RN-01 The Vallecito flood— a catastrophic flood on Los Pinos River, southern San Juan Mountains, Colorado and New Mexico

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About this report:

This report covers mapping of glacial and flood deposits in southwestern Colorado and northwestern New Mexico on the Los Pinos River and within the San Juan River basin. Flood boulders of the distinctive Vallecito Conglomerate, diagnostic of this flooding event, are recognizable from the Bayfield, CO area down through Farmington, NM.

For more information on glacial outburst floods in Colorado, please see related Research Notes publications: RN-02 Glacial outburst floods on the Animas River, Colorado and New Mexico, RN-03 Glacial outburst floods on the Uncompahgre River, Colorado, and RN-04 Glacial outburst floods on the Lake Fork of the Gunnison River, Colorado.

From the abstract:

A catastrophic flood ran down the length of Los Pinos River into the San Juan River valley. The flood originated in the southern San Juan Mountains of Colorado, and deposits from the flood are still recognized 72 miles downstream near Farmington, New Mexico. This catastrophic flood is here called the Vallecito flood. The cause of the flood is unknown, but it was likely a glacial outburst flood, like many other known glacial outburst floods in the San Juan Mountains. Glaciers flowed down the headwater valleys of Los Pinos River and its tributary Vallecito Creek, and they merged at Vallecito. The glacier that arrived first may have dammed the other fork creating a glacial lake, or a proglacial lake could have formed behind an end morainal dam during the merged glaciers’ retreat. Failure of the glacial dam led to an outburst flood whose path can be determined from the distribution of flood deposits. The time of the flood is likewise unknown, because no flood deposits have been dated. Flood gravels on high mesas in New Mexico suggest the flood occurred in the early Pleistocene.