Summary of Flooding in the Colorado Front Range
(Information compiled by Liz Gilliam)
In contrast to the Western Slope of Colorado, where the stream flows are most affected by spring snowmelt, the Colorado Front Range has differing hydrographic trends dependent upon elevation. Above 2300 m (7,500 ft), runoff generated by snowmelt has the greatest impact on the annual hydrograph, and rarely produces flooding. Below 2,300 m (7,500 ft), convective storms generated in the mountains can produce localized, high intensity rainfall events resulting in floods, which have a more significant impact on the hydrograph and morphology of streams than either snowmelt or rainfall above 2,300 m (7,500 ft).
Streams in the Front Range are normally stable with relatively low sediment loads, although they periodically exhibit dramatic responses to disturbance from floods and hillslope instability. Only the infrequent summer and autumn rainfall floods generate sufficient stream power to mobilize the coarse-surface streambed and to substantially reconfigure the morphology of the channel and the valley bottom (Wohl 2005). Floods also play a large role in the diversity of riparian vegetation.
Flooding can also be exacerbated by a hillslope disturbance, such as a forest fire, that introduces large quantities of sediment into the river via debris flows and landslides. In small basins above 2,300 m (7,500 ft), many large floods that were attributed to intense rainfall may be debris flows and not water floods (Costa and Jarrett 1981).
Anthropogenic impacts such as urbanization, channelization, diversion, and alteration of native species also affect the magnitude of floods and the damage incurred from them. Highways have been constructed directly adjacent to most of the major rivers of the Front Range, constricting the floodplain and preventing lateral migration. Recent flooding has caused large monetary losses from damaged infrastructure during these floods. Dams and diversions occur in every river system in the Front Range, altering the magnitude, frequency and timing of floods. Beavers have historically played a major role in the hydrologic regime in eastern Colorado (Westbrook 2006) and the near removal of beaver and their dams has also impacted the hydrology of the Front Range. Many streams and channels are now lacking the flood attenuation and groundwater recharge processes that the beaver dams once provided.
Hydroclimatology in the Front Range
The high intensity rainfall events that occur in the Colorado Front Range are the primary cause of high magnitude, low frequency floods. These rainfall storms are of short-duration but can precipitate several tens of centimeters of rain in a few hours, resulting in a spontaneous flood with a quick recession time (Follansbee and Sawyer 1948). These floods have a more significant impact on the hydrograph and morphology of streams than either snowmelt or rainfall above 2,300 m.
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Damaged structures along the Big Thompson River following the 1976 rainfall-generated flash flood.
(Photo by S. Schumm)
Historical Floods of the Colorado Front Range
Data compiled from NOAA, National Weather Service flood forecast page; Downton et al.. ‘Problems of climate variability and uncertain in flood hazard planning for the Colorado Front Range.’ National Center for Atmospheric Research, Boulder, CO; The Weather and Climate Impact Assessment Science Program.