128-10 Investigating the Effects of Human Alteration on Floodplain Carbon Storage in the Southwest and Midwest U.S.

Abstract:

River corridors, which include channels, floodplains, and the hyporheic zone, are key pipelines for carbon (C) transport from the landscape to the ocean. Previous research has shown that floodplain organic carbon (OC) is primarily stored in riparian vegetation, downed large wood, and soil. Across the U.S., floodplain ecosystem services and functions have been affected by human modifications, including dams, water withdrawals, agriculture, and land development. Current understanding of floodplain OC storage is limited, as few existing studies quantify OC stocks in all 3 sources, assess the impacts of alteration, or estimate OC across large spatial scales. To quantify OC stocks and understand how they have been affected by human alterations in the Southwest and Midwest U.S., two regions where floodplain OC has not been comprehensively studied, we combined geospatial analyses and field sampling techniques to assess OC stocks along the San Pedro River, AZ and the Rum River, MN.

For each floodplain, we classified high-resolution ecogeomorphic units based on vegetation, hydrology, and morphology using publicly available datasets and ArcGIS Pro. We used additional geospatial analyses to categorize reaches as altered or unaltered based on land cover, levees, and water withdrawal datasets. We collected soil cores, alluvial sediment depths, vegetation surveys, and downed wood surveys across ecogeomorphic units and alteration levels. To estimate wood volume, we measured wood jams, accumulations, downed wood, and porosity using the census method at each site. We collected sediment cores and analyzed them for grain size, OC content, soil moisture, and mineral-associated and particulate OC fractions. We measured tree dimensions within plots to calculate aboveground biomass from allometric relationships. We used these field data to calculate OC stocks at 36 locations within each floodplain. We then used our OC stock measurements to estimate biomass, downed wood, and soil OC stocks at the floodplain scale based on alteration and ecogeomorphic unit classifications. We compare our floodplain-scale OC stocks to existing large-scale estimations of OC, derived from the TreeMap and SSURGO databases. Preliminary results indicate that floodplain OC stocks differ by climate zone, and some stocks differ by level of alteration and ecogeomorphic unit. This work provides a replicable workflow that can be used to scale-up floodplain OC stock estimates over broad geographic extents. 

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