This data release contains data in support of "Diverging trends in Nitrate and Phosphorus Loads and Yields Across Illinois Watersheds, 1997–2022" (Kamrath and others, 2025). It contains input and output data used to estimate total nitrate + nitrite, dissolved phosphorous and total phosphorous loads for sites in Illinois from water years 1974 to 2022. The input data includes "input-data_WQ.csv," which contains water quality data for nitrate + nitrite, dissolved phosphorous and total phosphorous. The water quality data comes from (1) the Water Quality Portal (WQP), (2) the Illinois Environmental Protection Agency (IEPA), and (3) the Environmental Protection Agency (EPA) STORET warehouse. The WQP includes the most complete record of U.S. Geological Survey and IEPA water quality samples. The most recent IEPA water quality samples, which are not yet in the WQP, were provided directly from IEPA. Legacy IEPA data that is housed via the EPA STORET warehouse was used to fill in any gaps in the data record. See the Entity and Attribute section of the metadata for details. The output data provides results calculated using the water-quality method Weighted Regressions on Time, Discharge, and Season (WRTDS) from the R package 'EGRET' (Exploration and Graphics for RivEr Trends, available from https://doi-usgs.github.io/EGRET/). The file "model-runs.csv" includes a list of all the models run using WRTDS with additional information for each run; for each run, WRTDS stationary and generalized flow normalization (SFN and GFN, respectively) were used along with the WRTDS Kalman (WRTDS-K) post processing. The results of each WRTDS run are saved in 'eList,' the native output format from 'EGRET,' in "eLists.zip." There are two eLists per run, one with SFN results and the other with GFN and WRTDS-K results. For more information on 'EGRET' and the WRTDS method see https://doi-usgs.github.io/EGRET/index.html. Additional output data includes estimated annual and monthly loads and concentrations of nitrate + nitrite, dissolved phosphorous and total phosphorous for all sites. The files are broken up by constituent, nitrate + nitrite (N) or phosphorous (P), and time frame, annual or monthly.