Groundwater's Fingerprint in Stream Network Branching Angles

Branching river networks are prominent features of the Earth's surface, but the mechanisms that create branching river networks patterns remain elusive. Recent studies have suggested that climate, tectonics, and lithology may control both longitudinal profiles of channel incision and the planform geometry of stream networks. Here we show, by analyzing almost 1 million river junctions and over 4.2 million groundwater wells across the contiguous United States, that stream network branching angles vary systematically with the degree to which streams lose water to, or gain water from, nearby groundwater aquifers. Streams whose surfaces lie above nearby groundwater levels, and thus are likely to be losing flow to underlying aquifers, tend to have narrower branching angles than streams that lie below nearby groundwater levels, and thus are likely to gain flow from groundwater. This systematic relationship persists across several stream orders, and across a wide range in channel gradients. River networks are a striking reflection of the processes that shape Earth's surface, but it is still unclear what mechanisms created these distinctive geometric signatures. Branching angles of river networks tend to be narrower in regions with dry climate and wider in regions with humid climates, but the mechanisms behind this relationship remain unclear. By combining new continental data sets of groundwater well levels, river water levels, and river junction angle across the United States, we reveal the first continental-scale observational evidence on the potential contribution of groundwater to shaping river network branching angles, and thus to the development of large-scale drainage patterns. Stream network branching angles vary systematically with the degree to which streams lose water to, or gain water from, nearby aquifersStream branching angles correlate more strongly with streams' losing/gaining status than with other controls on stream network geometryThese continental-scale observations highlight the potential contribution of groundwater to the development of drainage patterns