is not firmly established, however, and the overflow might have occurred between 690,000 and 440,000 years ago or after 376,000 years ago. The glaciation was ending at that time, and meltwater from declining glaciers may have helped raise the water levels until the lake overflowed. Continuing uplift in the Southern Rocky Mountains may also have played a role. Lake Alamosa overflowed through the Costilla Plain into New Mexico, in a gap between the Fairy Hills and the Brownie Hills. 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. Faulting had weakened rocks at the overflow threshold, thus facilitating erosion of the threshold. More than one channel may have been active as overflow path before the flow became focused on the present-day path of the Rio Grande. The water would have reached the Culebra Creek and eventually the Red River at La Junta Point; the Red River constituted the headwater of the Rio Grande during that time. The overflow of Lake Alamosa into the Rio Grande expanded its catchment by about -, adding the high, glaciated San Juan Mountains, the upper Sangre de Cristo Mountains and Sawatch Range to its watershed. Water levels dropped quickly after the overflow began, preventing the formation of the repeated shorelines that are common at shrinking pluvial lakes where the decline in water levels is paced by evaporation. The decline probably was not as quick as during the Bonneville flood of Lake Bonneville, another example of a lake overflow event in North America, as the rocks at Lake Alamosa were more solid and there is no clear indication of a catastrophic overflow flood. Later research has indicated that the downcutting of the outlet may have continued for several hundred thousand years after the initial breach, and was accompanied by an integration of the Rio Grande all the way to the Gulf of Mexico. That the lake drained after groundwater sapping has also been postulated but is less likely. After the overflow After the drainage of the lake the lakefloor was eroded by streams, which redeposited its sediments, and soils formed. The lake would have been replaced by dry lakes and anastomosing stream networks. Creeks cut into the lake floor, forming valleys that later filled with alluvium. The Alamosa Formation, a geological formation, fills the basin of Lake Alamosa;