Can tributary in-flows improve the recovery of the dissolved organic carbon regime in a snowmelt river regulated by a large reservoir?

Although tributary inputs can accelerate the recovery of many physical and chemical gradients below large reservoirs, their contribution to the dissolved organic carbon (DOC) regime in regulated rivers remains poorly studied. In some regulated tributaries, flow volumes can be manipulated, potentially influencing DOC supply to the main stem. The present study examines how tributary water diversion affects DOC supply to a snowmelt river regulated by large reservoirs. DOC concentration was measured at tributary and main stem sites, and tributary DOC export was estimated under different tributary flow-diversion scenarios. Significant, positive correlations between DOC concentration and discharge were absent directly below the dam, but were present in the unregulated tributary, and re-emerged below the tributary confluence. Irrespective of water-diversion practices, tributary in-flows reconnected the regulated main stem to a more variable DOC regime driven by catchment flushing processes. However, tributary water diversion dampened the tributary signal by reducing DOC pulse frequency and total DOC export to the regulated river. These aspects of the DOC regime may influence basal resource availability and ecosystem functioning in the regulated main stem. The present study illustrates how an ecologically valuable tributary function can be addressed and quantified to guide the management and rehabilitation of a regulated river system