Walnut Creek IA 04-RAC-1120

Assessment Comments

Assessment is based on results of a DNR/UHL biocriteria sampling in 1998 (fish IBI= 48 (fair), BM-IBI= 53 (fair)).

Basis for Assessment

The Class A (primary contact recreation) uses remained "not assessed."  The Class B(LR) aquatic life uses were assessed as "partially supported."  EXPLANATION.   The Class A uses remained "not assessed" due to lack of data for indicator bacteria.   This stream reach has not been routinely monitored for indicator bacteria.   The Des Moines Water Works conducted special sampling for E.  coli on Walnut Creek following several rainfall runoff events during 1997 and 1998; the results of this sampling showed very high levels of E.  coli following rainfall events.   According to the Iowa Water Quality Standards, however, the criterion for indicator bacteria in Iowa's Class A waters does not apply "when the waters are materially affected by surface runoff."  Thus, the data from the Des Moines Water Works are of marginal use for developing an assessment of support for the Class A uses of this stream.   The Class B(LR) uses were assessed as "partially supported" due to results of a DNR/UHL biocriteria sampling in 1998.   Compared to results of Iowa ecoregion reference site sampling, both the fish and benthic macroinvertebrate communities were rated only "fair" (fish IBI= 48; BM IBI = 53).   Thus, the Class B(LR) aquatic life uses were assessed as "partially supported."