MULTI-DISCIPLINARY PALEOFLOOD RESEARCH IN THE SAN JUAN MOUNTAINS OF COLORADO, USA

The adverse human, environmental, and economic effects of major floods emphasize the need for a better understanding of the hydrometeorological processes involved and the effects of climatic variability on flooding. Poor understanding of complex hydrometeorology and a lack of data on extreme rainstorms and floods result in flood-frequency estimates with large uncertainties. Increases in recreational and residential development in mountainous areas have placed a greater number of people at risk from floods, therefore increasing the need for improved flood-warning systems, flood-plain management, and design of infrastructure located on floodplains. Floods in the San Juan Mountains in southwestern Colorado typify these problems. The most notable high-elevation rainstorm in the Colorado Rocky Mountains occurred October 4-5, 1911, in the Animas River basin, upstream from Durango, Colorado. The maximum recorded storm total in the basin was 205 mm at Gladstone, which is at an elevation 3,290 m, that produced record flooding in the region, but had little documentation.

Paleoflood hydrology is the science that investigates evidence of the movement of water and sediment in rivers before the time of hydrologic measurement and offers a way to extend short-term stream-gage records and flood data for ungaged basins. Knowledge of the number, magnitude, and ages of historical floods and paleofloods improves the reliability in subsequent hydrologic analysis. The presentation includes: 1) an overview of paleoflood hydrology; 2) recent paleoflood research; 3) the results of our multi-disciplinary regional paleoflood study in the upper Animas River basin and in the San Juan Mountain region, which includes analyses of precipitation, streamflow, and paleoflood data, and; 4) future research needs. In the Animas River study, flood-frequency relations using historical and paleoflood data were developed. The frequency relations, in turn, have been used to improve the understanding of how natural disturbances affect mountain hydrology and ecosystems. Regional paleoflood data indicate that Holocene climatic variability has had little effect on maximum flood magnitude in the San Juan Mountains. Paleoflood techniques provide a cost-effective, transferable approach to obtain hydrologic data in a short time frame.