Crawford Creek Impoundment IA 06-LSR-1583

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 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 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: For the 2012 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 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 60, and 63 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 moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column.  

Crawford Creek Lake was listed as "partially supporting" the Class A1 uses for the 2008 and 2010 assessment/listing cycles due to algal turbidity.   Based on IDNR's assessment methodology, median-based TSI values for both chlorophyll a and Secchi depth must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5).   The median TSI values for the 2010 assessment and listing cycle for chlorophyll a and Secchi depth were 63 and 62, respectively.   Because the median TSI values are less than or equal to 63 for a second assessment and listing cycle, the algae impairment at Crawford Creek Lake is suggested for de-listing for the 2012 Integrated Report.

The levels of inorganic suspended solids at this lake were low and do not suggest an impairment due to non-algal turbidity.   The median level of inorganic suspended solids in Crawford Creek Lake (2.0 mg/L) was the 10th lowest median of the 134 lakes sampled by the ISU and UHL surveys.

Data from the 2006-2010 ISU and UHL surveys suggest that a moderately large population of cyanobacteria exists at Crawford Creek Lake.   These data show that cyanobacteria comprised 77% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (31.0 mg/L) was also the 47th highest 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 2006-2010 show no violations of the Class B(LW) criterion for ammonia in 22 samples or pH in 22 samples.   There was one violation of the Class B(LW) criterion for dissolved oxygen in 22 samples (5%).   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.