Draft for Public Review and Comment Fox River TMDL
2-8
gravity through the natural channel to the Fox River. A COE 1978 inspection report concludes that the
Borah Lake reservoir and spillway are capable of passing and holding a 100-year flood (COE 1978b).
However, because of the extremely deteriorated condition of the spillway, it is possible that any
appreciable amount of flow could cause a total spillway failure as well as erosion of the natural channel
both upstream and downstream of the lake. In addition, the top of the spillway guide wall is only 2 feet
above the spillway crest. If flow over the spillway were to over-reach the top of guide wall, severe
erosion could result in further damage or loss of the spillway. Evidence of seepage at the downstream toe
of the dam has been noted. It was believed that water was entering the water supply intake tower and
seeping out through the embankment. This seepage could contribute flow to the Fox River (COE 1978b).
Other than potential seepages, discharges occur only when lake levels rise above the elevation of the
spillway (469.5 feet above mean sea level).
Because of lack of UGSG gauging stations throughout the Fox River watershed, a hydrologic model was
developed for the entire Little Wabash River watershed, which has USGS gauging stations. TMDL
development for the Fox River is based on the assumption that the Little Wabash River watershed
hydrologic model accurately predicts flows in the Fox River. Appendix B provides details on the Little
Wabash River hydrologic model development.
2.5 Growth Trends in the Watershed
Growth trends in the Fox River watershed are assumed to be the same as growth trends in Richland
County. Since a population peak of about 17,587 around 1980, the population in Richland County has
been steadily decreasing. The estimated population of Richland County was 16,545 in 1990 and 16,149
in 2000, which results in a growth change of - 2.4 percent. The population estimate predicted for 2001 is
16,042, which indicates that the population is expected to continue to decrease but at a slower rate (U.S.
Census Bureau 2002). For purposes of this study, the population in the Fox River Watershed was
assumed to remain relatively constant in the near future.
2.6 Biological Information
Biological information, including information on macroinvertebrate communities and habitat data, was
collected along the listed segment of the Fox River but not from the listed lakes. IEPA collected
biological samples and calculated the macroinvertebrate biotic index (MBI) at two stations along the Fox
River downstream of the Olney STP in 1988 and 1994. An MBI reflects the degree of tolerance of a
macroinvertebrate community to oxygen-demanding and other contaminants. MBI values reflect aquatic
community impairment as follows:
