A large influence on the water volume is local snow melt, especially from Steens Mountain, south of the lake.

For example, large snowpacks in the mid-1980s caused the lake to expand from approximately 67 to 160 square miles (170 to 410 km2) within three years, flooding usually dry areas and damaging a branch of the Oregon Eastern Railway.

Soon afterward, drought in the early 1990s reduced the lake size to just 200 acres (0.31 sq mi), exposing large mudflats and dusty playas.

[1][5] The size of this ancient lake, which existed during a wetter climate, has been estimated at 900 square miles (2,300 km2), with a maximum depth of 35 feet (11 m).

[6] The Great Basin redband trout (Oncorhynchus mykiss newberri) has reduced access to Malheur and Harney lakes due to irrigation diversions, channelization, draining of marshlands, and high alkalinities.

In these closed, high-desert basins, redband trout have evolved to survive in environments with vast extremes of both water flow and temperature.