Rathbun Reservoir - Main Basin IA 05-CHA-1309

Results of IDNR beach monitoring from 2012 through 2014 suggest that the Class A1 uses are "Fully Supported." Levels of indicator bacteria at Honey Creek Resort St. Park were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (15 samples), 2013 (15 samples) and 2014 (15 samples), as part of the IDNR beach monitoring program.   According to IDNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E.  coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E.  coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E.  coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S.   EPA guidelines (see pgs 3-33 to 3-35 of U.S.   EPA 1997b).

At Honey Creek Resort St. Park, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 9 E.  coli orgs/100 ml in 2012, 13 E.  coli orgs/100 ml in 2013 and 14 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 0% in 2012, 0% in 2013 and 0% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

However for the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Rathbun Reservoir are assessed (monitored) as "not supported" due to poor water transparency caused by high levels of non-algal turbidity that creates aesthetically objectionable conditions based on information from the ISU lake survey and the U.S. Army Corps of Engineers at Station RA-3. Additionally information from the IDNR Fisheries Bureau indicate that an estimate of more than 50% of the grassland in the 365,000 acre watershed has been lost within the last decade due to conversion to row crop. Using the median values from these surveys from 2010-2014 (approximately 40 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 50, and 63 respectively for Rathbun Reservoir. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Rathbun Lake in the Eutrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, poor water transparency, and moderately high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 25 samples.

The 2010-2014 TSI value for total phosphorus at Rathbun Reservoir (63) is much lower than the corresponding TSIs for Iowa’s other flood control reservoirs (Red Rock Lake (75), Saylorville Lake (73), and Coralville Lake (80)), thus suggesting that Rathbun Reservoir likely has fewer water quality problems related to excess algal production than do these other lakes.       

The level of inorganic suspended solids was moderately high at Rathbun Reservoir, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Rathbun Reservoir (6.2 mg/L) was ranked 98th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a small population of cyanobacteria exists at Rathbun Reservoir. These data show that cyanobacteria comprised 70% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (4.9 mg/L) was ranked 11th of the 138 lakes sampled.       

The Class B(WW-1) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey and the ACOE at station RA-3 from 2010-2014 show there were no violations of the criterion for ammonia in 44 samples(0%), 2 violations of the criterion for dissolved oxygen in 40 samples(5%), and no violations of the criterion for pH in 25 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the B(WW-1) uses of Rathbun Reservoir.

In addition, a recent comparison of water quality data to production of the primary sport fish at Rathbun Reservoir (crappie) suggest “full support” of the aquatic life uses of the main lake basin.   In 2006, the IDNR Fisheries Bureau summarized approximately 35 years of data on abundance of crappies at Rathbun Reservoir.   These data were combined with the available water quality data (707 Secchi depth readings) from that period to determine the spatial and temporal differences in crappie abundance and water quality conditions within Rathbun Reservoir.   Results of this analysis showed the following.   Compared to the near-dam and mid-lake areas, all tributary arms of the reservoir have relatively poor water quality (measured as water transparency) and relatively poor crappie production.   The greatest transparency at all locations occurred recently, in the period from 2000-2006.   These improvements in water transparency are attributed to implementation of best management practices in the Rathbun watershed, although the recent improvements may also be related to generally lower rainfall and runoff during recent years.    Based on the Secchi depth readings and the analysis of crappie production for the period 1970-2006, the best water clarity and crappie production occurs near the dam and in central portion of the main lake basin where Secchi depths average approximately one meter.   

Note: The Rathbun Lake watershed had several years of high rainfall between 2010-2014, which may contribute to the elevated levels of inorganic suspended solids and reduced water clarity observed in some recent years.   

However, the somewhat elevated levels of inorganic suspended solids as measured by the ACOE, ISU, and SHL monitoring during the 2010-2014 period does suggest concern that turbidity levels are increasing in the lower reservoir near the dam and that this increase may adversely impact the reservoir's fisheries and other aquatic life.     

Levels of atrazine in this segment of Rathbun Reservoir are low and do not suggest a threat to support of the drinking water (Class C) uses of the Chariton River downriver from Rathbun Dam.   Results of pesticide monitoring by ACOE and Syngenta from 2012-2014 in the reservoir near the dam (station RA-3) showed one violation of the MCL for atrazine in the 26 samples collected.  The mean, minimum, and maximum atrazine levels at the reservoir near the dam (station RA-3) during the 2012-2014 period were 0.75 ug/l, 0.31 ug/l, and 4.6 ug/l. The average level of nitrate in the 44 samples collected from station RA-3 during 2010-2014 was 0.75 mg/l; the maximum sample value during this period was 1.8 mg/l. These results are similar to those for previous assessment periods.  Results of ACOE and ISU monitoring suggest very low levels of nitrate in Rathbun Reservoir.       

Although average levels of atrazine as reported in the ACOE and Syngenta monitoring networks are, and have historically been, well below the MCL at the stations near the dam and at the outlet area (the vicinity of the drinking water intake), the atrazine levels at the ACOE’s uplake stations (RA-7 and RA-8) are occasionally higher, and levels in reservoir tributaries are routinely higher, thus suggesting a continued concern regarding atrazine and the drinking water uses of the Chariton River downriver from Rathbun Dam.   Based on approximately eight years of monitoring, the typical pattern for atrazine levels in Rathbun Reservoir and its watershed is occurrence of relatively low levels of atrazine near the dam (well-below the atrazine MCL), with progressively higher levels proceeding uplake and upstream into the lakes tributaries.     

The level of support of the Class HH-fish consumption uses is changed from “fully supporting” to “not assessed” due to the age of the data upon which the previous assessment was based.  The previous assessment was based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring at Rathbun Reservoir in 2004.    The composite samples of fillets from channel catfish and white crappie had low levels of contaminants.    Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.031 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.     Levels of primary contaminants in the composite sample of white crappie fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.   The fish contaminant data generated from the 2004 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.

[Note:  For the 2006, 2008, 2010, 2012, 2014 and 2016 305(b)/303(d) cycle, Rathbun Reservoir was subdivided into the following segments for purposes of developing water quality assessments:  main lake basin, South Fork Arm, Chariton Arm, Honey Creek Arm, and Buck Creek Arm.   These segments were created to better use the water quality data collected as part of the Rathbun Water Quality Project from monitoring stations located in the main lake basin and in the several arms of the reservoir.   Water quality data from the arms of Rathbun Reservoir suggest differing water quality conditions than do data collected from the main lake basin.   Thus, developing separate assessments for the main lake basin as well as for the reservoir’s arms represents a more accurate characterization of water quality conditions of the reservoir than did past assessments in which only one assessment was developed for the entire reservoir.]