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.