Abstract Coastal ecosystems are highly influenced by riverine freshwater inputs, but the nature of these influences is dynamic and hard to characterize. Here we present a method that leverages remote sensing data of river plumes to quantify land–sea connectivity. The method uses least‐cost routing across image‐derived cost surfaces, where connected paths have a directional water quality gradient characteristic of a fluvial plume. The central question this metric helps to answer is: What is the relative influence of different watersheds on a given point in the coastal zone? We demonstrate our approach with Sentinel‐3 OLCI satellite data to identify rivers originating large turbidity plumes along the California coast, then show that seasonal and inter‐annual connectivity patterns align with discharge time series. We anticipate applications of the proposed method will lead to improved understanding of linkages between the terrestrial and marine environment, which is needed to guide coastal management, conservation, and restoration.