What is the water temperature of Minnesota lakes in summer?

Most inland lakes have surface temperatures in the 70s through July and much of August, with shallow southern lakes sometimes reaching the low 80s in a hot spell. Deep northern lakes tend to peak in the low to mid 70s. These are typical ranges, not official averages, and any one lake varies with weather, depth and wind.

When do Minnesota lakes get warm enough to swim?

Usually mid to late June in the southern half of the state and late June to early July in the north. The DNR considers water under 70 degrees to be cold water, and most lakes cross that line sometime in June. A cold, late spring can push it back by a couple of weeks.

How cold is Lake Superior in summer?

Cold enough to be dangerous all year. The Minnesota DNR's Lake Superior water trail guidance says the lake seldom reaches 55 degrees even in the hottest summer weather. Shallow, protected water near shore can warm above that on calm sunny days, but an offshore wind can bring frigid water back within hours.

What temperature is considered cold water?

The Minnesota DNR uses 70 degrees as the line. Below that, sudden immersion can trigger cold shock and rapid loss of muscle control. That means spring, fall and even early summer on many northern lakes all count as cold-water season.

How long can you survive in cold water?

It depends on the temperature and whether you are wearing a life jacket. The DNR teaches the 1-10-1 principle: about one minute to get your breathing under control, about ten minutes of useful movement, and up to an hour before losing consciousness to hypothermia. Without flotation, many people drown in the first few minutes.

Are Minnesota lakes getting warmer?

Yes. The Minnesota DNR reports that lake surface temperatures have warmed in every season across the state, and that in July and August lakes average roughly 3 to 4 degrees warmer than they did 50 years ago.

How Cold Are Minnesota Lakes? Water Temperature by Month

Minnesota lakes are cold for most of the year and pleasantly warm for about ten weeks of it. A typical inland lake goes from just above freezing at ice-out in April to the 70s by early July, holds there through most of August, and slides back into cold-water territory by late September. Lake Superior never really joins in.

One caveat up front: no agency publishes an official statewide table of average lake temperatures by month. The figures below are typical ranges drawn from general patterns, anchored to a few facts the Minnesota DNR does publish. Treat them as planning guidance, and check a current reading for the lake you are heading to.

Typical surface temperatures by month

These are surface temperatures on inland lakes in an average year. A cold spring or a July heat wave can shift any row by a couple of weeks.

Month	Southern and metro lakes	Northern lakes	What it means
April	30s to 40s after ice-out	Ice, then 30s	Dangerous. Immersion gear territory.
May	50s, touching 60s late	40s to 50s	Fishing opener water. Still hazardous.
June	60s rising into the 70s	Upper 50s rising into the 60s	Swimmable late in the month.
July	Mid 70s, low 80s in heat waves	Low to mid 70s	Peak warmth.
August	70s	Upper 60s to mid 70s	Still good. Cooling late in the month up north.
September	Upper 60s falling through the 50s	60s falling into the 50s	Back to cold-water rules.
October	50s falling into the 40s	40s	Dangerous again.
November	Upper 30s, then ice	Ice forming	See ice safety guidance.

A few things to know about how lakes warm and cool:

Water is densest at 39 degrees. The DNR's lake FAQ explains that this is what drives spring and fall turnover: as the surface warms or cools to that temperature it sinks, and wind mixes the whole lake. Right after ice-out, the entire water column is close to 39 degrees.
Lakes stratify in summer. By late June, a deep lake has a warm surface layer over a much colder one. Dive down ten or twenty feet in a deep, clear lake and the temperature drops sharply. The number on your depth finder is the surface.
Wind rearranges things. A strong blow pushes warm surface water to the downwind shore and lets colder water rise on the upwind side. Two beaches on the same lake can differ by several degrees.

North versus south

Latitude, depth and size all matter, and they stack.

Small, shallow lakes warm first. Prairie lakes in southern Minnesota and the shallower Twin Cities lakes can be comfortable by early to mid June.
Big, deep lakes lag. Mille Lacs, Leech, Winnibigoshish, Vermilion and Rainy hold their winter cold longer, and wind keeps mixing them. Expect them to trail a nearby small lake by a week or two in spring.
The far north is on a different schedule. Ice typically leaves Lake Minnetonka around mid April, and Rainy Lake and Lake of the Woods around the first days of May, according to DNR median ice-out dates. That three-week head start carries through to swimming season.
Boundary Waters lakes vary enormously. A small, dark-water, shallow lake can be warm in July while a deep, clear lake trout lake a portage away stays bracing all summer.

On the back end, the north cools first. Swimming on a Brainerd-area lake is usually still fine around Labor Day. On a Cook County lake it is a quick dip.

Lake Superior, the outlier

Superior is so large and deep that summer barely dents it. The Minnesota DNR's Lake Superior water trail guidance states that the lake seldom reaches 55 degrees even in the hottest weather, and that average survival time in the lake without a life jacket is less than two hours.

What this means in practice:

North Shore swimming is a plunge, not a swim, at places like the river mouths and cobble beaches between Two Harbors and Grand Marais.
Sandy shallows near Duluth can warm considerably during a calm, sunny stretch, and people do swim comfortably there in late July and August.
It can reverse in an afternoon. When wind pushes the warm surface layer offshore, cold water wells up to replace it. A beach that was pleasant yesterday can be painfully cold today.
Kayakers and paddleboarders should dress for the water, not the air. An 85-degree day does nothing for you once you are in 50-degree water.

Cold-water shock: the real danger

The Minnesota DNR treats anything below 70 degrees as cold water. By that standard, most of the open-water season outside of July and August qualifies, including the fishing opener, most of the paddling shoulder season and all of the fall.

People assume hypothermia is what kills. Usually it is not. The DNR describes three stages:

Cold shock, the first two to three minutes. An involuntary gasp, then hyperventilation and panic. If your head is underwater when you gasp, you inhale water.
Swim failure, within about 30 minutes. Arms and legs cool quickly and stop working properly. This happens to strong swimmers too.
Immersion hypothermia, after 30 minutes or more. The body's core cools and consciousness fades.

At every stage, the DNR notes, the outcome without a life jacket is drowning. The department says more than 30 percent of Minnesota boating fatalities happen in cold water with a victim who was not wearing a life jacket, and that falls overboard and capsizing are the most common causes.

The DNR's boating guide teaches the 1-10-1 principle for someone wearing a life jacket:

1 minute to get your breathing under control. Do not thrash. The gasping passes.
10 minutes of meaningful movement to plan, find your companions, signal and attempt self-rescue.
1 hour, or more, of useful consciousness before hypothermia takes over.

If you go in

Keep your clothes and boots on. They trap some warmth and, according to the DNR, most clothing floats for a while.
Stay with the boat. A capsized boat is easier to spot than a head, and climbing onto the hull gets you out of the water.
Do not try to swim for a distant shore.
If you cannot get out, pull your knees to your chest and hold still to conserve heat, or huddle with others.

Before you go out in cold water

Wear the life jacket. Zipped and buckled, not stowed.
Dress in layers under it, and bring dry clothes in a waterproof bag.
Tell someone where you are going and when you will be back.
Avoid going alone, and keep the phone in a waterproof pouch on your body.

Full legal requirements are in our guide to Minnesota life jacket law and boating safety.

A warming trend

Minnesota lakes are measurably warmer than they used to be. The DNR reports that surface temperatures have risen in all seasons statewide, and that July and August lake temperatures now average roughly 3 to 4 degrees higher than 50 years ago. Ice seasons are shorter as well. Warmer water means a longer swimming season, and also more stress on cold-water fish such as lake trout and tullibee and a longer window for blue-green algae blooms.

Bottom line

Comfortable swimming on inland lakes runs roughly from late June to late August, a little longer in the south and shorter in the north.
Below 70 degrees is cold water. That includes the opener in May and nearly all of September and October.
Lake Superior is cold every day of the year. Treat it that way no matter what the air feels like.