7050

• UCREFRP Catalog Number: 7050
• Author: Andrews, E. D.
• Title: Effective And Bankfull Discharges of Streams In THe Yampa River Basin, Colorado and Wyoming
• USFW Year: 1980
• USFW - Doc Type: Journal of Hydrology
• Copyright Material: YES

324

skew accounts for slightly more than one-half (r2 = 0.52) of the variation in
effective-discharge duration. Although the scatter of data is appreciable a T-
test indicates that th~ relation is significant at the 99% level.
The apparent insignificance of median bed-material size and unit stream
power upon the effective-discharge duration probably is due to several factors.
Although the median size of bed material varied greatly between sites - rang-
ing from 0.35 to 86 mm - a majority of the sediment load at all sites was sus-
pended load and thus relatively fine-grained. Consequently, the thresholds of
sediment movement were relatively small and did not vary much between sites.
The difference in bed-material size primarily influenced the fraction of bed
material in transport at a given discharge.
The Little Snake River near Dixon is typical of the streams with relatively
coarse bed material. The relation between bed-material size and the discharge
necessary to initiate motion is shown in Fig. 9. This relation was determined
by modifying the Shields criterion for critical-shear stress. Shields (1936) used

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Effect i ve
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Maximum
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Mean annual
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10 50 100
DISCHARGE. IN CUBIC METERS PER SECOND

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Fig. 9. Relation between discharge and the grain size of bed material at the threshold of
transport at the Little Snake River near Dixon, Wyoming.

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