It existed from the Pliocene to the middle Pleistocene in the San Luis Valley, fed by glacial meltwater from surrounding mountain ranges.

The overflow cut down a valley that eventually drained the lake, leaving only the San Luis Closed Basin as a remnant.

[3] Westwards, Lake Alamosa spread to the San Juan Mountains close to Monte Vista to the west.

The lake was nourished by glacial meltwater coming from the San Juan, Sawatch, and Sangre de Cristo Mountains.

[23] Many lakes in western North America are cyclical, becoming deep and large during stadials and shallow and small during interstadials.

Lake Alamosa briefly reached elevations of 2,292–2,304 meters (7,520–7,559 ft) at Hansen Bluff four times during its history.

[24] According to modelling, ongoing sedimentation in the lake basin would have led to gradually increasing water levels during each stadial over time.

[25] Sedimentation occurred in the basin floor, although slip along the Sangre de Cristo fault to the east created accumulation space.

[18] It overflowed about 440,000 years ago,[3] when it reached a highstand during oxygen isotope stage 12, one of the major glaciations of the Northern Hemisphere.

[27] The glaciation was ending at that time, and meltwater from declining glaciers may have helped raise the water levels until the lake overflowed.

From there water would have flowed across the Costilla Plain and the Taos Plateau to join the Rio Grande and Red River west of Questa, New Mexico.

The overflow of Lake Alamosa into the Rio Grande expanded its catchment by about 22,000 square kilometers (8,500 sq mi)[16]-18,000 square kilometers (6,900 sq mi), adding the high, glaciated San Juan Mountains, the upper Sangre de Cristo Mountains and Sawatch Range to its watershed.

[32] Later research has indicated that the downcutting of the outlet may have continued for several hundred thousand years after the initial breach,[34] and was accompanied by an integration of the Rio Grande all the way to the Gulf of Mexico.

Decades before the region was settled and earlier than other geologic expeditions such as those of John Wesley Powell,[30] in 1811-1812 Jacob Fowler recorded the following:[29]I Have no doubt but the River from the Head of those Rocks up for about one Hundred miles Has once been a lake of about from forty to fifty miles Wide and about two Hundred feet deep—and that the running and dashing of the Watter Has Woren a Way the Rocks So as to form the present Chanel.