Assessment Comments
Assessment based on 2007 IDNR/SHL biological sampling data (CBI = 43; CW BIC = 60) and on a 2012 summary of trout reproduction in Iowa streams prepared by the IDNR Fisheries Bureau.
Basis for Assessment
Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(CW) aquatic life uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is also now presumptively designated for Class A1 (primary contact recreation) and Class A2 (secondary contact recreation) uses. This segment remains designated for coldwater aquatic life use (now termed Class B(CW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption) uses.
SUMMARY: The presumptive Class A1 (primary contact recreation) uses and presumptive Class A2 (secondary contact recreation) uses remain "not assessed" (IR Category 3a) due to the lack of data for indicator bacteria upon which to base an assessment. The Class B(CW1) coldwater aquatic life uses are assessed (evaluated) as "partially supported" (IR Category 3b-c) based on 2007 IDNR/SHL benthic macroinvertebrate sampling and on the February 2012 summary of trout reproduction in Iowa streams as prepared by the IDNR Fisheries Bureau. Fish consumption uses remain "not assessed" (IR Category 3a).
EXPLANATION: The assessment of the Class B(CW1) aquatic life uses was based on data collected in 2007 as part of the IDNR/SHL coldwater stream sampling project. A series of biological metrics which reflect coldwater stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa that were collected in the stream sampling reach. The biological metrics were combined to make a coldwater benthic index (CBI). The index ranks the biological integrity of a coldwater stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2007 CBI score was 43. The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the CBI score with biological impairment criteria (BIC) established for the 2012 Section 305(b) report. The biological impairment criteria were determined from a statistical analysis of data collected at coldwater stream reference sites from 1994-2011. The CW BIC is 60 and this segment passed the CW BIC 0/1 times in 2007.
This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology. IDNR now requires a segment have two or more biological samples collected from the segment in multiple years in a five-year period to be considered “monitored”. This segment had a single sample collected in 2007. Additionally, these data are now greater than five years old. 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).
In contrast to the benthic macroinvertebrate sampling, the updated summary of trout reproduction in Iowa streams prepared by the IDNR Fisheries Bureau in February 2012, suggests the Class B(CW1) uses be assessed as "fully supported". According to the updated summary, Twin Springs Creek (aka Spring Falls) is in the group of Iowa coldwater streams that supports natural reproduction of trout. Based on results of surveys by the IDNR Fisheries Bureau, this stream is placed in the category of Iowa trout streams that exhibit recent, but inconsistent, reproductive success and are not capable of maintaining a viable population of brown and brook trout at this time.
Fish consumption uses remain "not assessed" due to lack of fish tissue monitoring in this stream reach.
