Sny Magill Creek IA 01-YEL-430

Assessment Comments

Assessment is based on (1) results of at Sny Magill project sites monitored for water quality, fish, macroinverts, and habitat at varying frequencies and (2) a DNR summary of trout reproduction.

Basis for Assessment

SUMMARY:  The Class B(CW) coldwater aquatic life uses were assessed as "fully supported / threatened" (minor impacts).   Fish consumption uses remain "not assessed."  EXPLANATION:  Based on a summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau, the Class B(CW) uses were assessed as "fully supported / threatened."  According to Moeller (1999), Sny Magill Creek is in the category of Iowa trout streams that have at least some documented natural reproduction but that are generally unable to maintain a viable trout population at this time.   Monitoring of chemical water quality, habitat quality, fish populations, and aquatic macroinvertebrate populations has been conducted at Sny Magill Creek since 1992 as part of the Sny Magill Creek Nonpoint Source Pollution Monitoring Project.   Results of this monitoring show good chemical water quality and aquatic life.   Results of weekly chemical water quality monitoring from October 1996 through September 1999 at stations on the lower and upper reaches of this stream show (1) no violations of the Class B(CW) water quality criterion for dissolved oxygen (7.0 mg/l) in the 312 samples collected and (2) two estimated violations of the Class B(CW) chronic criterion for ammonia-nitrogen in 311 samples collected (these violations occurred at the upper reach station on February 18, 1997 and November 11, 1997).   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), these two violations suggest an impairment of the aquatic life uses.   For the following reasons, however, these violations were not used in the assessment of the aquatic life uses:  (1) violations of the temperature/pH-dependent water quality criteria for ammonia-nitrogen were based on "worst case" estimated values of pH (pH was not measured during sample collection at the Sny Magill project sites); (2) the estimated violations were relatively close to the corresponding water quality criteria (February 18, 1997:  NH3-N = 0.5 mg/l; WQ criterion = 0.39 mg/l;  November 11, 1997:  NH3-N = 2.1 mg/l; WQ criterion = 2.03 mg/l).   Results of monthly monitoring at the mid-reach station show no violations of these criteria for either dissolved oxygen or ammonia-nitrogen in the 36 samples collected.   The episodes of low dissolved oxygen that occurred in the Sny Magill and Bloody Run watersheds from 1992-1994 (see assessments for the 1996 and 1998 reports above) were due, at least in part, to problems with field equipment and certain reagents used to measure dissolved oxygen.   Since July 1995, measurements have been made with a YSI portable dissolved oxygen meter.   During the period October 1996 through September 1999, none of the samples collected at monitoring sites in the Sny Magill Nonpoint Source project have violated the Class B(CW) water quality criterion for dissolved oxygen of 7.0 mg/l.   Based on the Hilsenhoff Biotic Index, results of biological monitoring for benthic macroinvertebrates suggest "very good" water quality.   Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this stream reach.   For more information on the Sny Magill Creek Nonpoint Source Pollution Monitoring Project visit the DNR-Geological Survey Bureau web site at http://www.igsb.uiowa.edu/inforsch/waterag.htm.