Streams derive base flow from groundwater but are less frequently connected to the uplands that dominate catchments. This disconnection is profound in semi-arid systems where full connection to the drainage network is infrequent. While uncommon, these connections are critical for supplying the carbon and nitrogen that drive stream biogeochemistry. Here we discuss a developing conceptual model of river scale catchment biogeochemistry. An important element of biogeochemical linkages in semi-arid river systems is that dissolved constituents are more reactive and thus uplands, given their pulsed flow and infiltration hydrology, contribute little to the river system scale. Suspended and particulate loads travel greater distances and contribute large fluxes during flood flows to the river system. While some fraction of this suspended load is transported through a system, a significant percentage remains in the stream and near stream environment following floods. This sediment load represents a key resource for biogeochemical processing and nutrient availability as the stream and near stream environments are subjected to continual interaction due to stream-groundwater exchange and diurnal ET pumping of the stream bank environment. These nutrients sources are key for the sustainability of surface waters in arid and semi-arid rivers.
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