Jeffrey A. Coe jcoe@usgs.gov (1), Jonathan W. Godt jgodt@usgs.gov (1) and Mario Parise cerimp06@area.ba.cnr.it (2)

1. U.S. Geological Survey, Denver, Colorado, U.S.A.
2. CNR - CERIST, Bari, Italy

Preliminary results from a U.S. Geological Survey study of small (<0.5 km²) fans along a 23-km portion of Interstate Highway 70 show a moderate potential for hazardous stream and debris flow activity. The study area is west of Denver, Colorado, one of the fastest growing metropolitan areas in the U.S., and transects the Front Range portion of the southern Rocky Mountains along Clear Creek, an east-flowing, formerly glaciated drainage. Morphometric analysis utilizing Melton’s basin ruggedness number and fan slopes, combined with fan classification based on sedimentological observations, suggests that about 60% of fans are of debris-flow origin, 30% of stream-flow origin, and 10% of mixed origin. Fan slopes become steeper and drainage basins become more rugged above the line of maximum glacial extent. Historical records from the last 100 years indicate multiple depositional events on fans from south-flowing tributaries. Detailed subsurface mapping and radiocarbon dating of fans from north-flowing tributaries suggest that the last major phase in depositional activity was between 1250 and 1450 AD. Results from several fans suggest that late Holocene activity is distinguished from early Holocene activity by lower accumulation rates (0.2-0.4 vs. 0.4-0.6 mm/yr.) and longer recurrence intervals (200-5000 vs. 100-2000 yrs.)