Sources and transport of organic carbon in an Arizona river-reservoir system

Sources and transport of organic carbon were studied in two large, Sonoran desert watersheds (Salt and Verde rivers) and a two-reservoir system on the Verde River. Total organic carbon (TOC) concentrations in the unregulated rivers above the reservoirs did not follow a simple relationship with flow. TOC concentrations usually declined during spring runoff and reached low concentrations (1–3 mg L−1) by early summer. In most years, distinct peaks (10–30 mg L−1) occurred in late summer, coincident with small but abrupt increases in flow associated with the first monsoon rains. A two-reservoir mass balance showed that 72% of the particulate organic carbon (POC) input was retained, probably due largely to simple sedimentation. Production of dissolved organic carbon (DOC) within the reservoir was 41% of the inflow loading, even though the reservoirs accounted for only 0.14% of the watershed area. Reservoir DOC production comprised a large fraction of total watershed production because upstream DOC production was extremely low (0.2 g C m−2 yr−1), the reservoirs were moderately productive, and water residence times were fairly long during most of the year. We postulate that reservoirs are major contributors to total watershed DOC production in arid regions.