In September 2009, a 99.4-m (326-ft) deep well was drilled proximal to Great Sand Dunes National Park and Preserve to explore the history and subsurface geology of the San Luis Valley, Colorado. Of particular interest was deciphering the evolution of ancient Lake Alamosa, which filled much of the San Luis Valley in the Pliocene and Pleistocene. Thick intervals of massive clay, recovered at the well (BP-3-USGS [U.S. Geological Survey]), represent deposition as part of this extensive lacustrine environment. Oriented paleomagnetic samples from the BP-3-USGS core allow for estimation of sediment deposition ages. Paleomagnetic analyses show magnetic reversals correlated to the Brunhes-Matuyama boundary (0.78 Ma) at about the 75.2-m (247-ft) depth and the start of the Jaramillo subchron (1.07 Ma) at the 90.9-m (298-ft) depth. These paleomagnetic age ties imply an average sediment accumulation rate of 0.10 mm/yr for the sediments above the Brunhes-Matuyama reversal and 0.05 mm/ yr for the sediments below the reversal. The relatively low average sedimentation rate (0.05 mm/yr) from below the reversal roughly corresponds with an observed lithological change at 70.7 m (232 ft) from deep-water lacustrine clays below the reversal, to increasing littoral and basin-margin deposits above. Magnetic reversal ages provide estimates for when the sedimentary environment at the well site shifted away from a deep-water lacustrine-dominated system to an alluvial system. This shift in depositional environment may correspond to the draining of Lake Alamosa and indicate when the San Luis Valley was incorporated into the downstream Rio Grande drainage network. The last extensive deposit of lacustrine clay at the well site is encountered at the 36.3–40.8 m (119–134 ft) depth, which corresponds to 423–376 ka. A lack of broadly distributed lacustrine clays above this layer suggests that Rio Grande incorporation and onset of Lake Alamosa draining occurred about 376 ka, similar to lake drainage estimates from ³He cosmogenic nuclide dating of Lake Alamosa and Rio Grande Gorge landforms. As such, our analyses suggest that the San Luis Valley likely became incorporated into the Rio Grande approximately 376 ka. Local, shallower lake systems appear to have existed at the well site until about 250 ka. This may corroborate the results of previous works that propose the drainage of Lake Alamosa began about 400 ka and was ultimately finished by 200 ka.