Flow-Dependent Color Patches in a Great Plains River

Ecosystem structure and its heterogeneity shape ecosystem processes. Ecosystem heterogeneity has been characterized in smaller stream ecosystems dominated by benthic processes. However, in larger river ecosystems structured by water column characteristics including suspended sediment and phytoplankton, ecosystem heterogeneity has not been directly observed. We assessed flow-dependent ecosystem structure along 230 km of a large, highly managed Great Plains river (The Kansas River) by analyzing 1-dimensional, downstream color profiles across flow conditions derived from satellite imagery. River color is a robust metric that reflects the combined state of several important large-river habitat features, specifically suspended sediment, chromophoric dissolved organic matter, and phytoplankton. We found that at flows above a flow threshold that we call Q_patch (240 m³ s⁻¹), the entire river was uniformly yellow. At flows below Q_patch, the river was generally greener and often had patches of very green water that occurred upstream of run-of-river dams. Comparing color with in situ data showed the color patches were likely areas of elevated chlorophyll-a concentrations from phytoplankton accumulation, indicating that the patches reflected biological processes. Flows were below Q_patch on 77% of days during the period of record (1985–present), indicating that the ecosystem spends significant time in a patchy state. Our findings uniquely demonstrate that the water column characteristics structuring temperate, large-river ecosystems can be patchy.