Impacts of Streambed Dynamics on Nutrient and Fine Sediment Transport in Mountain Rivers

Authors

Elowyn Yager¹ (eyager@uidaho.edu), Nicole Hucke¹, Rachel Watts², Andrew Tranmer¹, Janice Brahney², Joel Rowland³*, George Perkins³, Rose Harris³

Institutions

¹University of Idaho–Boise, ID; ²Utah State University–Logan, UT; ³Los Alamos National Laboratory, Los Alamos, NM

Abstract

In mountainous watersheds, rivers typically have an armor layer of coarse sediment that protects the finer subsurface from erosion. In theory, armor layer motion during high magnitude flows could release the subsurface fine sediments that are often enriched in phosphorus and particulate organic carbon (POC). Hysteresis in POC, soluble reactive phosphorus (SRP), particulate phosphorus (PP), total phosphorus (TP), and suspended sediment (SS) may therefore be partly controlled by armor layer motion. In addition, streambed concentrations of these constituents may depend on whether a reach is losing or gaining. The project tests whether armor layer motion and streambed concentrations influence hysteresis patterns during summer monsoon flows in one gaining and one losing reach of La Jara Creek in Valles Caldera National Preserve, NM. Researchers measure the amount and timing of armor layer motion, streambed and river concentrations of POC, PP, SRP, TP, and fine sediment, as well as surface flow and groundwater exchange in these two reaches. In addition, the team conducted artificial floods that isolated the effects of armor layer removal on nutrient and fine sediment concentrations in the water column. Hysteresis in SS, TP, and PP occurred during these artificial floods, demonstrating the ability of the armor layer to control hysteresis because all other potential sources of these constituents were eliminated. Equilibrium experiments suggested that streambed sediments are a potential source of SRP to the water column, which was observed in natural flow events. Preliminary results also demonstrate that SS, turbidity, PP, TP, and POC often follow the same hysteresis pattern in a given natural flow event, implying that they may have a similar source. Hysteresis of these constituents changes between clockwise and counterclockwise in different natural flow events and may be related to the amount or timing of armor layer breakup or the streambed concentrations, which the team is currently investigating. The final results of this work will determine how perturbations, such as the sequence and magnitude of droughts and floods, constrain biogeochemical nutrient cycling and impact subsequent temporal variations in nutrient and fine sediment export from mountainous watersheds.