While riparian zones have been shown to be important denitrification 'sinks' for nitrate in agricultural watersheds, there is considerable uncertainty about, and variability in, their performance in forested and urban/suburban watersheds. In forests, variability is controlled by hydrologic controls on flow paths from uplands to streams, microtopographic and seasonal controls on biological activity, and by the low levels of nitrate common in forested watersheds. In urban and suburban watersheds, impervious surface and stormwater infrastructure increase certain hydrologic fluxes (e.g, runoff), decrease others (e.g., infiltration, evapotranspiration) and alter stream channels and riparian zones. Urban stream channels are often highly incised, which in combination with reduced infiltration in impervious uplands, can reduce riparian groundwater levels. The reduction in groundwater levels leads to 'hydrologic drought' in urban riparian zones that causes changes in soil, vegetation and biogeochemical processes, leading to a reduction in denitrification. We suggest that the key to understanding variation in riparian zone denitrification in both forested and urban/suburban watersheds is to focus on connections between the riparian zone and the uplands, the stream and the subsurface. Flowpath analysis can be used to determine just how much of the water moving from uplands passes through riparian zones. Recent research demonstrates that riparian zones and streams are strongly connected, with an active hydrobiogeochemical ‘edge' at the riparian/stream interface. Conditions at this interface are strongly influenced by different sources of carbon in the subsurface, including buried horizons and deep roots. Understanding connections to the stream is also critical to efforts to restore riparian functions in urban watersheds.