Big Wall Lake IA 02-IOW-656

Results of water quality monitoring conducted at Big Wall Lake indicate that the aquatic life uses should be assessed (monitored) as "fully supported."  Guidelines for wetland assessment from the Upper Mississippi River Conservation Committee’s Water Quality Technical Section (2003) state that an average total suspended solids concentration during the growing season of less than 30 mg/L is necessary to provide sufficient water clarity to support the growth of submersed aquatic vegetation in wetlands and shallow lakes.   High levels of total suspended solids impede the growth of submersed aquatic vegetation (SAV).   Submersed aquatic vegetation is critical to the health of wetland and shallow lake ecosystems.   The elimination of SAV can degrade habitat quality such that undesirable aquatic species such as cyanobacteria, common carp and fathead minnows dominate.   As such, the suppression of SAV constitutes a violation of Iowa’s narrative water quality criteria protecting against undesirable or nuisance aquatic life.  

Using the overall median values from the IDNR shallow lakes and wetlands monitoring program from 2012 through 2014 (approximately 15 samples), the median growing season total suspended solids concentration was 3 mg/L; this median value is well below the UMRCC guideline and suggests "full support" of the aquatic life uses.  

In addition, the level of algae also suggests "full support" of the aquatic life uses at Big Wall Lake. The Carlson’s (1977) trophic state index value for chlorophyll a was 41. According to Carlson (1977), the index value for chlorophyll a places Big Wall Lake in the mesotrophic category.  This value suggests extremely low levels of chlorophyll a and suspended algae in the water and "full support" of the aquatic life uses.   

NOTE:  The TSI value for Secchi depth is not used to evaluate the attainment of aquatic life goals in shallow lakes.   Due to the depth of these shallow lakes, TSI values for Secchi depth can be misleading.   In some instances the Secchi disk remains visible at the bottom of the lake and the depth of the lake is recorded as the Secchi depth.   In these instance, water clarity may be sufficient to support the Class B(LW) uses, but the index value is limited by the depth of the lake.   Thus the combination of total suspended solids and chlorophyll a will be used to determine whether or not the Class B(LW) uses are impaired in these shallow systems.

A watershed project now exists for Big Wall Lake (see the project web site at http://www.iowadnr.gov/water/watershed/bigwalllake/about.html).   This is a cooperative project between Iowa DNR, the Iowa Department of Agriculture & Land Stewardship/Division of Soil Conservation, and the federal Natural Resources Conservation Service.   The following information is from the project web site:

"In 2006, the Wright County Soil and Water Conservation District completed an inventory of the potential causes of watershed degradation of Big Wall Lake, and presented possible solutions to landowners, cabin owners and recreational users of the lake.  A partial drawdown of the lake in the winter of 2005-2006 was the jump-start of the project, and was followed by a series of chemical treatments in June 2007 to remove the carp.  The design of the project included the installation of a water control structure with stop logs to control the water levels in the lake.  Rodent guards were placed on both ends of the structure and a second channel was constructed to extend the lake."

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this wetland.   In general, Iowa wetlands are not managed for recreational fisheries; thus, fish contaminant monitoring is not typically conducted at these waterbodies.

NOTE: Information from IDNR staff indicate that a control structure was installed in early 2007, thus allowing the ability to manipulate the water level and thus improve water quality conditions at this wetland.   Therefore the impairment for flow alteration has been removed at this lake.