Sponsor:

Speaker: Dr. Karen Gran

Organization: UMD Dept. Geological Sciences

Topic: Evolving Sediment Sources in the Minnesota River Basin: Implications for Resource Management

AIPG Luncheon Speaker - 3 April 2012 - Dr. Karen Gran - UMD Dept. Geological Sciences

Abstract:  Evolving Sediment Sources in the Minnesota River Basin: Implications for Resource Management
 
Management and restoration of sediment-impaired streams requires an understanding of sediment sources and pathways of transport. In the predominantly-agricultural Minnesota River basin, the primary sources of sediment are generally classified as field vs. near-channel sources, with near-channel sources including stream banks, bluffs, and ravines. Our research group has been exploring the relative importance of each source in the Le Sueur watershed and how these sources have changed through time, from changes evident over the past few decades to changes associated with valley evolution over the past 13,500 years.

The Minnesota River valley was carved ~13,500 years ago due through catastrophic drainage of glacial Lake Agassiz. As glacial River Warren incised, knickpoints were created on tributaries to the Minnesota River. These knickpoints have been migrating upstream since then, carving out deep valleys where many of the near-channel sediment sources are derived. Work on a modern sediment budget shows that the majority of the fine sediment load in the Le Sueur River comes from bluffs and other near-channel sources. Using a combination of aerial lidar analyses, terrace dating, and numerical modeling, we were able to determine a pre-settlement rate of erosion from near-channel sources, finding it to be 3-5 times lower than modern erosion rates.

Records from Lake Pepin downstream show a more dramatic 10-fold increase in deposition rates from pre-settlement times to the present. Sediment fingerprinting shows that pre-settlement loads were dominated by near-channel sediment sources. As deposition rates rose in the late 1800s and 1900s, the sources included an increasing percent coming from fields. In the last few decades, deposition rates have remained high, but sediment loads increasingly dominated by near-channel sources once again, with significant implications for turbidity management in the Minnesota River basin.

Speaker Bio
Dr. Karen Gran is an assistant professor of geological sciences at the University of Minnesota Duluth and a member of the National Center for Earth-surface Dynamics, an NSF-funded Science and Technology Center based at the St. Anthony Falls Laboratory at the University of Minnesota. She obtained her B.A. in geology from Carleton College in 1996, M.S. in geology from the University of Minnesota in 2000, and Ph.D. in geology from the University of Washington in 2005. Her research in fluvial geomorphology focuses on how riverine landscapes respond to disturbances, varying from widespread post-glacial incision and more recent land use change in the Minnesota River valley to large pyroclastic eruptions at Mount Pinatubo, Philippines.