Judy's Branch watershed, a small basin (8.64 square miles) in the St. Louis Metro East region in Illinois, was selected as a pilot site to determine suspended-sediment yields and stream-channel processes in the bluffs and American Bottoms (expansive low-lying valley floor in the region). Suspended-sediment and stream-chan-nel data collected and analyzed for Judy's Branch watershed are presented in this report to establish a baseline of data for water-resource managers to evaluate future stream rehabilitation and manage-ment alternatives. The sediment yield analysis determines the amount of sediment being delivered from the watershed and two subwatersheds: an urban tributary and an undeveloped headwater (pri-marily agricultural). The analysis of the subwater-sheds is used to compare the effects of urbanization on sediment yield to the river. The stream-channel contribution to sediment yield was determined by evaluation of the stream-channel processes operat-ing on the streambed and banks of Judy's Branch watershed. Bank stability was related to hydrologic events, bank stratigraphy, and channel geometry through model development and simulation.
The average suspended-sediment yield from two upland subwatersheds (drainage areas of 0.23 and 0.40 sq.mi. was 1,163 tons per square mile per year (tons/sq.mi.-year) between July 2000 and June 2004. The suspended-sediment yield at the Route 157 station was 2,523 tons/sq.mi.-year, near the outlet of Judy's Branch watershed (drainage area = 8.33 sq.mi.). This is approximately 1,360 tons/sq.mi.-year greater than the average at the upland stations for the same time period. This result is unexpected in that, generally, the suspended-sediment yield decreases as the watershed area increases because of sediment stored in the channel and flood plain. The difference indicates a possible increase in yield from a source, such as bank retreat, and supports the concept that land-use changes increase stream-flows that may in turn result in higher rates of bank retreat. Utilizing both bank-rod data and resurveyed cross-section data, it was determined that approxi-mately half of the suspended- sediment yield at Route 157 during July 2000-June 2004 came from bank retreat.
Given that bank retreat can be a substantial portion of the sediment yield, understanding bank stability processes is important. Bank stability can be assessed mathematically by computing the factor of safety, which is defined by the ratio of the shear strength (resisting force) along the failure surface and the shear stress (driving gravitational force). Once the factor of safety falls below one, the bank theoretically becomes unstable. Bank-stability conditions were related to hydrologic events, bank type, and channel geometry through model develop-ment and simulation. The most common type of bank in the watershed consists of cohesive alluvial soil deposits overlying a stiff glacial till. A stabil-ity chart for different bank types was developed using a bank-stability analysis. Banks steeper than 70 degrees and higher than from 10 to 11.5 feet (depending on bank type) become at risk for mass failure in the watershed under conditions that pro-mote saturation of the bank and a sudden drop in the river level.
Additional publication details
USGS Numbered Series
Suspended-sediment yields and stream-channel processes on Judy's Branch watershed in the St. Louis Metro East region in Illinois