Developing Regional Curves Relating Bankfull-Channel Geometry and Discharge to Drainage Area

What is Bankfull Channel Geometry and Why is it important to the Wyoming Landscape Conservation Initiative?

Good water quality is necessary for healthy aquatic life, drinking water, recreation, and agriculture. Streams must be biologically functional and geomorphically stable to support high water quality. Changes in land use and watershed development activities can lead to changes in flow and sediment regimes. This can result in major disruptions to stream channel stability. The cumulative effects of channel and watershed disturbance can cause persistent instability problems with long-term consequences, including: accelerated streambank erosion and lateral channel migration rates; channel incision or excess sediment deposition; frequent flooding; loss of land, productive soil, buildings, and road crossings; and impairments to water quality, biological function, and scenic value. Numerous state agencies, including the Wyoming Department of Environmental Quality, Wyoming Game and Fish Department, and Wyoming Department of Transportation expressed the need for regional curves related to bankfull flows for a number of applications, such as structure (for example, water diversions) design and placement, streamflow regulation, habitat monitoring and assessment, and designing restoration or habitat enhancement projects.

 

Key Findings

  • Cross-sectional area has the strongest relation to drainage area of the parameters measured for all streamgages and streamgages at streams receiving more than 25 in. mean annual precipitation (0.85 and 0.91, respectively).
  • For all streamgages, we found that the R2 values for the other geometry parameters indicated that most of the variability in each regional curve was explained by drainage area for bankfull discharge (77%), bankfull width (80%), and bankfull mean depth (48%).
  • For streamgages with greater than 25 in. mean annual precipitation the R2 values for the other geometry parameters indicated that most of the variability in each regional curve was explained by drainage area for bankfull discharge (87%), bankfull width (83%), and bankfull mean depth (64%).

Study Objectives

  • Develop regional curves relating bankfull-channel geometry and discharge to drainage area for the state of Wyoming, including the WLCI region.

How were Regional Curves Developed?

USGS scientists collected data characterizing bankfull channel geometry at 37 active and discontinued streamgages between July 2010 and September 2011. We developed regional curves by relating cross-sectional area, bankfull discharge, bankfull width, and bankfull mean depth to drainage area through regression analysis based on precipitation zones. We used simple linear regression techniques to develop regional curves for the Rocky Mountains Hydrologic Region in Wyoming. Cross-sectional area, bankfull width, bankfull mean depth, and estimated bankfull discharge—the response variables—were regressed against drainage area—the explanatory variable—to show the relation between drainage area and each response variable. We developed two sets of regional curves for each response variable based on the range of mean annual precipitation for all streamgages and for streamgages with greater than 25 in. mean annual precipitation.

How Do Regional Bankfull Curves Help With Conservation Planning?

The intended use of regional curves is to validate estimates of bankfull discharge needed to conduct stream channel stability analyses and classify stream types as well as other uses such as stream channel restoration.These equations are useful for identifying the bankfull channel in areas with similar runoff characteristics. Regional curves are used to determine channel departure from reference conditions and to plan stream restoration. Regional curves are useful aids for estimating bankfull discharge and related channel geometry at sites without streamgages, particularly where field indicators of bankfull stage are not apparent, such as actively incising or degrading stream channels.