About The Illinois State Water Survey

Hydrologic and Nutrient Monitoring for the Lake Decatur Watershed

Principal Investigators: Laura Keefer and Erin Bauer

Project Staff: Mary Richardson

Sponsor: City of Decatur, Illinois

Project Period: 1993-2009

Lake Decatur is the water supply reservoir for the City of Decatur. The reservoir was created in 1922 by impounding the flow of the Sangamon River in east-central Illinois, has a surface area of 4.4 square miles, and maximum storage capacity of 28,000 acre-feet (9,125 million gallons). Total water withdrawal from the lake currently averages 39 million gallons per day for a population of 86,705. The drainage area of the Sangamon River upstream of the Lake Decatur dam is 925 square miles. The watershed includes portions of seven counties in east-central Illinois and agriculture is the predominant land use in the watershed, comprising over 80 percent of the land area. Lake Decatur has high concentrations of nitrate-nitrogen (nitrate-N) and has been periodically exceeding the Safe Drinking Water Act standard of 10 milligrams per liter since 1979. This created a serious situation for the drinking water supply of the City of Decatur, since nitrate-N cannot be removed from finished drinking water through regular water purification processes. From 1993 to 2008, the Illinois State Water Survey monitored the Lake Decatur watershed for trends in nitrate-N concentrations and loads to identify any significant changes in the watershed. Even though the City of Decatur constructed an ion exchange facility to remove nitrate from the drinking water which came on-line in June 2002, the ISWS continued to monitor for the purpose of collecting reliable hydrologic and water quality data throughout the watershed for use by city planners to efficiently operate the nitrate removal facility and by resource managers to develop watershed management alternatives based on scientific data.


Based on the compilation and analyses of the 15-year dataset, the following observations can be made:

  • Based on the 100-year streamflow record at the Monticello station, 4 of the top 11 total annual discharges occurred during the project monitoring period (WY1993 - 2nd, WY2008 - 3rd, WY1994 - 10th, and WY1998 - 11th), further confirming that the monitoring period was a wet period.
  • Tributary stations had nitrate-N concentrations consistently higher than river stations, which in turn were higher than Lake Decatur concentrations at the South Water Treatment Plant. As watershed areas increased, nitrate-N concentrations decreased.
  • Nitrate-N yields decreased as drainage area increased, were dispersed throughout the watershed, and varied from year to year.
  • Annual flow-weighted nitrate-N concentrations vary year-to-year for all stations, but no longer-term trends are apparent over the course of the 15-year monitoring period
  • There are no significant trends in discharge and nitrate-N yields for the three stations in the Lake Decatur watershed, implying that there was no change in these parameters over the 15 years.
  • Trends analysis detected a statistically significant increasing trend in annual average nitrate-N concentration at Monticello (111) for the monitoring period. This corresponds to an increase of 1.3 mg/L between 1993 and 2008. This trend did not have a corresponding increasing trend in nitrate-N yields, where as no trends were detected at the other stations in the Sangamon River watershed. Also, it is unclear whether an increasing trend would continue into the future due to the unknown nature of climate cycles and variability relative to the shorter time interval (15 years) for which nitrate-N data are available.
  • Similarity in trends for nitrate-N yields between the project and Oakford stations during both data periods could imply a similarity in trends during the longer-term period (WY1975-2006). Therefore it can be speculated that there has been no significant change in nitrate-N yields in the Lake Decatur watershed since 1975.


Keefer L. L., E. Bauer and M. Markus. 2010. Hydrologic and Nutrient Monitoring of the Lake Decatur Watershed: Final Report 1993-2008. Illinois State Water Survey Contract Report 2010-07.

Keefer L. L. and E. Bauer. 2008. Watershed Monitoring for the Lake Decatur Watershed, 2003-2006. Illinois State Water Survey.

Keefer, L.L. and E. Bauer. 2005. Watershed Monitoring for the Lake Decatur Watershed, 2000-2003. Illinois State Water Survey.

Keefer, L. and M. Demissie. 2002. Watershed Monitoring for the Lake Decatur Watershed, 1999-2000. Illinois State Water Survey.

Keefer, L. and M. Demissie. 2000. Watershed Monitoring for the Lake Decatur Watershed, 1998-1999. Illinois State Water Survey.

Borah, D.K., M. Bera, S. Shaw, and L. Keefer. 1999. Dynamic Modeling and Monitoring of Water, Sediment, Nutrients, and Pesticides in Agricultural Watersheds during Storm Events. Illinois State Water Survey Contract Report 655, Champaign, IL, 81p.

Keefer, L. and M. Demissie. 1999. Watershed Monitoring for the Lake Decatur Watershed, 1997-1998. Illinois State Water Survey.

Keefer, L., M. Demissie, S. Shaw, S. Howard. 1997. Watershed Monitoring for the Lake Decatur Watershed, 1996-1997. Illinois State Water Survey.

Keefer, L., M. Demissie, S. Shaw, and K. Nichols. 1996. Watershed Monitoring for the Lake Decatur Watershed. Illinois State Water Survey.

Demissie, M., L. Keefer, D. Borah, V. Knapp, S. Shaw, K. Nichols, and D. Mayer. 1996. Watershed Monitoring and Land Use Evaluation for the Lake Decatur Watershed. Illinois State Water Survey Miscellaneous Publication 169.

Demissie, M., L. Keefer, A. Akanbi, V. Knapp, S. Shaw, and E. Brown. 1994. Watershed Monitoring and Land Use Evaluation for the Lake Decatur Watershed. Illinois State Water Survey Miscellaneous Publication 159.

Borah, D.K., M. Demissie & L. L. Keefer, 2002. "AGNPS-based Assessment of the Impact of BMPs on Nitrate-Nitrogen Discharging into an Illinois Water Supply Lake". Water International, vol. 27, no. 2, p. 255-265, June 2002.

Demissie, M. and L. Keefer, 1998. Watershed Approach for the Protection of Drinking Water Supplies in Central Illinois (U.S.A). Water International, International Water Resources Association, Volume 23, Number 4, December 1998, p. 272-277.

Borah, D.K., M. Bera, L. Keefer, M. Demissie, and S. Shaw. 2000. Watershed models analyzing agricultural watersheds in Illinois. Proceedings of the West North Central Region Soil and Water Conservation Society's Conference on The Watershed Approach to Improving Water Quality: Fact or Fantasy? March 28-30, 2000, La Crosse, WI: 70-72.

Keefer, L. and M. Demissie, 1997. Drainage and Water Quality Characteristics of the Upper Sangamon River Watershed in Central Illinois. Proceedings of the Conference on Management of Landscapes Disturbed by Channel Incision, May 19-23, 1997. S.S.Y. Wang, E. J. Langendoen, and F. D. Shields, Jr., editors. Center for Computational Hydroscience and Engineering, University of Mississippi, Oxford, MS, p. 101-106.

Demissie, M., L. Keefer, S. Shaw, and K. Nichols. 1996. Variability and Trends of Nitrate Concentrations in the Lake Decatur Watershed. Proceedings, Biennial Conference, Illinois Section, American Water Resources Association, Bloomington, IL, October 22, 1996, pp. 71-80.

John, S.F., K. Alexander, T. Hoffman, L. Keefer. 1996. Lake Decatur, Illinois, Case Study: Nitrate Reduction for SDWA Compliance in an Agricultural Watershed. Proceedings of WATERSHED'96, A National Conference on Watershed Management, Baltimore, MD, June 8-12, pp. 262-267.

Related Publications

Brown, Carl B., J.B. Stall, and E.E. DeTurk, 1947. The Causes and Effects of Sedimentation in Lake Decatur ISWS B-37

Agersborg, H. P. K., and W. D. Hatfield, 1929. The Biology of a Sewage Treatment Plant, a Preliminary Survey, Decatur, Illinois ISWS C-5

Bogner, William C., 2002. Sedimentation Survey of Lake Decatur's Big and Sand Creek Basins, Macon County, Illinois ISWS CR 2002-09

Illinois State Water Survey

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