<p>Groundwater is a critical component of the terrestrial water cycle, yet the distance and depth of its connections with streamflow remain unquantified at large scale. Here we conducted a backward-particle-tracking simulation across the continental United States. We quantified the lateral length and vertical depth of groundwater flow discharged to streams as baseflow. Our simulation results suggest that water may travel longer distances underground before emerging in a stream than previously thought, and that deep groundwater sourced from consolidated sediments, aquifers typically 10–100 m below the ground surface, contribute more than half of the baseflow in 56% of the subbasins. Water-balance approaches may underestimate inter-basin groundwater flow due to concurrent groundwater exportation and importation of a watershed. Unexpectedly stronger connections of streamflow with deep and inter-basin groundwater flow paths found here have important implications for watershed resilience to climate change and persistence of contamination.</p>