Assessment Comments
Assessment is based on results of (1) IDNR/UHL biological (biocriteria) monitoring in 2001 and (2) an IDNR investigation of a fish kill in September 2003.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses remain assessed (monitored) as "partially supported" based on results of IDNR/UHL biological (biocriteria) sampling in 2001. In addition, the occurrence of two fish kills on this stream in September 2003 also suggests that the Class B(LR) uses should be assessed as “partially supported”. No cause for either kill was identified.
EXPLANATION: This assessment is based on the results of IDNR/UHL biological (biocriteria) monitoring conducted in 2001. 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 2001 FIBI score was 22 (poor) and the BMIBI score was 63 (good). The aquatic life use support was assessed as partially supporting (= PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54.
The previous (2002) assessment of the aquatic life uses ("partially supported") was based, in part, on the occurrence of a fish kill in 1998 (see assessment developed for the 2000 reporting cycle). According to IDNR's assessment methodology, the absence of a fish kill in the three-year period following a fish kill suggest that impacts from the kill have dissipated. Thus, due to the lack of a subsequent kill in this stream reach, the occurrence of the 1998 fish kill is not considered in development of the current (2004) assessment.
In addition to results of biological monitoring that suggest impairment of the aquatic life uses of this river segment, two fish kills were reported on this stream in September 2003. The first of the two kills occurred on or before September 11, 2003. A cause of this kill could not be identified (cause = unknown). An estimated 40 fish were killed (five common carp and 35 “minnow” species). No estimated value of the fish killed was provided. No estimate of the length of this stream affected by the kill was provided. No responsible party was identified. The location point for this kill is the SE ¼ of the SE ¼ of Section 19, T96N, R44W, Sioux County. According to the IDNR investigation, the Sioux Center area recently received heavy rains.
The second of the two kills occurred on or before September 13, 2003. No cause of the kill was determined, possibly due to the time that elapsed between the kill and the IDNR investigation. An estimated 100 fish were killed; all fish were non-game fish (“minnows”). No estimated value of the fish killed water provided; no estimate of the length of stream affected was provided. No responsible party was identified. The location point for the kill is Section 17 of T94N, R45W, Sioux County. According to the IDNR investigation, approximately 100 dead chubs and minnows were observed and appeared to have been dead for several days. Although a cause of the kill was not identified, IDNR staff reported that water levels were extremely low at that time due to drought conditions, thus making fish susceptible to any kind of pollution that might have washed into the stream from the recent rainfall.
These kills and others that occurred in small streams in northwest Iowa from September 11-15, 2003 were believed related to a rainfall event that followed a prolonged dry period. None of the investigations of these kills, however, suggested a specific pollutant-related cause. Thus, this impairment is appropriate for Category 3b of Iowa’s 2006 Integrated Report (=Section 303(d) list). Based on the summaries of the kills provided by IDNR field staff, the possibility exists that both kills occurred at the same time and were related to the same runoff event.
According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
