Assessment Comments
Assessment based on (1) 2000 IDNR biocriteria sampling data: 7/8, 6/8, and 3/8 coldwater biological indicators met the criteria and (2) fish kill in March 2000.
Basis for Assessment
SUMMARY: The Class B(CW) aquatic life uses are assessed (evaluated) as "partially supported"; this assessment is based on occurrence of a fish kill in March 2000 and on results of IDNR/UHL biological monitoring in August 2000. Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this stream segment.
EXPLANATION: A fish kill occurred on this stream reach 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 Sec. 17. A total of 12 species of fish were affected. After visually inspecting the habitat in the area that the fish kill occurred, we would rate the habitat as excellent. The bottom substrate of much of the stream is composed of gravel and cobble. Deeper water areas for fish overwintering locations are also available. The Smith Rd. Bridge is the upstream extent of the area in which we have stocked fingerling brown and rainbow trout. No trout were seen in any of the counts of dead fish. The total number of fish that were observed was very low for a stream of this size and habitat quality. In previous work by Paragamian (1986), between 3000 and 5000 fish/acre were seen in northeast Iowa streams of comparable sizes. We feel that there has been an unknown factor at work that has severely reduced the total fish community in this section of the Yellow River prior to the total kill that this incident has caused to occur."
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. Thus, the Class B(CW) aquatic life uses of this stream were assessed as "partially supported."
The aquatic life use assessment was also based on data collected in 2000 as part of the DNR/UHL stream biocriteria project. A series of eight (8) coldwater biological indicators, which reflect stream water quality and habitat integrity, were calculated from the biocriteria sampling data and supplemental IDNR Fisheries Bureau data. The coldwater biological indicators are based on the numbers and types of coldwater benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. Sites that score 7 or 8 are Fully Supporting (FS), sites that score 4-6 are Fully Supporting/Threatened (FS/T), sites that score 2-3 are Partially Supporting (PS), and sites that score 0-1 are Not Supporting (NS). The 8/14/2000 coldwater biological indicator scores were 7/8 and 6/8 which equates to Fully Supporting/Threatened (= FS/T), and the 8/15/2000 biological indicator score was 3/8 which equates to Partially Supporting (= PS) based on a comparison of the coldwater biological indicator score with biological assessment criteria established for this Section 305(b) report. The coldwater biological indicators were determined from a statistical analysis of data collected at coldwater stream ecoregion reference sites from 1994-2002 and with assistance from the IDNR Fisheries Bureau.
Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this stream segment.
