Quantifying Greenhouse Gas Emissions From the Upper Mississippi and Illinois Waterway System Navigation Pools

ABSTRACT The upper Mississippi and Illinois rivers were impounded for the facilitation of commercial river navigation in the 1930s using a series of low‐head dams and navigation pools which receive elevated nutrient loads. The impacts of impoundment and nutrients on greenhouse gas (GHG) emissions have not been fully quantified at navigation pool to waterway scales along these rivers. This study seeks to fill this knowledge gap using the GHG reservoir tool which quantifies emissions from a reservoir at both the annual and design‐lifetime scales (100 years). This tool also distinguishes between emissions attributable to impoundment versus elevated nutrient loads. The navigation pools' total GHG emissions over the 100‐year assessment period is estimated to be 80.0 (million metric tons of CO 2 equivalents [MT CO 2 e]) with 60% of the emissions ascribed to impoundment and the remaining 40% credited to elevated nutrient loads. The modeling also indicated the emissions from the navigation pools peaked in 1941 at 2.4 MT CO 2 e year −1 , which was equivalent to 9% of the estimated global emissions from reservoirs in this year, and since then emissions have decreased reaching a post‐impoundment baseline of 0.7 MT CO 2 year −1 by 2040. This new GHG emission baseline reflects conditions with the navigation pools remaining in place and is not the same as pre‐impounded conditions with forested floodplain which was estimated to be a net sink of 0.2 M MT CO 2 e year −1 . These findings indicate the navigation pools are likely to continue to be a substantial GHG emission source in the future.