Why Do Lakes Turn Over in Autumn? The Hidden Upheaval Under Every Still Lake
TL;DR: Lake turnover is the mixing of a lake from surface to bed. In summer, sun warmed water floats on colder, denser water and the two layers stay apart for months, while the deep water slowly loses oxygen. In autumn the surface cools until it weighs the same as the water below, and the wind can stir the whole lake at once. In the Lake District that mixing begins in October and November.
Last updated: 16 September 2026
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Key takeaways
- Lake turnover is the mixing of a lake from surface to bed, once the surface water has cooled to the same temperature and density as the water beneath it.
- In summer, warm water floats on colder, denser water, and the boundary between them acts as a barrier that keeps the layers apart for months.
- Sealed off from the air, the deep water loses oxygen. UKCEH’s records show it at its lowest before the Lake District lakes mix again in October and November.
- Windermere has no freezing period and mixes once a year. Lakes that freeze over turn over twice, in spring after the ice melts and again in autumn.
- Windermere’s surface has warmed by around 1.7°C in 70 years, and its summer layering now starts earlier and ends slightly later.
What is lake turnover?
Lake turnover is the moment a layered lake mixes from top to bottom. Through the summer a lake sits in layers, warm water floating on cold. In autumn the surface cools until it matches the water below, and once the layers weigh the same, the wind can stir the whole lake in one go.
The University of Minnesota Duluth’s Water on the Web primer on density stratification puts it simply: when surface and bottom waters approach the same temperature and density, autumn winds can mix the entire lake, and the lake is said to turn over.
It is one of the biggest events in a lake’s year, and nobody standing on the shore can see it happen. The surface looks exactly as it did in August. Underneath, water that has been trapped at the bottom since spring is rising to meet the air.
Why does cold water stay at the bottom of a lake?
Because across most of a lake’s temperature range, colder water is heavier. Water is at its densest at about 4°C and gets lighter as it warms, so in summer the warm water floats and the cold water stays down. Our water fact on why ice floats explains the twist below 4°C, when water starts to get lighter again.
The density difference is tiny, but it is powerful. Water on the Web notes that the relatively large differences in density at higher temperatures are very effective at preventing mixing. A warm surface layer resists being pushed down in the same way oil resists sinking through vinegar, and it takes energy to force the two together.
It follows that the warmer the top of the lake gets, the harder the wind has to work to mix it.
What happens to a lake in summer?
It splits into layers. The UK Centre for Ecology & Hydrology explains that spring sunshine and warmer air heat the surface water and make it less dense, while deep water that the heat never reaches stays cool and dense, so a warm layer forms on top of a cool one.
Deep lakes settle into three layers. The warm, well mixed top layer is the epilimnion. Beneath it, the thermocline is a band where temperature falls quickly with depth. The cold bottom layer is the hypolimnion, cut off from the surface. Water on the Web describes the density change at the thermocline as a physical barrier that stops the upper and lower layers mixing for several months.
That barrier has a cost. The deep water can no longer take in oxygen from the air, but microbes keep using oxygen as they break down organic matter sinking from above. UKCEH’s monitoring of the Cumbrian lakes shows oxygen in deep water declining through spring and summer to its lowest point, until lake turnover begins in October and November.
Why do lakes turn over in autumn?
Because autumn takes the barrier away. As the air cools, the warm surface layer loses heat, grows denser and sinks. Each cold night and windy day mixes the lake a little deeper, and the thermocline moves steadily down.
Eventually the surface and the bottom reach the same temperature and the same density. With nothing left holding them apart, Water on the Web explains, very little wind energy is needed to mix the lake completely. That is lake turnover: the whole water column circulates, and bottom water that has been sealed away since spring comes into contact with the air again, raising its oxygen content.
So the timing is set by the weather rather than the calendar. Lake turnover needs both ingredients: enough cooling to erase the density difference, and enough wind to do the mixing.
Do UK lakes turn over twice a year?
Not usually. The textbook picture of lake turnover happening twice a year belongs to lakes that freeze. Water on the Web describes the classic cycle for such temperate lakes: turnover in spring, layers in summer, turnover in autumn, then a weak layering under the ice in winter. Lakes with two mixing periods are called dimictic.
Windermere, England’s largest lake, follows a simpler rhythm. The International Lake Environment Committee’s World Lake Database entry for Windermere lists no freezing period and a monomictic mixing type, meaning it mixes once a year, and records the thermocline forming from June to early November.
Without an ice lid, the lake simply stays mixed through the winter until spring sunshine builds the layers again. For the Lake District, then, lake turnover is an autumn event, and it happens once.
Is lake turnover changing?
Its timing is. The Freshwater Biological Association’s Windermere information note reports that the lake’s average annual surface water temperature has risen by around 1.7°C over the last 70 years, and that this has made its seasonal layering start earlier and end slightly later.
A longer summer and a later lake turnover mean the deep water is sealed off for longer. The FBA says this increases the risk of oxygen depletion, as microbes decompose organic matter in water that cannot reach the air. UKCEH found that all the lakes in the Windermere catchment it monitors have seen their minimum oxygen saturation decline over time.
Few places can measure this so precisely. Windermere has systematic records that began in the 1930s with the Freshwater Biological Association and have been continued by UKCEH and its forerunners since 1989. Our explainer on algal bloom monitoring covers the other danger of warm, still water: the oxygen crash that follows a bloom.
Frequently asked questions
What is lake turnover?
Lake turnover is the mixing of a stratified lake from surface to bed. It happens when the surface water cools to the same temperature and density as the deep water, so the wind can circulate the whole lake.
Why do lakes turn over in autumn?
Because the surface water cools, becomes denser and sinks, until it matches the water below. With the summer density barrier gone, very little wind energy is needed to mix the lake completely.
Do all lakes turn over?
Not in the same way. Water on the Web notes that many shallow lakes never stratify in summer, or do so only briefly, and some mix and restratify many times in a single summer. Deep lakes generally form three distinct layers.
Does Windermere turn over twice a year?
No. The World Lake Database lists Windermere as monomictic with no freezing period, so lake turnover happens once a year. Lakes that freeze over usually turn over twice, in spring and autumn.
What is a thermocline?
A thermocline is the layer in a stratified lake where temperature drops quickly with depth. It separates the warm surface water from the cold bottom water and acts as a barrier to mixing through the summer.
One last thought
Lake turnover is a reminder that a still lake is never really still. Every autumn the whole lake quietly trades places, and the deep water meets the air again for the first time since spring.
We watch water all day. It still surprises us. You can find the rest of the series, and the monitoring work behind it, across our water quality monitoring pages.