Lake vs. Reservoir: What's the Difference in Minnesota?

A lake holds water because of the shape of the land. A reservoir holds water because of a dam. In most of the country that is a clean distinction, but in Minnesota it is not: the state has thousands of natural glacial lakes, a good number of built reservoirs, and a large middle group of natural lakes that had a dam added at the outlet a century ago. Knowing which kind of water you are on explains a lot about its levels, its clarity and how it fishes.

The three kinds of water

Natural lakes

Most of Minnesota's 11,842 lakes are natural basins left by glaciers, many of them kettle lakes formed where buried ice blocks melted. The DNR's lake FAQ describes a lake as an enclosed basin filled or partly filled with water, with or without an inlet and outlet. Levels on a natural lake rise and fall with rain, snowmelt and groundwater, usually slowly. The full glacial story is in why Minnesota has so many lakes.

Reservoirs and flowages

A reservoir, impoundment or flowage is the pool behind a dam. The DNR uses flowage and impoundment interchangeably. Minnesota has more than 1,150 dams, according to the DNR dam safety program; about 650 are public, and the state owns more than 300 of those. They were built for navigation, logging, hydropower, flood control, water supply and, in a few dry corners of the state, simply to have a lake.

Raised natural lakes

The largest group of Minnesota "reservoirs" by acreage are lakes that already existed. A low dam at the outlet raised the level a few feet and gave someone a valve to turn. On a map they look like any other lake. On the water, the giveaways are a dam at the outlet, stumps in the shallows and a level that follows a schedule.

The Mississippi Headwaters reservoirs

The best-known examples sit near the top of the Mississippi. Between 1884 and 1912 the US Army Corps of Engineers built six dams in north-central Minnesota to store spring runoff and release it later, keeping the river deep enough for boats and mills downstream toward St. Paul. Lake Winnibigoshish was dammed first, in 1884, and Gull Lake last, in 1912. The Corps' St. Paul District still operates all six as the Mississippi River Headwaters Reservoirs.

Reservoir	Dam	County	Area (acres, DNR LakeFinder)
Leech Lake	Leech Lake Dam	Cass	about 103,400
Lake Winnibigoshish	Winnibigoshish Dam	Cass, Itasca	about 56,500
Gull Lake	Gull Lake Dam	Cass, Crow Wing	about 10,000
Pokegama Lake	Pokegama Dam	Itasca	about 6,700
Big Sandy Lake	Sandy Lake Dam	Aitkin	about 6,100
Cross Lake (Whitefish Chain)	Pine River Dam	Crow Wing	about 1,800 for Cross Lake itself

Every one of these was a natural lake first. The dams raised and steadied them, and in the case of the Pine River Dam at Crosslake, tied a string of lakes into what is now the Whitefish Chain. The locks and dams built downstream in the twentieth century took over the navigation job, and today the pools are managed with flood control, recreation, fish and wildlife in mind. The history is not all benign: the reservoirs flooded Ojibwe villages, gardens, burial sites and wild rice beds around Leech Lake and Winnibigoshish, and wild rice still depends on how steady the water is held.

What it means for you:

• The Corps draws the reservoirs down gradually through fall and winter to make room for snowmelt, then refills in spring. Docks and shallow launches can be high and dry early or late in the season.
• Corps recreation areas at the dams (Leech, Sandy, Cross, Gull and Pokegama among them) have campgrounds and boat launches that are some of the better public facilities in the Brainerd Lakes area.
• The fishing is lake fishing. Winnibigoshish and Leech are managed by the DNR as walleye lakes like any other, with their own special regulations.

Hydropower flowages near Duluth

North of Duluth the picture is different. Island Lake Reservoir, Boulder Lake, Fish Lake Flowage and Whiteface Reservoir are storage pools on the Cloquet, Otter, Beaver and Whiteface rivers. Minnesota Power holds water in them and releases it to feed hydroelectric stations on the St. Louis River, especially in winter when natural flow is low.

These fish and feel like true reservoirs:

Water	Area (acres)	Max depth (feet)	Average clarity (feet)
Island Lake Reservoir	8,001	94	5.2
Whiteface Reservoir	4,567	35	4.5
Boulder Lake	3,260	18	5.5
Fish Lake Flowage	3,259	36	7.6

Figures are from DNR LakeFinder. The water is tea-colored from bog drainage, the shorelines are irregular, and there are stump fields and rock reefs that were hilltops before the valley was flooded. Levels drop noticeably over winter as water is released. Go slowly in unfamiliar bays, especially in spring before the pools refill. The upside is a lot of undeveloped shoreline and good walleye, pike and crappie fishing within a half hour of the city, covered in the fishing near Duluth guide.

Lakes built from scratch

A few Minnesota lakes exist only because someone needed one.

• Lake Bronson in Kittson County was created after the drought of the 1930s dried up local wells. Construction of the dam on the South Branch Two Rivers began in 1936, and the lake is now the center of Lake Bronson State Park.
• Hayes Lake in Roseau County was formed by damming the north fork of the Roseau River when Hayes Lake State Park was developed. Only electric motors are allowed.
• Split Rock Lake in Pipestone County is a dammed creek. Pipestone is one of four Minnesota counties with no natural lakes, so this small reservoir is the local swimming and fishing hole.
• Rochester's reservoirs in Olmsted County, another lakeless county, hold floodwater first and anglers second.

Built lakes on small streams tend to be shallow, fertile and murky, and they slowly fill with sediment. They are also the only flat water for many miles, and locals love them accordingly.

River lakes that are not reservoirs

Wide spots on big rivers confuse the issue. Lake Pepin looks like a reservoir but the DNR describes it as a naturally occurring lake, 21 miles long, formed where the Chippewa River's delta partly dams the Mississippi. On the other hand, the navigation pools between the Corps' locks and dams from Minneapolis downstream are true impoundments, and their backwater lakes rise and fall with the pool.

Mine pit lakes

The third category is neither glacial nor dammed. When iron mining ended on the Cuyuna Range and parts of the Mesabi, pumps were shut off and the pits filled with groundwater. The results are small in acreage and enormous in depth: the DNR lists the connected Mahnomen, Alstead and Arco pit near Crosby at 525 feet, the deepest inland water in the state. With steep walls, little shallow habitat and very clear water, pits behave unlike any natural lake. Many are stocked with trout. See the mine pit lakes guide.

Why the difference matters

• Water levels. A natural lake changes slowly. A reservoir changes when the operator opens a gate. Check levels before trailering a big boat to a flowage in late fall or early spring.
• Hazards. Flooded timber, stumps and old roadbeds do not show on every map. Reservoir ice is also less predictable where current runs under it; see ice thickness safety.
• Clarity and color. River-fed pools carry sediment and bog stain. Do not expect Itasca County clarity on a St. Louis County flowage.
• Fish. Old river channels, flooded wood and current near the inlet concentrate fish. Reservoir walleyes often relate to the original riverbed more than to shoreline structure.
• Rules. Fishing regulations do not care whether water is natural. Special regulations are set lake by lake, so check the DNR fishing regulations either way.

Bottom line

• If there is a dam at the outlet, someone manages the level, and you should expect it to move with the seasons.
• Minnesota's biggest "reservoirs" are natural lakes with a federal dam added, and they fish like lakes. The flowages near Duluth fish like flooded river valleys.
• Mine pits are their own thing: deep, clear and cold.