Assessing the effects of land use on water turbidity in a fluvial lake floodplain using Sentinel-2 imagery

Lake Saint-Pierre (LSP), the largest fluvial lake in the St-Lawrence River system, is a vital freshwater floodplain with rich biodiversity. However, expanding agricultural activities have disrupted the landscape, increasing turbidity and threatening the ecosystem. In response, the Quebec government launched the LSP strategic research cluster in 2018 to encourage sustainable land and water management. This study examines how different land-use types affected turbidity during the spring floods of 2019, 2020, and 2022, using Sentinel-2 satellite imagery. Land types included conventional and improved agriculture, cultivated and natural grasslands, and flooded forests. A new empirical model for flooded forests showed strong accuracy (adjusted R2 of 0.88, RMSE (root mean square error) of 10.91 FNU (Formazin Nephelometric Unit)). Combined with an existing model for open water, turbidity maps were created across all land types. Using a linear mixed model, we found that conventional and improved practices increased turbidity in the LSP floodplain by up to 600% compared with natural forests. Grasslands also contributed to higher turbidity, though to a lesser extent. The findings underscore that even improved practices cannot fully mitigate turbidity. Effective control requires integrated watershed management, including upstream inputs and targeted best practices to protect the LSP floodplain.