Assessment Comments
Assessment is based on (1) results of approximately 18 water quality monitoring events conducted at Rathbun Reservoir during the years 2006 through 2008 by the U.S. Army Corps of Engineers at up-lake Station RA-7 in the Chariton Arm of the reservoir, (2) an IDNR Fisheries analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir, and (3) results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004.
Basis for Assessment
[Note 1: For the 2006, 2008, and 2010 Section 305(b)/303(d) cycles, 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.]
[Note 2: Previous Section 305(b) assessments for this subsegment of Rathbun Reservoir have included an assessment of the support of Class C (drinking water) uses. According to Iowa’s Surface Water Classification, however, Rathbun Reservoir is not designated for Class C uses (see http://www.iowadnr.gov/water/standards/files/swcdoc2.pdf). The segment of the Chariton River immediately downriver from Rathbun Dam is, however, designated for Class C uses. Thus, the Class C assessments and any related Section 303(d) [impaired waters] listings developed for this subsegment of Rathbun Reservoir have been in error and will be removed for the current (2010) Integrated Reporting cycle.]
SUMMARY: The Class A1 (primary contact recreation) uses of the Chariton Arm are assessed (monitored) as “not supporting” due to very poor water transparency caused by both algae and non-algal turbidity that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(WW-1) (aquatic life) uses are assessed (monitored) as "partially supported” due to impacts of water quality (poor transparency) on the reservoir’s fish populations. Fish consumption uses remain assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant monitoring in 2004. The sources of data for this assessment include (1) results of approximately 18 water quality monitoring events conducted at Rathbun Reservoir during the years 2006 through 2008 by the U.S. Army Corps of Engineers at up-lake Station RA-7 in the Chariton Arm of the reservoir, (2) an IDNR Fisheries Bureau analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir, and (3) results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004.
EXPLANATION: The Class A1 (primary contact recreation) uses of this arm of Rathbun Reservoir are assessed (monitored) as "not supported" due to poor water transparency that violates Iowa’s narrative water quality criteria protecting against “aesthetically objectionable conditions.” Data collected by the U.S. Army Corps of Engineers at the up-lake station (RA-7) in the Chariton Arm of Rathbun Reservoir suggest that extremely high levels of turbidity impair the Class A1 uses of this arm of Rathbun Reservoir. Using the median values from the approximately 18 samples collected as part of the ACOE survey during 2006 through 2008, Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll a, and Secchi depth are 70, 64, and 75, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth place this arm of the reservoir in the hypereutrophic category, while the index value for chlorophyll a places this lake in between the eutrophic and hypereutrophic categories. These index values suggest very high levels of phosphorus in the water column, extremely poor water transparency, and moderately high levels of chlorophyll a and suspended algae in the water. The relatively low TSI value for chlorophyll a compared to Secchi depth suggests that inorganic suspended solids are likely the cause of poor water transparency and the impairment in this segment of the lake.
For information on cyanobacteria at Rathbun Reservoir, see the assessment developed for the main lake basin (IA 05-CHA-0020-L-1).
The Class B(WW-1) aquatic life uses of this arm of Rathbun Reservoir are assessed as "partially supported" due to the impact or poor water quality (water transparency) on the lake’s fish populations. A comparison of water quality data to production of the primary sport fish at Rathbun Reservoir (crappie) suggests only “partial support” of the aquatic life uses of the Chariton Arm. 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.
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. Of the main arms of the reservoir, the Honey Creek Arm typically has the best crappie production and best water transparency (average Secchi depth of 0.66 m), followed by the Buck Creek and Chariton arms (average of 0.5 m), and South Fork Chariton arm (average of 0.3 m). The poorest crappie production occurs in the Chariton and South Fork arms. The reduced water transparencies and relatively poor crappie production in the arms of the reservoir is attributed primarily to greater influence of nonpoint source runoff from the reservoir’s watershed on the water quality in the reservoir’s arms. Additional sources of turbidity in the reservoir’s arms include re-suspension of bottom sediments and wave action. In summary, results of the analysis of fisheries and water quality data at Rathbun Reservoir suggest that (1) poor water quality (transparency) in adversely affects the reservoir’s sport fish populations, (2) the aquatic life uses of the Chariton and South Fork arms are impaired due to high levels of turbidity, and (3) improving water transparency through control of nonpoint source runoff (i.e., through implementation of best management practices (BMPs)) could provide significant benefits to the sport fishery of this reservoir. Based on this information, the Class B(WW-1) aquatic life uses of the Chariton Arm are assessed (monitored) as “partially supported.”
In contrast to the assessment based on water transparency, the ACOE lake monitoring data from 2006-2008 show relatively good chemical water quality in this arm of Rathbun Reservoir. These data show one violation of the Class B(WW-1) criteria for dissolved oxygen in 16 samples (6%) and one violation of the Class A1,B(WW-1) criterion for pH in the 16 samples collected (6%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(WW-1) uses.
Results of monitoring for atrazine and nitrate suggest low levels of these contaminants in the Chariton Arm of Rathbun Reservoir. The average level of atrazine in the 12 samples collected from ACOE station RA-7 from 2006-08 (1.8 ug/l) was well-below the MCL of 3 ug/l. Only one of the 12 samples exceeded the atrazine MCL (maximum value was 4.9 ug/l on May 21, 2007. The average level at Station RA-7 in the Chariton Arm was somewhat higher than the average at station RA-3 near the dam (1.4 ug/l) and likely reflects inputs of atrazine from the watershed. Based on approximately eight years of monitoring, the typical pattern for atrazine levels in the Rathbun Reservoir and its watershed is occurrence of relatively low levels of atrazine near the dam, with progressively higher levels proceeding uplake and upstream into the lakes tributaries.
Results of ACOE monitoring suggest very low levels of nitrate in Rathbun Reservoir. The average level of nitrate in the 19 samples collected at Station RA-7 from 2006-08 was 0.7 mg/l; the maximum sample value during this period was 2.2 mg/l. These results are far below the nitrate MCL of 10 mg/l. The results of monitoring for atrazine and nitrate in this segment of Rathbun Reservoir do not suggest any threats to full support of the Class C uses of the Chariton River downriver from Rathbun Dam.
Fish consumption uses of the entire reservoir remain assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (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. 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.
