Remote Sensing of Visible Dye Concentrations During a Tracer Experiment on a Large, Turbid River

Understanding dispersion in rivers is critical for numerous applications, such as characterizing larval drift for endangered fish species and responding to spills of hazardous materials. Injecting a visible dye into the river can yield insight on dispersion processes, but conventional field instrumentation yields limited data on variations in dye concentration over time at a few, fixed points. Remote sensing can provide more detailed, spatially distributed information on the dye's motion, but this approach has only been tested in clear‐flowing streams. The purpose of this study was to assess the potential of remote sensing to facilitate tracer studies in more turbid rivers. To pursue this objective, we injected Rhodamine WT dye into the Missouri River and collected field spectra from a boat, videos from a small unoccupied aircraft system (sUAS), and orthophotos from an airplane. Applying an optimal band ratio analysis (OBRA) algorithm to the field spectra revealed strong correlations (R2 = 0.936) between a spectrally based quantity and in situ concentration measurements. OBRA also performed well for broadband RGB (red, green, blue) images extracted from the sUAS‐based videos; the resulting concentration maps were used to produce animations that captured movement of the dye pulse. Spectral mixture analysis of repeat orthophoto coverage yielded relative concentration estimates that provided a synoptic perspective on dispersion of the dye throughout the entire 13.8 km reach over the full 2.5‐hr duration of the experiment. The results of this study demonstrate the potential to remotely sense tracer dye concentrations in large, highly turbid rivers. Plain Language Summary The movement of organisms and contaminants through a river system is influenced by flow patterns within the channel. Understanding the dispersion processes that create these patterns can facilitate a number of important applications, including predicting travel times for spills of toxic materials and simulating larval drift for endangered species of fish. An effective and intuitive means of examining dispersion are to conduct a tracer experiment by injecting a visible dye into a river and then measuring changes in dye concentration over time. Typically, these data are collected by sensors deployed at a few, fixed monitoring locations, but this approach yields only limited insight. Remote sensing methods, in contrast, can provide more detailed, spatially distributed information on the dye's mov