Maria Crawford successfully defended her M.S. in Geology this morning, Congrats Maria! Her project focused on small streams like the one pictured below (photo from Carlson Recreation Area in Webster County, IA), whose beds are littered with large cobbles and boulders that are likely never transported by the streams that flow over them. She specifically targeted streams whose valleys are carved entirely into Wisconsinan tills of Iowa, and whose streambeds are therefore composed entirely of particles eroded out of the till.
Maria sought to find out whether the concentration of coarse particles on the streambed (sometimes called a "lag deposit") could be predicted by knowing the particle-size distribution of the till and the configuration of the valleys and their catchments. Since till tends to exhibit fractal particle-size distribution (which she confirmed with a bulk sample taken from a nearby borrow pit), she was able to estimate the number of particles of any given size class that would be contained in a volume of till eroded from a given section of valley. When combined with estimates of bankfull streamflow characteristics and a model of selective particle entrainment from mixed-sized streambeds, she predicted that these streams would have bed particle sizes that would usually coarsen as you move down-valley. Observations from her study valleys showed that this trend was only observed in the uppermost reaches, but she had some interesting ideas about why the model and observations diverged further downstream. Stay tuned for more details, to be presented in her thesis soon!
Congrats again, Maria, and thanks for your great work with AGL!
