Crawford Creek Impoundment IA 06-LSR-1583

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported."  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  Fish consumption uses are “not assessed.”  Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: For the 2014 reporting cycle, the Class A1 (primary contact recreation) uses for Crawford Creek Lake are assessed as “fully supported."  Using the median values from the ISU statewide survey of lakes and the UHL ambient lake monitoring program from 2008-2012(approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 57, and 59 respectively for Crawford Creek Lake.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus index values all place Crawford Creek Lake in the between the eutrophic and hypereutrophic categories.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and relativley low levels of phosphorus in the water column.  

The levels of inorganic suspended solids at this lake were relativley low and do not suggest an impairment due to non-algal turbidity.   The median level of inorganic suspended solids in Crawford Creek Lake (2.5 mg/L) and ranked 23rd of the 134 lakes sampled by the ISU and UHL surveys.

Data from the 2008-2012 ISU and UHL surveys suggest that a relatively small population of cyanobacteria exists at Crawford Creek Lake.   These data show that cyanobacteria comprised 56% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (16.9 mg/L) and ranked 60th of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses for Crawford Creek Lake are assessed (monitored) as “fully supported.”  Data from the ISU and UHL lake surveys from 2008-2012 show no violations of the Class B(LW) criterion for ammonia in 14 samples or pH in 14 samples.   There was one violation of the Class B(LW) criterion for dissolved oxygen in 14 samples (7%).   Based on IDNR's assessment methodology, the single violation is not significantly greater than 10% of the samples, and therefore these results suggest “full support” of the Class B(LW) uses at Crawford Creek Lake.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.