Estimating the magnitude of bankfull flows for streams in Idaho

Methods for estimating magnitudes of peak
flows with recurrence intervals of 1.5 and 2.33
years were developed for ungaged sites on streams
throughout Idaho. These peak flows represent the
magnitudes at and near bankfull stage and are
needed for quantification of water rights required
to maintain or restore fish and wildlife habitats and
riparian vegetation. Data from a previous report
detailing methods for estimating magnitudes with
recurrence intervals of 2 to 500 years were used in
this study.

Generalized least-squares regression techniques
were used to calculate the final coefficients
and measures of accuracy for the regression equations
for each of nine regions. The equations relate
basin and climatic characteristics to peak flows
with recurrence intervals of 1.5 and 2.33 years. The
basin and climatic characteristics used to develop
the equations included drainage area, mean basin
elevation, forested area, mean annual precipitation,
basin slope, north-facing slopes greater than 30 percent,
and slopes greater than 30 percent. Average
standard errors of the regression model ranged from
+150 to -60.1 percent, and average standard errors
of prediction ranged from +165 to -62.2 percent.
The range of prediction errors was narrowest,
-48.9 to -32.9 percent, for region 5.
A computer program was developed to automate
the calculations required for the regional
regression calculations. Results from this program
comprised calculated peak flows, site-specific standard
errors of prediction, and the 90-percent confidence intervals for the estimates.