Riverine Freshwater Connectivity Among Major Tributaries in a Large Estuary

Riverine freshwater plays a central role in estuarine systems by regulating estuarine circulation, stratification, and the transport of waterborne materials. While much attention has been given to exchange between individual tributaries and the main stem, system-wide freshwater connectivity among multiple tributaries remains understudied. Here, we quantify inter-tributary freshwater connectivity in Chesapeake Bay using a calibrated three-dimensional hydrodynamic model and two quantitative metrics: the Percentage Metric, which describes the spatial distribution of freshwater from a supplying tributary across the system, and the Contribution Metric, which quantifies its influence on water composition in receiving tributaries. Model results show that most freshwater constituents are retained within their source tributary and the main stem, while inter-tributary exchange is generally low but non-negligible (<0.01% to 6.39%), yet contributions to water composition in some receiving tributaries can approach 50%. We further show that the Contribution Metric correlates strongly with river discharge, while the Percentage Metric is influenced more by tributary volume and location. Intrusion lengths of freshwater constituents closely matched those of salinity, reflecting shared transport pathways governed by estuarine circulation and mixing. To evaluate the role of mixing, we applied mixing diagrams of freshwater constituent concentration versus salinity, which revealed how incomplete mixing near confluences creates spatial variability in freshwater–salinity relationships that stabilize only after sufficient transport. This study highlights the importance of viewing estuaries as hydrodynamically connected networks rather than isolated tributaries and provides a transferable framework for quantifying freshwater connectivity and its implications for water resource and quality management.