Assessment Comments
Assessment based on results of (1) IDNR/UHL biological (biocriteria) monitoring in 2000: FIBI = 15 (poor), BMIBI = 52 (fair) and (2) the IDNR investigation of a fish kill in March 2000.
Basis for Assessment
SUMMARY: The general aquatic life uses remain assessed (monitored) as "not supporting" due to a combination of results from the IDNR/UHL biological monitoring in August 2000 and information from the fish kill investigation in March 2000, both of which suggest impairment of aquatic life uses.
EXPLANATION: The assessment of the general aquatic life uses was based on data collected in August 2000 as part of the IDNR/UHL stream biocriteria project and fishkill follow-up sampling. 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 (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2000 FIBI score was 15 (poor) and the BMIBI score was 52 (fair). The aquatic life use support was assessed as not supporting (=NS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61. Even though this site failed both the FIBI and BMIBI BICs, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it is a small general use stream and doesn’t fall in the calibrated watershed size. Note that results of IDNR/UHL biocriteria monitoring are not typically used to develop "monitored" (I.e., high confidence) assessments because the assessment metrics are based on historical data from Class B(LR) and Class B(WW) streams and rivers and are thus not strictly appropriate for assessing ecological conditions in the typically intermittent general use-only streams.
This stream was also investigated as part of a fish kill that occurred on the Yellow River on March 17, 2000. An estimated 4,800 fish were killed; no cause or source of the kill was identified. The following account is from the IDNR fish kill database and includes comments from the IDNR staff that investigated the kill: "Dead fish were observed both upstream and downstream of the Smith Rd. bridge on the Yellow River, but only below the confluence with the unnamed tributary in section 17. North of Postville. Water turbid and green, with high flow. The kill affected a 3.1 mile segment of the Yellow River. Dead fish were not observed in the unnamed trib that enters into the Yellow River in Section 17." 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 (2000-2002) indicates impairment of the aquatic life uses. Pollutant-caused kills where the source of pollution was not identified justify addition of the affected stream segment to Iowa's Section 303(d) list of impaired waters.
