The Work Done by the September 2013 Flood

 John Pitlick, Geography Department, University of Colorado

Geomorphologists use the term “work” to describe the amount of sediment transported by individual flows.  It 
turns out that, in many rivers, most of the work is done by intermediate flows that provide an optimum 
combination between magnitude and frequency of sediment transport.  Rare floods have the capacity to 
transport large amounts of sediment, but because they occur so infrequently, it is difficult to determine 
their long-term significance in the denudation of landscapes.  In this talk I will present estimates of the 
work done by the September 2013 flood.  These estimates were developed from measurements at a select number 
of sites located along Front Range streams.  The results suggest that the amount of work done by the 2013 
flood was quite variable: At one locality on Left Hand Creek, the 2013 flood appears to have transported the 
equivalent of 100 years worth of bed load, with half of that being transported in only two days.  On Boulder 
Creek, transport rates were much lower, but still significant, with roughly 25 years-worth of bed load being 
carried through the section near the USGS gaging station at 75th St.  I will, of course, address the 
uncertainties in these estimates, but will also suggest that, in the Colorado Front Range, and perhaps in 
other places, the uncertainty in estimating the “work” done by individual flows has more to do with how we 
determine the average annual sediment discharge, not the sediment discharge associated with rare floods.