LATE PLEISTOCENE EQUILIBRIUM LINE ALTITUDE TRENDS AND PRECIPITATION DISTRIBUTION IN THE SANGRE DE CRISTO RANGE, COLORADO

Using primarily field observations, we reconstructed the late Pleistocene extents and equilibrium line altitudes (ELAs) of 31 glaciers across the Sangre de Cristo Range of southern Colorado. Reconstructed ELAs range from 3,185 m for the Hunts Glacier at the northeast end of the range to 3,707 m for the Willow Glacier on the west side near the town of Crestone. In general, reconstructed ELAs are 100-150 m lower for the east side of the range for glaciers at similar latitudes. Along strike, reconstructed ELAs on both sides of the range increase to a maximum near the highest topography in the vicinity of Crestone and the Blanca Massif and then decrease slightly into the Culebra Range toward the Colorado-New Mexico border.

Modern precipitation varies considerably across the range, and precipitation gradients between the extremely dry San Luis Valley and the crest of the range are very steep. Meteorological stations show that the east side of the range likely receives ~35% more precipitation during the winter months than any other part of the range, but given the lack of stations at higher elevations, we cannot accurately determine vertical precipitation gradients for most of the range. To circumvent this limitation, we regressed the PRISM (Parameter-elevation Regressions on Independent Slopes Model) precipitation data against topographic data, resulting in a basic precipitation-elevation relationship. Normalized residuals for mean winter precipitation show a 10-30% enhancement in precipitation across the east side of the range and a deficit of similar magnitude on the west side of the range. Areas of enhanced and deficient precipitation correspond closely with lower and higher reconstructed ELAs, respectively. The similarities between the reconstructed ELA trends and the modern precipitation distribution suggests that during the late Pleistocene, winter precipitation from upslope storms delivering moisture from the southeast exerted a strong positive influence on the mass balances of glaciers on the east side of the range. Storms delivering Pacific-derived moisture were, like today, probably not the dominant source of precipitation for the Sangre de Cristo Range. Such a late Pleistocene precipitation distribution may also explain the dramatic geomorphic differences between valleys on opposite sides of the range.