Seminal research conducted in the temperate city of Baltimore, MD, showed that urban stream channel incision is associated with lowered riparian water tables and drier riparian soils, leading to decreased denitrification rates and more upland tree species in riparian forests. This phenomenon, termed “urban riparian hydrologic drought,” is now considered a general impact of urbanization. However, in precipitation-limited systems, urban water use and wastewater can also dramatically increase stream flows in historically intermittent streams, potentially creating an opposing trend of increased moisture in urban riparian zones. We asked whether channel incision interacted with increased dry-season stream flow to influence riparian vegetation and soils along small streams in Sacramento, CA. We hypothesized that incision would limit the impact of increased dry-season flow. In Sacramento, which has a long summer dry season, highly localized urban water inputs and heterogeneous land management histories have created a mosaic of intermittent and perennial stream reaches across a gradient of channel incision. We chose stream reaches with contrasting dry-season flow conditions across a range of channel depths to investigate riparian woody plant communities (66 sites), water stress in riparian trees (n = 96), and riparian soil and understory vegetation characteristics (40 sites).
Results/Conclusions
Similar to patterns in Baltimore, we found that riparian zones along deeply incised channels supported primarily upland trees, while wetland species were more common along streams without incision. However, seedlings of wetland species were found only on the banks of flowing streams, indicating that urban water subsidies may be key for the establishment of wetland trees along these small streams. We also found that the presence of dry-season stream flow could substantially reduce midday water stress in two focal riparian tree species, but only at low levels of channel incision. Similarly, dry-season stream flow was associated with increased riparian soil moisture, soil organic matter, and perennial vegetation cover only in the absence of incision. Our work shows that in a precipitation-limited city, dry-season water subsidies can increase moisture in riparian zones, potentially enhancing riparian ecosystem services such as primary production, carbon storage, and nutrient cycling. However, channel incision limits the influence of dry-season flows and may create persistently drier conditions that shape riparian forest composition. Urban management and design practices that influence the prevalence of dry-season flows, as well as the magnitude of storm flows that cause incision, can thus substantially affect riparian ecosystem function, with consequences for downstream ecosystems.