A GIS-BASED ANALYSIS OF LATE PLEISTOCENE GLACIAL OUTBURST FLOODING IN THE COLORADO ROCKY MOUNTAINS

During the late Pleistocene, the San Juan and Sawatch Mountain Ranges of Colorado were heavily glaciated. In several valley floors, ice- and/or moraine-dammed lakes were present in the Late Pleistocene. Some lakes subsequently experienced glacial outburst floods, during the LGM retreat. This study reconstructs two lakes using ArcGIS, and estimates lake area, volume and peak discharge in areas with documented paleo-lake and outburst flood events.

The presence of ice-dammed lakes in the upper portion of the Rio Grande drainage, west of Creede, CO, has been noted by previous workers. Past paleomagnetic work and radiocarbon dating indicates that a large ice- or moraine-dammed lake (Lake Atwood) existed in the drainage from at least 16,380 to 13,500 BP, before draining catastrophically between 13,500 and 7,610 BP. At its maximum, Glacial Lake Atwood had a surface elevation between 3080 m and 3090 m. Using these lake surface elevation estimates and detailed digital topography we determined lake geometry and the probable magnitude of outburst flooding from the lake. At its maximum, the lake covered between 51x10⁵ and 71x10⁵ m³, with a maximum depth of between 14 and 18 m and a volume of between 70x10⁶ and 128x10⁶ m³. Using various jokulhlaup discharge models and the range of calculated lake volumes the maximum discharge of the flood was calculated in this study to be between 1,300 and 9,300 m³/s.

In the Upper Arkansas River Valley, between the Sawatch and the Mosquito Mountains, north of Buena Vista, a moraine- or ice-dammed lake occurred at the maximum extent of several proximal glaciers, with evidence of a series of glacial outburst floods. Based on moraine geometry and large boulder flood deposits downstream, several lake surface elevations have been proposed. Surface elevation estimates were determined from field, theoretical and DEM analyses. The lake surface elevation is between 2800 and 2930 m, with min / max volumes of 30.1 and 4200x10⁶ m³, with estimated peak discharge between 4920 m³/s and 20x10³ m³/s, based on field evidence. The field-based dam elevation, the maximum lake magnitude, exceeds the max depth of 200 m of an ice-dammed lake, as determined by Paterson, 1994, thus the lower estimate for lake surface elevation.