ARMORED TRIBUTARIES: INFLUENCE OF TILL TEXTURE ON POSTGLACIAL DRAINAGE INTEGRATION

As streams adjust to the local base level fall that accompanied the end of the last glacial period, many have carved steep-sided valleys into the surrounding till-covered landscape. The tills often include particles far too coarse for small streams to transport, so they accumulate along the streambed while finer sediments are removed around them. The accumulation of immobile particles forms a static armor that is characteristic of small semi-alluvial streams incising into cohesive till. Past studies in southern Minnesota suggest that armoring may have been partly responsible for delaying response to late glacial incision of the Minnesota River valley. These armored streams differ from drainage systems in older glaciated landscapes by their downstream-coarsening bed particle sizes and extensive knickzones. These traits suggest incomplete adjustment, but their connection to till particle size distribution and armor formation is uncertain.

Observations across 5 field sites in central Iowa valleys revealed many of the same characteristics. Sites were selected based on simple adjacent till stratigraphy, the presence of streambed armor, and drainage area size ranging from 1.6 km² to 50.5 km². One of the five field sites is located within the Middle Wisconsinan Sheldon Creek surface, while the remaining four sites are distributed across the Late Wisconsinan Des Moines Lobe region. Field measurements included channel surveys and streambed pebble counts. Additionally, a large (0.25 m³) till sample was collected for particle size analysis using a combination of particle-size analysis methods including sieve analysis and laser diffraction using a Malvern 3000 Mastersizer. Preliminary stream profile, valley, and armor evolution have been modeled in MATLAB using site data. A key research objective is to test the hypothesis that stream long profiles and bed particle-size distributions are interdependent and can be predicted with valley erosion models given a till particle size distribution. This study builds on prior research to improve our understanding of rate-limiting factors in drainage network evolution in low-relief, postglacial landscapes.