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EPA in Arkansas

Water in Arkansas

Illinois River Watershed

The Illinois River is a multi jurisdictional tributary of the Arkansas River, approximately 100 miles (160) km) long, between the States of Arkansas and Oklahoma. The Illinois River begins in the Ozark Mountains in the northwest corner of Arkansas (Washington County) and flows west into northeast Oklahoma. Once the Illinois River enters Oklahoma, it then flows southwest and south through the mountains of eastern Oklahoma into Tenkiller Ferry Lake. The Illinois River in Oklahoma is currently on the 303(d) list for total phosphorus, while tributaries to the Illinois River (e.g. Osage Creek, Muddy Fork, and Spring Creek) in Arkansas are also on the 303(d) list for total phosphorus.


Project Updates

Current Phase – Watershed and Lake Calibration/Validation Modeling

The calibration/validation modeling is complete for both Illinois River and Lake Tenkiller. The purpose of the calibration/validation modeling is to ensure a scientifically sound modeling system is developed to represent the entire Illinois River Watershed. Calibration modeling was performed using data obtained from the time period of 2001 to 2009, whereas data obtained from the 1992 to 2000 period of record was used for validation purposes. Calibration and validation of the Illinois River Watershed models was achieved by considering qualitative and quantitative measures, involving both graphical comparisons and statistical tests.

Events and Activities 

Modeling complete September 2015

Technical Memorandum-Procedures and Methodologies for the Sensitivity and Uncertainty Analyses for Illinois River Watershed and Tenkiller Ferry Lake, Oklahoma describes the model sensitivity analysis and uncertainty analysis performed for Illinois River Watershed HSPF model and Tenkiller Ferry Lake EFDC model.
Final SA UA Technical Memo_08_07_2015

The Response to Statement of Findings explains the findings from the two peer reviewers based on their evaluation of the final reports developed by U.S. EPA contractors for the Agency’s Illinois River Watershed (IRW) TMDL Modeling Program.
Response to Statement of Findings- Final_8_4_2015

The Final Watershed Lake Model Main Report contains the Final Setup, Calibration, and Validation for Illinois River Watershed Nutrient Model and Tenkiller Ferry Lake EFDC Water Quality Model.
Final-Watershed Lake Model_Main Report 08_07_15

Appendices to the Final Watershed Lake Model Main Report

Calibrated Modeling Files
To obtain the calibrated modeling files contact:

Taimur A. Shaikh, Ph.D
Water Quality Division
EPA Region 6