Assessment Summary
The Class A1 use was assessed (monitored) as “not supported” due to extremely poor water transparency caused by non-algal turbidity that violated Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class BWW1 use was assessed (monitored) as “not supported” due to impacts of water quality (poor transparency) on the reservoir’s fish populations. All other designated uses were “not assessed.”
Note: A TMDL for turbidity at four segments of Rathbun Lake was prepared by DNR and approved by EPA in 2019. Because all Section 303(d) impairments identified for the current assessment/listing cycle are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
Assessment Explanation
Data Sources:
|
Data Source
|
Data Source ID
|
Data Type
|
Data Age
|
Site ID
|
Site
Name
|
Site
Description
|
|
USACE
|
12
|
WQ
|
CY 2016-2020
|
99990025
|
South
Fork Chariton River Bay Station (RA-8) in the South Fork Arm of the reservoir
|
|
Class A1 - Indicator Bacteria: No Data
Class A1 - pH:
|
Site ID
|
Data Source ID
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Mean Value
|
# Violations
|
Violations needed for impairment
|
Violates Significantly >10% Rule?
|
Assessment Type
|
Support Level
|
|
99990025
|
12
|
5 / 2
|
7.88
|
8.59
|
8.28
|
0
|
2
|
NO
|
Evaluated
|
Full
|
Class A1 - Lake TSI's:
|
Site ID
|
Data Source ID
|
Parameter Name
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Median Value
|
Median TSI Violation?
|
Assessment Type
|
Support Level
|
|
99990025
|
12
|
Chlorophyll a TSI
|
28 / 5
|
44
|
74
|
62
|
NO
|
Monitored
|
Full
|
|
99990025
|
12
|
Secchi TSI
|
26 / 5
|
70
|
93
|
83
|
YES
|
Monitored
|
Not
|
|
99990025
|
12
|
Total Phosphorus TSI
|
28 / 5
|
72
|
89
|
80
|
NA
|
NA
|
NA
|
Class BWW1 - Conventional Parameters:
|
Site ID
|
Data Source ID
|
Parameter Name
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Mean Value
|
# Violations
|
# Acute / Chronic Violations
|
Violations needed for impairment
|
Violates Significantly >10% Rule?
|
Assessment Type
|
Support Level
|
|
99990025
|
12
|
Ammonia
|
5 / 2
|
0.01
|
0.16
|
0.08
|
NA
|
0 / 0
|
2
|
NO
|
Evaluated
|
Full
|
|
99990025
|
12
|
Dissolved Oxygen
|
26 / 5
|
3.83
|
13.21
|
7.44
|
2
|
NA / NA
|
6
|
NO
|
Monitored
|
Full
|
|
99990025
|
12
|
pH
|
5 / 2
|
7.88
|
8.59
|
8.28
|
0
|
NA / NA
|
2
|
NO
|
Evaluated
|
Full
|
|
99990025
|
12
|
Temperature
|
27 / 5
|
12
|
29.5
|
22.7
|
0
|
NA / NA
|
6
|
NO
|
Monitored
|
Full
|
Class BLW - Additional Information:
The Class BWW1 aquatic life use of this arm of Rathbun
Reservoir was assessed (monitored) as “not 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 an impairment of the aquatic life use 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 BWW1 aquatic life use of the South Fork Arm was assessed (monitored) as “not supported.”
Class HH: No Data
