Rock River IA 06-BSR-1534

The Class A1 (primary contact recreation) are assessed as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at the Rock River near Hawarden were as follows: the 2012 geometric mean was 201 orgs/100 ml, the 2013 geometric mean was 110 orgs/100 ml, and the 2014 geometric mean was 149 orgs/100 ml.  Two of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml.  Nine of the combined 24 samples (38%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml.  According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).  Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as "partially supported".  

Regarding support of the Class B(WW1) aquatic life uses, monitoring at the IDNR/SHL ambient monitoring station near Hawarden showed no violations of Class B(WW1) water quality criteria for 36 Ammonia samples (maximum = 1.8 mg/L), 36 Dissolved Oxygen samples (minimum = 6.7 mg/L), 36 pH samples (range = 7.3 to 8.6), 36 Temperature samples (maximum = 26.7° c), 36 Chloride samples (maximum = 59 mg/L), or 36 Sulfate samples (maximum = 140 mg/L) occurred during monitoring from January 2012 to December 2014.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

Results of biological monitoring in 2003 and 2011-2014 suggest “partial support” of the Class B(WW1) aquatic life uses of this river segment.   This assessment is based on biological sampling conducted in 2003 and 2011-2014 as part of the IDNR/SHL stream REMAP and large river projects.   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 2003 FIBI score was 57 (good).   The 2011-2014 BMIBI scores were 21 (poor), 34, 42, 44 (all fair).   The aquatic life use support was assessed (evaluated) as partially supporting (=PS), 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 1994-2008.   The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54.   This segment passed the FIBI BIC 1/1 times in 2003 and passed the BMIBI BIC 0/4 times in the last five years.   This assessment is considered evaluated because the drainage areas (1591 and 1680 mi2) above the sampling sites were greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site passed the FIBI BIC (1/1) and failed the BMIBI BIC (0/4), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the sites used for the assessment don’t fall in the calibrated watershed size.   According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report).   IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Despite results of ambient water quality monitoring that suggest “full support” of the Class B(WW1) uses, the occurrence of a fish kill in this river segment in August 2002 suggests that these uses should remain assessed (evaluated) as only “partially supported.”  A fish kill occurred on this stream on August 8, 2002.   The kill followed heavy rainfall; no specific cause was determined, but runoff from feedlots was suspected as the cause and source of this kill.   Approximately 1.75 miles of river was affected, and an estimated 930 fish were killed.   According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill indicates an impairment of the aquatic life uses.   Because the data upon which these assessment are based are now more than five years old, the assessment type is changed from “monitored” (a higher confidence assessment) to “evaluated” (a lower confidence assessment).   As water quality data age, they are less able to represent current water quality conditions.   Despite the age of the data and the change from a “monitored” to an “evaluated” assessment type, any impairments previously identified for this assessment segment remain on Iowa’s section 303(d) list.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.