Lake Alamosa is a former lake in Colorado. It existed from the Pliocene to the middle Pleistocene in the San Luis Valley, fed by glacial meltwater from surrounding mountain ranges. Water levels waxed and waned with the glacial stages until at highstand the lake (high water level in the lake) reached an elevation of and probably a surface of over , but only sparse remains of the former waterbody are visible today. The existence of the lake was postulated in the early 19th century and eventually proven in the early 20th century. The lake eventually overflowed into the Rio Grande river system during the middle Pleistocene. The overflow cut down a valley that eventually drained the lake, leaving only the San Luis Closed Basin as a remnant. The Alamosa Formation is a rock formation left by the lake. Groundwater resources are contained trapped between sediments left by the former lake. Description The lake was in southern Colorado, at high altitude, covering most of the San Luis Valley/San Luis Basin north of the San Luis Hills. It reached an elevation of at highstand. Westwards, Lake Alamosa spread to the San Juan Mountains close to Monte Vista to the west. Northwards, it almost reached the present-day location of Saguache. It was about long in north–south direction and reached a maximum width of , making it one of North America's largest high elevation lakes, comparable only to the historic Lake Texcoco in Mexico. The surface area may have exceeded at highstand. It is likely that some water seeped out of the lake before its final overflow as groundwater. Saddleback Mountain was an island in Lake Alamosa, and its northern side was swept by waves during storms owing to the lake's long fetch. Additional islands were located in its southern basin; today they are hills. The city of Alamosa and possibly also the town of Center, Colorado are located within the former lake's floor. San Luis Lake presently occupies a small basin in the former lake floor; the basin was previously occupied by a leftover of Lake Alamosa, Lake Sipapu. Because of the old age of Lake Alamosa, shorelines and shoreline features are often buried or eroded; in many places former shorelines are only highlighted by changes in vegetation patterns or slope angle. Barrier bars, lagoons and spits formed in Lake Alamosa when it reached its highest stand. In particular, two spits at Saddleback