East Nishnabotna River IA 05-NSH-1817

The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supporting” based on biological data collected in 2002 as part of the DNR/SHL REMAP project. 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 2002 FIBI score was 45 (fair) and the BMIBI score was 42 (fair). The aquatic life use support was assessed as partial 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 31 and the BMIBI BIC for this ecoregion is 54. This assessment is considered evaluated because the drainage area (781 mi2) above this sampling site was 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 and failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the site used for the assessment doesn’t fall in the calibrated watershed size. According to DNR’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). DNR 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).

This assessment is also considered of lower confidence (evaluated) as opposed to higher confidence (monitored) because the data from the 2002 biological sampling are now older than five years; thus, these data are considered less able to accurately characterize current water quality conditions than would be more recent data.

As part of the REMAP project, diurnal dissolved oxygen/temperature monitoring conducted during September 2002 found substantial fluctuation of dissolved oxygen levels associated with high levels of seston and periphyton chlorophyll, and very high estimates of primary production. Levels of BMIBI metrics that are sensitive to organic enrichment suggest a nutrient enrichment/algal growth-related water quality problem. Habitat modification that has resulted in a wide and shallow stream channel with high solar input may contribute to algal growth. Additional sampling data are needed to evaluate the extent, causes and sources of this water quality problem.