Carbon export by rivers draining the conterminous United States

Material exports by rivers, particularly carbon exports, provide insight to basin geology, weathering, and ecological processes within the basin. Accurate accounting of those exports is valuable to understanding present, past, and projected basin-wide changes in those processes. We calculated lateral export of inorganic and organic carbon (IC and OC) from rivers draining the conterminous United States using stream gaging and water quality data from more than 100 rivers. Approximately 90% of land area and 80% of water export were included, which enabled a continental-scale estimate using minor extrapolation. Total carbon export was 41–49 Tg C yr⁻¹. IC was >75% of export and exceeded OC export in every region except the southeastern Atlantic seaboard. The 10 largest rivers, by discharge, accounted for 66% of water export and carried 74 and 62% of IC and OC export, respectively. Watershed carbon yield for the conterminous United States was 4.2 and 1.3 g C m⁻² yr⁻¹ for IC and OC, respectively. The dominance of IC export was unexpected but is consistent with geologic models suggesting high weathering rates in the continental United States due to the prevalence of easily weathered sedimentary rock.