Assessment Comments
Assessment is based on (1) occurrence of a fish kill in September 2001 & (2) results of biocriteria sampling in 1999: Fish IBI= 33 (fair), BM-IBI= 48 (fair).
Basis for Assessment
SUMMARY: Despite results of biological monitoring that suggest that the Class B(LR) aquatic life uses are "fully supported / threatened," these uses are assessed (evaluated) as "partially supported" due to occurrence of a fish kill. EXPLANATION: A biological assessment was conducted in 1999, prior to the fish kill in 2001, as part of the IDNR/UHL stream biocriteria project. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (F-IBI) and a benthic macroinvertebrate index (BM-IBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 1999 Fish IBI score was 33 (fair) and the BM-IBI score was 48 (fair). These results suggest that the aquatic life uses are "fully supported / threatened." The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001. A fish kill, however, occurred on this stream segment south of Royal on September 19, 2001. The kill was caused by the discharge of cattle manure from an open feedlot; an estimated 8,900 fish were killed. No estimate of the length of stream affected is available. Water quality monitoring conducted with a field kit during the fish kill investigation showed a dissolved oxygen concentration of zero and an ammonia-nitrogen concentration of 12.5 mg/l. According to DNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill during the most recent three-year period indicates "partial support" of the aquatic life uses. This stream has a history of reoccurring fish kills (see assessments for the 1996 and 1998 reports above); the most recent fish kill in IDNR files occurred approximately five years ago in 1996. Thus, depsite the results of biological monitoring in 1999 that suggest full support, the occurrence of the September 2001 fish kill suggests that the the Class B(LR) aquatic life uses of this stream should be assessed as "partially supported."
