Paleoflood Investigations for Cherry Creek Basin, Eastern Colorado

In 1950 when Cherry Creek dam, which is located in Denver, Colorado, was completed, the design flood was 5,126 m³/s. Two recent probable maximum flood (PMF) estimates for the dam range from 14,840 to 18,750 m³/s demonstrate the uncertainty in estimating extreme flooding in eastern Colorado, PMF difference is due in part to a lack of extreme rainfall and flood data in eastern Colorado. A paleoflood study was conducted to assist dam-safety officials in assessing the risk of large floods in Cherry Creek basin. An envelope curve encompassing maximum contemporary floods (19 sites) and paleofloods (99 sites) was developed for Cherry Creek basin streams; paleoflood data reflect maximum flooding during the last few hundred to many thousands of years. Maximum paleofloods in Cherry Creek range from about 1,050 m³/s near Franktown (in about 5,000 to at least 10,000 years), about 2,100 m³/s near Melvin (in about 1,500 to 5,000 years), and about 2,270 m³/s at Cherry Creek Reservoir (also in about 1,500 to 5,000 years). Flood-frequency relations for Cherry Creek, which incorporate paleoflood data, indicate the 10,000-year flood (10⁻⁴ annual exceedence probability) ranges from about 1,200 m³/s (near Franktown) to about 2,200 m³/s (near Melvin). PMF estimates are about six to eight times larger than paleofloods in Cherry Creek basin. Additional research in flood hydrometeorology is needed to help dam safety officials evaluate potential safety problems related to large floods in Cherry Creek basin.