Assessment Summary
The Class A1 use of the Chariton Arm was assessed (monitored) as “not supported” due to very poor water transparency caused by algal and 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
|
99990024
|
Chariton
River Bay (RA-7) in the Chariton 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
|
|
99990024
|
12
|
5 / 2
|
7.85
|
8.96
|
8.48
|
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
|
|
99990024
|
12
|
Chlorophyll a TSI
|
28 / 5
|
32
|
81
|
59
|
NO
|
Monitored
|
Full
|
|
99990024
|
12
|
Secchi TSI
|
26 / 5
|
59
|
87
|
76
|
YES
|
Monitored
|
Not
|
|
99990024
|
12
|
Total Phosphorus TSI
|
28 / 5
|
63
|
92
|
74
|
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
|
|
99990024
|
12
|
Ammonia
|
5 / 2
|
0.01
|
0.15
|
0.06
|
NA
|
0 / 0
|
2
|
NO
|
Evaluated
|
Full
|
|
99990024
|
12
|
Dissolved Oxygen
|
26 / 5
|
5.99
|
11.72
|
8.84
|
0
|
NA / NA
|
6
|
NO
|
Monitored
|
Full
|
|
99990024
|
12
|
pH
|
5 / 2
|
7.85
|
8.96
|
8.48
|
0
|
NA / NA
|
2
|
NO
|
Evaluated
|
Full
|
|
99990024
|
12
|
Temperature
|
27 / 5
|
11.4
|
29.7
|
22.6
|
0
|
NA / NA
|
6
|
NO
|
Monitored
|
Full
|
Class BWW1 - Additional Information:
The Class BWW1 aquatic life use of the Chariton Arm of Rathbun
Reservoir was assessed 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 impairment of the aquatic life
use of the Chariton 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 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 Chariton Arm was assessed (monitored) as “not supported.”
Class HH: No Data
