Sponsor:

Speaker: Karen Jackson

Organization: Macalester College

Topic: Bed Sediment Grain size Distribution and Flow Dynamics- Indianhead Reservoir St. Croix River

Invited Luncheon Speaker - May 3, 2011 - Karen Jackson - Macalester College.

Bed Sediment Grain Size Distribution and Flow Dynamics of Indianhead Reservoir, St. Croix River, MN/WI

Abstract 
Dams alter sediment transport and flow dynamics in rivers by acting as a physical barrier to the downstream movement of bed sediment, decreasing water velocity, and allowing suspended material to settle. The St. Croix River, a National Scenic Riverway located along the border of Minnesota and Wisconsin, is subject to such alterations. Interstate Park, located several kilometers below the St. Croix Falls Dam, is host to a large population of native mussels, including threatened and endangered species. Over the past 20 years Hornbach and others (2009) have documented a ~90% decrease in the juvenile mussel population at this site, concurrent with a gradual decrease in river bed grain size, from mixed sand/gravel to sand. One hypothesis is that the Indianhead Reservoir above the dam may be a significant source of the fine sediment found at Interstate Park as a result of extensive reservoir infilling. In this study we focus on characterizing spatial and temporal variability in hydrology and sediment transport in the Indianhead Reservoir, and the impoundment behind the St. Croix Falls Dam. Our objective is to determine if the sediment at Interstate Park could originate from the river upstream of the dam.

To quantify grain size distribution at the bed, measure suspended and bedload sediment in transport across a range of water discharges, and characterize flow dynamics of Indianhead Reservoir, samples were collected across a 2 km stretch of the reservoir just above the dam in 2009 and 2010. Grab samples and gravity cores of bed sediment, suspended sediment concentration (SSC) samples, vertical water velocity profiles using an Acoustic Doppler Current Profiler (ADCP), and high resolution bathymetry readings were collected at 3 points along 13 transects. Bathymetric data show depths ranging from 2-15 m, confirming significant infilling in the last 40 years. Vertical velocity profiles reveal flow velocities between 0 and 32 cm/s during the summer low flows. Bed sediments fine from coarse sand to silt and clay closer to the dam, however, reservoir cores spanning the last century reveal stochastic deposition of sand and woody debris within several hundred meters of the dam. As water discharge increases, calculated shear stresses increase, thus increasing the likelihood of sand transport. One site under moderate water discharge (8,050 cfs) exhibits basal shear stress measurements similar to fluvial systems (15 dynes/cm2). However, at low discharges (3,100 cfs) ADCP profiles show little vertical variation in water velocity. During spring or flood stages (~25,000 cfs), the larger shear stresses would allow the sand grain size fraction to likely reach and breach the dam. Results demonstrate sand is present in the reservoir close to the dam. Calculations of settling velocities for available bed sediment, and estimates of residence time for water in the reservoir will further illuminate our understanding of sand transport behind the dam. Additional electron microscopy to compare reservoir and Interstate Park bed sediment will improve our understanding of possible sediment transport from the reservoir to mussel beds downstream.

Contributors in this study were Kelly R. MacGregor and Daniel J. Hornbach - Macalester College, St. Paul
Geology and Biology Departments

 
Bio
Karen Jackson is a senior at Macalester College. In addition to taking geology courses at Macalester, Karen spent a term aboard a scientific research vessel. Karen conducted an independent project comparing carbon dioxide outgassing at the Equatorial Undercurrent to the Intertropical Convergence Zone using carbonate chemistry. Learning to sail an 134 foot brigantine was part of the program. As geology major, Karen has been very interested in the Earth’s surface processes. More specifically, Karen is interested in the relationship between hydrologic processes and the biosphere, and geological and environmental factors intrigue her academically; however, Karen's professional interests are focused on human impacts on the environment. Karen hopes to continue her education with a Masters and Ph.D. in the geomorphology field.