Assessment Comments
Assessment is based on results of an IDNR investigation of a fish kill in July 2005.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses of this waterbody are assessed (evaluated) as “partially supported” due to a fish kill in July 2005. The kill was attributed to natural causes. This impairment is appropriate for Category 3b of Iowa’s 2006 Integrated Report.
EXPLANATION: This kill occurred on or before July 18, 2005 and was attributed to natural causes (water temperature). Approximately 3 miles of stream were affected. Results of the IDNR investigation show that levels of dissolved oxygen were high and levels of ammonia were very low. The kill was attributed to water temperatures that remained in the upper 70s (F) despite a recent rainfall event. The kill affected non-game species; no estimate of the number of fish killed was provided. The observation of live fish suggested that this was only a partial kill. Although believed unrelated to the kill, the Rockwell City area received from 1 to 2-inches of rainfall the evening prior to the kill.
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.
