The Wolf River and Rock River Watersheds: Developing a Regional Curve for Bankfull Stage

Abstract

Regional hydraulic geometry curves are graphical plots of discharge and channel geometry that can be used to calculate bankfull discharge and the bankfull channel geometries (bankfull), the channel-forming flood. Specifically, a regional curve provides information to estimate bankfull discharge, mean depth, width, and cross-sectional area at ungauged sites within given watersheds, (Mistak and Stille, 2007). Bankfull forms the average or natural stream channel and can be used to guide stream restoration. Bankfull is frequently assumed to be associated with the Q1.5 year flood, but varies between the 1.0 and 2.5-year flood (Copeland et al., 2000). Land use practices within a watershed have been found to affect bankfull discharge (Reidel et al., 2005). Areas that are primarily forest cover tend to produce less runoff and consequently less mass wasting, while areas that are primarily agricultural tend to produce more runoff and consequently more mass wasting (Reidel et al., 2005). Stream type affects the geometry and morphology of a stream and consequently the stream discharge. The purpose of this study is to develop a regional curve for bankfull stage by determining bankfull discharge using field techniques and historical gauging station data. For the purpose of this study, the 1.5-year recurrence 1.0-year recurrence intervals are used as surrogates for bankfull stage. This study also examines landcover and stream type to determine if these variables affect the relationship between hydraulic geometries and watershed area. This study conducts hydraulic geometry surveys in the Wolf River Watershed in Northeastern Wisconsin and the Rock River Watershed in Southeastern Wisconsin. These sites are chosen due to similarities in topography and an abundance of USGS gauging stations, which are used for historical data. Seven sites within the Wolf River and four sites within the Rock River are examined and compared to each other. This study finds a strong correlation between surveyed data and historical data within the Wolf River Watershed, and a strong relationship between the 1.5-year recurrence interval and watershed area. A strong relationship is also found between surveyed data and historical data within the Rock River Watershed, where a strong relationship exists between the 1.0-year recurrence interval and watershed area. This study did not find a consistent, significant relationship between landcover and discharge, which may be due to many factors, including relatively homogenous percentages of landcover, and heterogeneous watershed areas. Stream types are found to be relatively homogeneous across both watershed regions. Published regional curves are not available for the state of Wisconsin. This study uses survey data and historical data to develop a regional curve for the Wolf River Watershed and Rock River Watershed. A regional curve that is suitable for stream restoration was not developed for the Rock River due to challenges associated with historical data and survey calculations; however, these challenges provide many insights into the development of a regional curve and could aid in stream restoration efforts in the future.