RN-04 Glacial outburst floods on the Lake Fork of the Gunnison River, Colorado

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This report covers mapping of glacial and flood deposits in southwestern Colorado along the Lake Fork of the Gunnison River, where the catchment heads in the circular basin formed by the Lake City caldera. The author discusses two glacial outburst floods resulting from dam failures on proglacial Lake Hinsdale, associated with the latest Pinedale glacial advance and subsequent retreat.

For more information on glacial outburst floods in Colorado, please see related Research Notes publications: RN-01 The Vallecito flood— a catastrophic flood on Los Pinos River, southern San Juan Mountains, Colorado and New Mexico, RN-02 Glacial outburst floods on the Animas River, Colorado and New Mexico, and RN-03 Glacial outburst floods on the Uncompahgre River, Colorado.

From the abstract:

Glaciers from the San Juan icefield in southwestern Colorado formed in the circular topographic basin of the Lake City caldera and flowed down the Lake Fork of the Gunnison River. Only the latest glacier, the Pinedale, or Last Glacial Maximum, left a record. Pinedale glaciers in the basin converged at Lake City and ran about 22 miles down to the north. A late Pinedale recessional moraine three miles below Lake City dammed a proglacial lake, Lake Hinsdale, that was fed by all streams in the basin. Erosion of the morainal dam at the right abutment led to a glacial outburst flood that scoured the outwash train in the valley below. Much of the moraine remained intact, however, and alluvial fan gravels plugged the outlet gap. Lake Hinsdale persisted until failure of the left abutment caused a second outburst flood that tore out most of the moraine and further scoured the valley below. One of the floods left a large boulder 70 ft above the valley floor.