Accurate measurement of TP concentrations in rivers is needed to quantify loads and evaluate the progress of nutrient reduction strategies. We evaluated the relation of water quality surrogates, turbidity, ortho-phosphorus (OP), chlorophyll a, chloride and discharge to TP concentrations at 43 different river monitoring sites over a 15-year period. New hydrological insights for the region: TP concentrations were highly correlated to turbidity (0.78 ± 0.20) and OP (0.69 ± 0.13) across all sites and less correlated to chlorophyll a (0.07 ± 0.15), chloride (−0.10 ± 0.24) and discharge (0.41 ± 0.23). When the regression models included OP as a variable, the mean r2 for all 43 sites was 0.90 ± 0.08 and ten of the 43 sites had r2 values greater than 0.95. When OP was excluded in the regression model, the overall mean r2 values decreased to 0.72 ± 0.14 and for six of the river sites, the r2 value decreased by 50%. Other variables (discharge, chlorophyll a, chloride) were included in the regression equations on a case-by-case basis. Including OP in the regression models was critically important for rivers draining the tile-drained Des Moines Lobe region.