out there.

Where are the fish before it turns?

On the thermocline edge. In August the usable lake is the top layer plus the first few feet of the thermocline, and the fish and their food crowd into it: the coolest water that still has oxygen. Largemouth hold in a 3–4 ft (1 m) zone just above it and seldom cross into the oxygen-poor depths. Crappie school 2–3 ft (0.6–0.9 m) above the low-oxygen floor, often in 22–24 ft (6.7–7.3 m). Walleye feed in their 60–72 °F (16–22 °C) band where it touches humps, points and deep weedlines; trout and salmon live in or under it wherever the oxygen holds.

Find the band on the sonar; the fish finder guide shows it. Then fish the structure that cuts through that depth with a jig or drop-shot counted down to it. The water temperature hub has a 26-species chart and a thermometer tool for the band each fish wants.

What happens under the ice and in spring?

The layers come back upside down. Once the surface drops below 39.2 °F (4 °C) it is lighter than the water beneath, so the coldest water floats on top, ice forms at the surface, and the bottom holds about 39 °F all winter. Pyzer notes that fish gather in that stable bottom layer. Decay uses up the bottom oxygen as in summer, and heavy snow on the ice that blocks the light can winterkill a pond.

Spring turnover runs the film backward. The ice melts, sun and breeze warm the top layer to 39 °F, and the lake mixes; for a few weeks fish can be at any depth. It ends when the surface warms past the bottom and the summer layers form again.

Does turnover kill fish?

Not usually. Fisheries biologist Gene Gilliland’s summary: most of the time turnover does not kill fish, but it does stress them.

The fish kills that make the news are pond kills, and most happen in summer, not fall. A cold rain or a thunderstorm’s wind mixes a fertile pond whose bottom water has been without oxygen for weeks. Mississippi State Extension warns that a severe mixing event can kill nearly every fish larger than an inch or two in one night. That water can also carry hydrogen sulfide, toxic on its own.

In a lake the cost is a slow week or two, and then the lake is more open than it has been since June. A September cold front gets blamed for the same slow days: it starts the cooling that leads to turnover and drops the bite for a day or two on its own.