In semi-arid regions, the growth and survival of cottonwoods (riparian Populus species) depend on river water supplementing the limited precipitation. Indicators of growth and productivity are needed to assess how altered streamflow regimes on regulated rivers impact cottonwood trees and the riparian forest ecosystems they support. Satellite imagery from the Landsat program was used to make historical assessments of ecosystem productivity in a riparian cottonwood forest along a regulated prairie river in southern Alberta, Canada from 1984 to 2020, with an environmental flow regime that increased the minimum flows implemented in 1993. A version of the near-infrared reflectance of vegetation scaled with incoming sunlight (NIRvP) was calculated from Landsat images to  provide a proxy for primary production. NIRvP was validated against gross primary production measurements from eddy covariance and cottonwood basal area increment measurements from tree ring analyses. Streamflow and weather data were used to assess what  environmental conditions drive year-to-year variations in NIRvP.