Magnitude-frequency of bed load transport in mountain streams in Colorado

This study examines magnitude-frequency relations for bed load sediment transport in 12 gravel-bed streams in the Rocky Mountains of Colorado. Channel and sediment characteristics measured in the field are used to model flow and bed load transport over a range of discharges in order to determine the volume of sediment carried by individual flows. The results indicate that these streams respond similarly to precipitation and runoff. The threshold for bed load transport in these streams occurs, on average, at 34% of the bankfull discharge; flows up to and including the bankfull discharge transport an average of 69% of the total volume of bed load. The discharge that is most effective for transporting bed load occurs 1.1% of the time (4 d/yr); this flow is within 20% of the bankfull discharge in eight of the 12 streams. The duration of the effective discharge does not appear to vary with drainage-basin area, channel slope or sediment size, as suggested in several previous studies. The consistency in magnitude-frequency relations observed here reflects regional similarities in hydro-climatology and geomorphology, resulting in channels that are formed and maintained by a common set of processes. (C) 2003 Elsevier Ltd. All rights reserved.