SEDIMENT AS MODULATOR OF PHOSPHORUS DYNAMICS IN POST-GLACIAL CHANNELS

Much of the once-glaciated Upper-Midwestern United States is characterized by high agricultural land use and heavy erosion – resulting in interactions between agricultural runoff and large sediment inputs in these systems. In order to improve our understanding of the dynamics of in-stream processing of phosphorus, a nutrient with a high affinity for sorption to sediment, we examine sediment-P interactions across a gradient from flat agricultural uplands to highly incised lower valleys in the Le Sueur River, a tributary to the Minnesota River, and present preliminary results of a phosphorus-sediment budget for the Le Sueur River Basin.

The Minnesota River valley formed over the course of multiple drainage events of glacial Lake Agassiz between 11,500 and 8,200 radiocarbon years before present, incising up to 70 meters through an otherwise relatively planar landscape comprised largely of glacial till. As occurs in many post-glacial channels, tributaries to the Minnesota River are still making a substantial base-level adjustment, incising deeply and leaving behind massive bluffs which are highly vulnerable to erosion. This landscape, which is geologically primed for erosion, is also predominantly in agricultural land use, and subsequently also has strongly altered land cover and hydrology. Row crop agriculture, the dominant land cover in this basin, requires significant application of manure and fertilizers, resulting in major inputs of nutrients including phosphorus.

In incising agricultural rivers like the Le Sueur River, fresh glacial till exposed by river incision interacts with dissolved phosphorus runoff from agriculture, potentially resulting in the accumulation of legacy phosphorus stores downstream. By examining the phosphorus content and sorption characteristics of sediments from distinct sources ranging from agricultural upland sediment to stream banks to glacial till bluffs, we can begin to tease apart the role of sediment in driving basin-scale phosphorus behavior, an important step in optimizing mitigation strategies that prevent both erosion and nutrient transport in post glacial channels.