Phosphorus pollution in Illinois watersheds is a growing concern, threatening water quality, aquatic ecosystems, and human health. Excess phosphorus acts as a limiting nutrient in freshwater systems, causing eutrophication, algal blooms, hypoxia, and fish mortality. Illinois watersheds also contribute to nutrient loading in the Gulf of Mexico, worsening hypoxic conditions. This study aims to analyze phosphorus load trends and identify key drivers, including watershed characteristics, climate variables, hydrology, land use, soil composition, and anthropogenic influences. Historical water quality data will be assessed using non-parametric tests such as Mann–Kendall and Sen’s slope to detect trends, followed by machine learning to evaluate factor importance. We hypothesize that watersheds with high precipitation, urban runoff, and intensive agriculture will show increasing phosphorus loads. Findings will inform targeted strategies to mitigate pollution and protect ecological health.