Mine Pit Lakes of Cuyuna and the Iron Range: Clear, Deep and Cold

Minnesota's mine pit lakes are flooded open-pit iron mines, and they are unlike any natural lake in the state: turquoise on a sunny day, clear enough to see 30 feet down, and hundreds of feet deep a short cast from shore. The best place to experience them is Cuyuna Country State Recreation Area near Crosby and Ironton in Crow Wing County. A second group is scattered along the Mesabi Iron Range from Coleraine to Aurora. Depths, acreages and clarity figures here come from DNR LakeFinder surveys and the DNR's Cuyuna Country pages, checked in October 2026.

How a mine becomes a lake

Open-pit mines go well below the water table, so they have to be pumped constantly. When mining stops, the pumps stop, and groundwater fills the hole. The Cuyuna Range, the DNR recounts, boomed during the two world wars with twenty to thirty mines operating, then shut down almost completely by the early 1960s. What remained were pits 100 to 525 feet deep and rock stockpiles up to 200 feet high. Forest grew back over the stockpiles, and the area became a state recreation area in 1993.

Three things make pit lakes different:

• Almost no shallows. Portsmouth Mine Lake covers about 149 acres and LakeFinder lists zero acres of it as shallow littoral zone. The walls are the shoreline.
• Very little inflow. Without streams bringing in silt and nutrients, algae stays sparse.
• Depth out of proportion to size. A pit of 100 acres can be deeper than Leech Lake.

Cuyuna Country State Recreation Area

The recreation area takes in about 5,000 mostly undeveloped acres, with 15 former mine pits, six natural lakes and roughly 25 miles of shoreline. It is best known for more than 50 miles of red-dirt mountain bike trail, but the water is the reason the place looks the way it does.

Mine lake	Area (acres)	Max depth (feet)	Average clarity (feet)	Notes
Mahnomen, Alstead and Arco	272	525	32.3	Reached by channel from the Pennington access; lake trout present
Portsmouth	149	395	30.5	Campground, swimming beach, public access
Pennington	62	259	26.0	Public access with picnic area
Huntington	106	258	38.5	Carry-in trail and unimproved access
Sagamore	119	210	25.7	Public access near Riverton

A vehicle permit is required, sold online or at the office in Ironton. The Portsmouth Campground has electric and non-electric sites but no RV dump or fill station. Two yurts sit along Yawkey Mine Lake. Details on booking are in the camping reservations guide.

You may still see Portsmouth described as 450 feet deep and the deepest lake in Minnesota. The DNR's current figures give Portsmouth 395 feet and hand the record to the Mahnomen group at 525.

Iron Range pits worth knowing

The Mesabi Range has many more pits, but most are on mining company land, still active, or without legal access. These have public access listed by the DNR:

Pit	Near	Area (acres)	Max depth (feet)	Notes
Gilbert Pit (Lake Ore-be-gone)	Gilbert	243	443	City park with beach, picnic shelter, boat landing; average clarity about 23 feet
Sabin (Embarrass Mine)	Aurora	156	465	DNR concrete ramp with a steep approach; designated trout lake with lake trout; clarity near 39 feet
Kinney	Kinney	45	160	Rainbow trout
Judson Mine Pit	Buhl	20	64	Carry-in; designated trout lake; clarity about 33 feet
Tioga Mine Pit	Cohasset	49	225	Designated trout lake with rainbow trout

Lake Ore-be-gone, a pun on a certain fictional Minnesota town, is the Range's swimming hole and a regular scuba site. Canisteo Pit near Coleraine is the giant of the group at roughly 1,458 acres and 311 feet, but LakeFinder lists no public access, so it is not a place to plan a launch. Do not assume any pit you can see from the road is open. Many are posted, and active mine property is dangerous as well as off limits.

Trout fishing in the pits

Cold, clear, well-oxygenated water is trout water, and the DNR stocks the pits accordingly. The agency says approximately 50,000 rainbow and brook trout go into the Cuyuna mine lakes each year. Some pits also hold lake trout, and the warm upper layer supports largemouth bass, northern pike, bluegills and crappies along the walls.

Rules to know (2026 regulations; confirm in the current DNR fishing regulations):

• Trout stamp. Portsmouth, Pennington and the Mahnomen group, Huntington, Sagamore, Yawkey, Manuel and several smaller Cuyuna pits are designated trout lakes under state rule. Anglers 18 to 64 need a trout and salmon stamp to fish them at all.
• Seasons. On designated stream trout lakes, the 2026 summer season runs May 9 to October 31 and the winter season ran January 17 to March 31. Designated trout lakes are closed to all fishing outside those seasons.
• Limit. Five stream trout combined, with no more than three over 16 inches.
• Bait and lines. Live minnows are prohibited on designated stream trout lakes; worms, leeches and preserved minnows are fine. One line only, winter or summer.
• Lake trout. On the Mahnomen, Alstead and Arco pit, lake trout must be at least 20 inches, with a possession limit of one.

Because Cuyuna is a state recreation area rather than a state park, the resident license exemption for state parks does not apply. Bring a license. For tactics, see the stream trout page; small spoons and a worm under a slip bobber near the drop both work from shore, and the edge is usually within casting distance. How the stocking program works is covered in fish stocking explained.

Paddling and scuba

Paddling. Pit lakes are sheltered, small and scenic, which makes them ideal for kayaks, canoes and paddleboards. Portsmouth, Pennington and the channel into Mahnomen are the usual starts. Calm mornings give the best view down the walls. More ideas are on the paddling page.

Scuba. The DNR describes the Cuyuna mine lakes as offering some of the best visibility for divers in the Midwest. Divers come for the visibility and for the fish; the DNR notes that Alstead, at about 140 feet, is loaded with a variety of species to watch. Below the surface layer the water is cold in every month, light fades fast along the walls and the bottom may be far beyond recreational limits. Go with a shop or club that knows the site.

Safety

Mine pits deserve more respect than their size suggests.

• The drop-off is immediate. A child wading three steps from shore can be in water over an adult's head. Life jackets for kids are not optional here.
• Cold water below. The surface warms in July, but below that warm top layer the temperature falls sharply. A jump or dive puts you straight into it, and cold shock is how strong swimmers get in trouble. See how cold Minnesota lakes get.
• No cliff jumping. Submerged ledges, rock and old timber sit just under clear water that hides how shallow or deep a spot is. Cliff jumping at Lake Ore-be-gone has been enough of a problem that Gilbert police have publicly looked for ways to stop it.
• Unstable edges. Stockpile slopes and pit rims can slough. Stay on trails and behind fences.
• Ice. Deep pits hold heat and can freeze later than nearby shallow lakes. Check thickness yourself; see the ice safety guide.

Protecting the water

Zebra mussels are already in several Cuyuna pits, including Pennington and Huntington, and in Gilbert Pit and Sabin on the Range. Others, Portsmouth among them, were not listed as infested when we checked. Paddlers hop between pits in a single day, which is exactly how mussels move. Drain and dry boats, boards and dive gear between lakes, as laid out in clean, drain, dispose.

Before you go

• Buy a vehicle permit for Cuyuna and a trout stamp if you plan to fish; most pits are designated trout water.
• Treat every pit as deep, cold and steep, because it is.
• Stick to pits with listed public access. The Range has many more holes than it has legal places to launch.