The arid lands around Summer Lake were once lush. During the Pleistocene Epoch, vast areas of this region of south central Oregon were covered by lakes and wetlands. As the last ice age was ending, rain and runoff from melting snow filled the lowlands throughout this region of the Great Basin, creating an immense, freshwater lake called Lake Chewaucan. The lake covered at depths of up to. Lake Chewaucan covered the Summer Lake basin and drainage system much of the late Pleistocene. The last high water period is thought to have occurred about 13,000 years ago. There is archaeological evidence of human use of Lake Chewaucan during this time. The oldest known human remains in the Western Hemisphere come from the Paisley Caves, wave-cut caves currently 10 miles southwest from Summer Lake. Lake Chewaucan began to dry up at the close of the Pleistocene. As it shrank, salts and alkali were concentrated in its remaining waters. The exposed lake bottom sediments were blown by the prevailing westerly winds to form the sand dunes that still lie on the east side of Summer Lake. Today, Summer Lake and Abert Lake are separated by and are the only remnants of Lake Chewaucan. At high water, Summer Lake is about long and wide; however, water levels fluctuate dramatically, and sometimes the lake is almost dry in summer. The main source of fresh water is the small spring-fed Ana River, which is used for irrigation.
Fremont expedition
Summer Lake was spotted and named by Captain John C. Frémont during his 1843 mapping expedition through central Oregon. Fremont and his Army topographical team were mapping the Oregon Territory from The Dalles on Columbia River to Sutter's Fort in Sacramento, California. On 16 December 1843, the expedition struggled down a steep cliff from a snow-covered plateau to reach a large lake in the valley below. Fremont named the snow-covered rim "Winter Ridge" and the temperate waters "Summer Lake." Fremont described the discovery and naming of Summer Lake as follows:
Plumes of Summer Lake dust, lifted by winds of up to, are thought to be responsible for "dirty rain" that fell on February 6, 2015, across parts of three U.S. states: Oregon, Washington, and Idaho. Scientists at Washington State University concluded that an unusual combination of weather systems carried the dust up to north and northeast, where it mixed with rain, which turned "milky". Chalky residue coated cars and windows in Hermiston, Spokane, and at least 13 other cities.