Mann Lake, Lake in Harney County, Oregon, United States

Mann Lake is a shallow playa lake in the Alvord Valley, east of Steens Mountain in Harney County, Oregon. It is the largest of the playa lakes in the valley and is a remnant of a much larger Pleistocene lake that once covered much of the area.

The lake is a remnant of a large Pleistocene lake that once filled the Alvord Valley. In 2010, the Oregon Department of Fish and Wildlife treated the lake with rotenone to remove an illegally introduced goldfish population, killing about 200,000 goldfish and other unwanted fish while sparing most of the trout.

The lake is known among anglers for its Lahontan cutthroat trout, and fishing here is restricted to flies and artificial lures. This rule helps maintain the trout population and gives the place a particular character for those who practice catch-and-release or fly fishing.

The lake is reached by a combination of paved and unpaved roads, and the Fields-Denio road runs along its eastern side. Facilities are limited to toilets and boat ramps, so visitors should bring their own supplies and be prepared for basic conditions.

The lake has no outlet and loses water mainly through evaporation, which makes it hypereutrophic and prone to algae blooms. Despite this, it supports a population of Lahontan cutthroat trout that can reach 12 to 20 inches in length.