Mann Lake (Harney)

Reachcode: 17120009001467 | Area: 227.2 acres | Shoreline: 2.5 mi | View on Interactive Map

(From Atlas of Oregon Lakes, Johnson et al. 1985)  Mann Lake, named for a local rancher, is a shallow playa lake located in the northern portion of the Alvord Valley. This part of the valley is fairly narrow; therefore the alluvial material carried from the enclosing escarpments has formed barriers for a number of small playa lakes. Mann Lake is the largest of these, while Tudor Lake and Juniper Lake to the north are other examples. All are relicts of a larger lake that covered the area during the Pleistocene Epoch when precipitation was greater and evaporation less. Mann Lake is fed by small, intermittent streams that emanate from the east slope of Steens Mountain. There is also intermittent inflow on the east side of the lake, but the drainage basin boundary cannot be identified. Being a lake basin of interior drainage, water is lost by evaporation. The surrounding landscape is a semi-arid rangeland for the most part, although the valley floor south of the lake and most of the shoreline consists of marsh area. While the valley floor is mostly private land, the slopes to the east and west are under administration of the Bureau of Land Management. The lake contains a population of Lahontan cutthroat trout and is managed by the Oregon Department of Fish and Wildlife as a source for the eggs of these fish.

The lake basin is a simple oval shape with a maximum depth of about 14 feet, too shallow to develop any permanent thermal stratification. Lake surface area and depth decrease significantly during dry years. Extensive shallow areas support dense growths of submerged and emergent macrophytes, and unusually large aggregations of Cladophora (a planktonic alga) have been observed in the lake. The concentrations of major ions are exceptionally high (sodium and potassium are particularly high), due to high rates of evaporation in this arid region. The concentrations of phosphorus and chlorophyl are also exceptionally high, identifying Mann Lake as hypereutrophic; it is one of the most eutrophic lakes in the state. Surface water pH is well above average and there is significant depletion of oxygen near the bottom, both caused by the intense growth of plankton and macrophytes. The lake is naturally eutrophic, but nutrient inflow is enhanced by wastes from cattle which frequently graze in the vicinity.