Rathbun Reservoir - South Fork Arm IA 05-CHA-2027

For the 2018 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 U.S. Army Corps of Engineers at Station RA-8. Additionally information from the DNR 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 2012-2016 (approximately 30 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 82, 64, and 75 respectively for Rathbun Reservoir. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Rathbun Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest moderatley high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and very high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 13 samples.

For information on indicator bacteria, cyanobacteria and inorganic suspended solids 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 (monitored) 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 South Fork Arm. In 2006, the DNR 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 35-year 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 South Fork Arm are assessed (monitored) as “partially supported.”

Results of ACOE chemical/physical water quality monitoring in this arm of Rathbun Reservoir from 2012-2016 show generally good water quality. Results of this monitoring show there were no violations of the criterion for ammonia in 26 samples(0%), one violation of the criterion for dissolved oxygen in 28 samples(4%), and no violations of the criterion for pH in 13 samples(0%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(WW-1) uses of Rathbun Reservoir.

The average level of atrazine in the 12 samples collected from ACOE Station RA-8 in the South Fork Arm from 2012-2016 (2.25 ug/l) was below the atrazine MCL of 3 ug/l. Only one of the 12 samples exceeded the atrazine MCL; maximum value (3.6 ug/l). The average level at Station RA-8 in the South Fork Arm is higher than at station RA-3 near the dam (0.68 ug/l) and likely reflects inputs of atrazine from the watershed. 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 support of the Class C (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, with progressively higher levels proceeding uplake and upstream into the lakes tributaries.

Results of ACOE monitoring suggest very low levels of nitrate in the South Fork Arm. The average level of nitrate in the 29 samples collected from 2012-2016 was 0.81 mg/l; the maximum sample value during this period was 3.7 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.

The level of support of the Class HH-fish consumption uses is “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 / DNR 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 1: For the 2006, 2008, 2010, 2012, 2014, 2016, and 2018 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.]