Channel-Morphology Characteristics, Bankfull Discharge, and Various
Design-Peak Discharges in Western Montana
Sean M. Lawlor
U.S. Geological Survey
Scientific Investigations Report 2004-5263
Montana Department of Transportation, and the
U.S. Department of Agriculture-Forest Service
Lolo National Forest
Stream-restoration projects using natural
stream designs typically are based on channel configurations that can
accommodate a wide range of streamflow and sediment-transport conditions without
excessive erosion or deposition. Bankfull discharge is an index of streamflow
considered to be closely related to channel shape, size, and slope (channel
morphology). Because of the need for more information about the relation between
channel morphology and bankfull discharge, the U.S. Geological Survey (USGS), in
cooperation with the Montana Department of Transportation and the U.S.
Department of Agriculture-Lolo National Forest, conducted a study to collect
channel-morphology and bankfull-discharge data at gaged sites and use these data
to improve current (2004) methods of estimation of bankfull discharge and
various design-peak discharges at ungaged sites. This report presents
channel-morphology characteristics, bankfull discharge, and various design-peak
discharges for 41 sites in western Montana.
Channel shape, size, and slope and bankfull
discharge were determined at 41 active or discontinued USGS streamflow-gaging
sites in western Montana. The recurrence interval for the bankfull discharge for
this study ranged from 1.0 to 4.4 years with a median value of 1.5 years.
The relations between channel-morphology
characteristics and various design-peak discharges were examined using
regression analysis. The analyses showed that the only characteristics that were
significant for all peak discharges were either bankfull width or bankfull
Bankfull discharge at ungaged sites in most of
the study area can be estimated by application of a multiplier after determining
the 2-year peak discharge at the ungaged site. The multiplier, which is the
ratio of bankfull discharge to the 2-year peak discharge determined at the 41
sites, ranged from 0.21 to 3.7 with a median value of 0.84.
Regression relations between bankfull discharge
and drainage area and between bankfull width and drainage area were examined for
three ranges of mean annual precipitation. The results of the regression
analyses indicated that both drainage area and mean annual precipitation were
significantly related (p values less than 0.05) to bankfull discharge.
Purpose and scope
Description of the Study Area
Determination of Channel-Morphology Characteristics
Determination of Bankfull Discharge at Gaged Sites
Determination of Various Design-Peak Discharges at Ungaged Sites
Regression Relations between Various Design-Peak
Discharges and Channel Morphology
Use of Existing Flood-Frequency Equations
Regression Relations between Channel Morphology and Drainage Area and Bankfull
Discharge and Drainage
- Map showing location of the study
area, measurement sites, and two flood regions.
- Schematic diagram showing typical channel reach and cross section
for measurement of channel features.
- Photographs showing site and cross-section locations on South Crow
Creek and Mill Creek, western Montana.
- Graph showing longitudinal profiles for Boulder Creek at Maxville
and Eightmile Creek near Florence, Montana.
- Boxplot showing variation in recurrence interval of bankfull
discharge at 41 selected sites, western Montana.
- Boxplot showing variation of ratio of bankfull discharge to the
2-year design-peak discharge at 41 selected sites, western Montana.
- Graphs showing regional relations of bankfull-channel dimensions
and bankfull discharge to drainage area for different ranges of mean annual
characteristics and design-peak-discharge data for selected streamflow-gaging
stations, western Montana.
- Basin and climatic characteristics for selected
streamflow-gaging stations, western Montana.
- Three sets of regression equations for estimation of
design-peak discharge in western Montana.
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