The Rio Grande Gorge in Colorado and New Mexico formed during the middle Pleistocene period, around 400,000 years ago. This was due to a combination of reduced tectonic activity and increased glaciation, which caused Lake Alamosa to overflow and connect with other Rio Grande basins. This expansion added more drainage areas and led to the integration of tributary streams into their current canyons. The process of incision, or cutting into the land, occurred over a span of 200,000 years, with significant erosion of tectonic and volcanic barriers. This event allowed the Rio Grande to flow from the Rocky Mountains to the Gulf of Mexico, reshaping the landscape in the region.