Assessment Comments
Assessment based on (1) IDNR/UHL biological (REMAP) monitoring in 2004 [FIBI = 0 (poor), BMIBI = 28 (poor). FIBI BIC = 43; BMIBI BIC = 54] and (2) results of an IDNR investigation of a fish kill in September 2003.
Basis for Assessment
SUMMARY: The general uses of this waterbody are assessed (evaluated) as “not supported” based on results of biological monitoring in 2004 and 2005. In addition, a fish kill on this stream segment in September 2003 also suggests impairment of the general uses. The sources of data for this assessment include (1) results of IDNR/UHL biological (REMAP) monitoring in June 2004 and (2) results of an IDNR investigation of a fish kill in September 2003. The apparent lack of fish in this stream segment, as documented by IDNR/UHL biological samplings in 2004, as well as in 2005, suggests a severe water quality impact and overwhelming evidence of impairment that justifies addition to Iowa’s 2006 Section 303(d) list (IR Category 5b).
EXPLANATION: Results of biological monitoring in 2004 suggest that the general uses of this stream segment should be assessed as "not supported". This assessment was based on biological data collected in 2004 as part of the DNR/UHL stream REMAP project. A series of biological metrics that 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 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 2004 FIBI score was 0 (poor) and the BMIBI score was 28 (poor). The aquatic life use support was assessed (evaluated) 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 43 and the BMIBI BIC for this ecoregion is 54. 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. However, because IDNR/UHL biological data generated after the 2006 assessment period continue to indicate severe water quality problems in this stream segment, IDNR views the existing biological data as indicating overwhelming evidence of impairment. Thus, this stream segment will be placed on Iowa’s 2006 Section 303(d) list (IR Category 5b).
In addition to results of biological monitoring that suggest impairment, the occurrence of a fish kill in September 2003 also suggests impairment of the general uses of this stream. This kill occurred on or before September 13, 2003; the kill was attributed to low levels of dissolved oxygen in a oxbow/backwater of Dry Creek. An estimated 1,501 fish were killed. According to the fish kill report; all fish were common carp except one crappie. No estimated value of the fish killed was provided. No estimate was provided for the size of the waterbody affected by the kill. The kill occurred in a backwater of Dry Creek at the southwest edge of Hawarden, IA; the location point for the kill is the NW1/4 of the SW1/4 of Section 2, T94N, R48W, Sioux County. According to the IDNR investigation, dead fish were concentrated in a small backwater oxbow. Heavy rains in the area on September 13 may have allowed the carp to move into backwaters and oxbow; when stream level dropped the fish were trapped in the oxbow with no oxygen or water. A reporter noticed a muddy smell on 9/14 and noticed that fish were dying. During the investigation, oxygen levels were greater than 15ppm and ammonia levels ranged from 1.0 to 1.5 ppm. The water was clear to mud stained. The kill was attributed to natural causes. This kill and others that occurred in smaller streams in northwest Iowa from September 11-15, 2003 were believed related to a rainfall event or events that followed a prolonged dry period. None of the investigations of these kills, however, suggested a specific pollutant-related cause.
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.
