|Location||Harney County, Oregon|
|Primary inflows||Intermittent streams|
|Catchment area||41 square miles (110 km2)|
|Basin countries||United States|
|Surface area||276 acres (112 ha)|
|Average depth||6 feet (1.8 m)|
|Max. depth||14 feet (4.3 m)|
|Water volume||1,600 acre feet (2,000,000 m3)|
|Shore length1||2.4 miles (3.9 km)|
|Surface elevation||4,173 feet (1,272 m)|
|1 Shore length is not a well-defined measure.|
Mann Lake is the largest of the shallow playa lakes in the northern part of the Alvord Valley in Harney County in the U.S. state of Oregon. The lake is a remnant of a larger Pleistocene lake that covered much of the valley, east of Steens Mountain. Fields–Denio Road runs along the east side of the lake.
Inflow to Mann Lake comes from small, intermittent streams, mainly from Steens Mountain. The lake, which has no outlet, but loses water by evaporation, is hypereutrophic. An influx of cattle waste from nearby rangelands adds to the lake's natural eutrophic state. Large concentrations of Cladophora algae are common.
The Bureau of Land Management operates the Mann Lake Recreation Site, about 100 miles (160 km) southeast of Burns by highway and unpaved road. The site has vault restrooms and boat ramps, but no other amenities. Recreation at or near the site includes fishing, camping, hiking, wildlife viewing, and amateur geology.
Mann Lake, stocked with Lahontan cutthroat trout, has been called "one of the premiere trout fisheries in southeast Oregon". Trout in the lake range in length from 12 to 20 inches (30 to 51 cm). Fishing is restricted to artificial flies and lures.
A large population of goldfish, introduced illegally, have at times multiplied in Mann Lake and competed for food with the smaller trout. In 2010, the Oregon Department of Fish and Wildlife treated the lake with rotenone, a piscicide, killing around 200,000 goldfish and fathead minnows while sparing nearly all the trout.
- Johnson, Daniel M.; Petersen, Richard R.; Lycan, D. Richard; Sweet, James W.; Neuhaus, Mark E., and Schaedel, Andrew L. (1985). Atlas of Oregon Lakes. Corvallis: Oregon State University Press. p. 232. ISBN 0-87071-343-4.
- "Atlas of Oregon Lakes: Mann Lake (Harney County)". Portland State University. 1985–2012. Retrieved December 20, 2012.
- "Mann Lake". Geographic Names Information System. United States Geological Survey. November 28, 1980. Retrieved December 20, 2012.
- "Alvord Desert/Mann Lake". Bureau of Land Management. Retrieved December 20, 2012.
- Sheehan, Madelynne Diness. Fishing in Oregon: The Complete Oregon Fishing Guide (10th ed.). Scappoose, Oregon: Flying Pencil Publications. pp. 282–83. ISBN 0-916473-15-5.
- Monroe, Bill (December 3, 2010). "Mann Lake Gets a Second Round of Rotenone for Cutthroat Restoration". The Oregonian. Oregon Live LLC. Archived from the original on December 28, 2012. Retrieved December 20, 2012.
- Mann Lake Campground – Public Lands Information Center