EXPRESSION OF VARIATIONS IN WATER AND SEDIMENT DISCHARGE OF THE MAINSTEM MISSISSIPPI RIVER IN THE ZUMBRO RIVER TRIBUTARY, MN

Since the Last Glacial Maximum, ice-sheet-induced fluctuations in water & sediment supply to the Upper Mississippi River have caused it & its tributaries to aggrade & incise. These changes in river bed elevation are preserved in a suite of 1 to 7 terraces expressed in tributaries throughout the Upper Mississippi. These terrace surfaces formed as a result of a decrease in slope of the mainstem channel leading to incision in the main channel & creating a base level fall & knickpoint migration in tributaries. This migration led to the abandonment & preservation of the terrace surfaces seen in the tributaries today. The goal of this ongoing study is twofold: (1) to map terraces throughout the Upper Mississippi Valley and its tributary valleys, and (2) to determine what the expression of terraces in the tributaries & mainstem reveals about climate impacts on river long profile evolution. We mapped the terraces from statewide LiDAR data using the automated “LSD TopoTools” terrace extraction algorithm, & extracted the terrace elevations compared to the modern channels. We fit long profiles to the terrace surfaces along the Zumbro River in MN, a tributary of the main channel that was shaped by late Pleistocene incision & Holocene aggradation of the mainstem Mississippi. The terrace long profiles of the Zumbro River were then compared to those in the mainstem channel to identify any variations in the number of distinct terrace surfaces, long profile slopes, & terrace elevations above the channel, and to develop possible explanations for the differences or similarities between these. Grain size data from sediment samples of distinct terrace surfaces were used alongside slopes from the terrace long profiles to obtain an estimate of paleo-discharge. Changes in water & sediment conditions decreased the slope of the main channel, causing it to incise below its terrace surfaces, & caused base-level fall in the tributary valleys, where knickpoints continue to migrate upstream. This knickpoint migration is responsible for abandonment of the terrace surfaces in the tributaries, leaving behind a signature in the long profile. We use a combination of digital terrace mapping & sediment grain analysis to observe how changes in fluvial & climatic conditions along the mainstem Upper Mississippi are expressed & preserved in the terraces of tributaries.