Desoto Bend IA 06-WEM-1716

Harrison County S21T78NR45W 5 mi. W of Missouri Valley.

Open Data Collection Period Modal

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2018 - 2020
Stream/River Water Quality Data	CY 2018 - 2020
Lake Water Quality Data	CY 2016 - 2020
Biological Sampling Data	CY 2016 - 2020*
Fish Kill Data	CY 2016 - 2020
Fish Tissue Sampling Data	CY 2016 - 2020
Continuous Dissolved Oxygen Sampling Data	CY 2016 - 2020
*Biological sites sampled in both 2015 and 2019 were also assessed.

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Cycle: 2022
Release Status: Final
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Hypereutrophic
Trend: Stable
Created: 1/4/2022 2:51:38 PM
Updated: 1/28/2022 8:04:28 AM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Turbidity

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

Moderate

Cycle Added

2004

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier IV

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Natural: Internal Nutrient Recycling (Primarily Lakes)

Source Confidence

Low

Cycle Added

2016

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier IV

Class BLW

Aquatic Life - Lakes and wetlands

Fully Supported

Class HH

Human Health -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed (monitored) as “not supported” due to poor water transparency caused by the presence of large amounts of suspended sediment and aesthetically objectionable conditions caused by algae blooms. The Class BLW use was assessed (monitored) as “fully supported” although siltation remains a concern. All other designated uses were “not assessed.”

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2016-2019	22430001	DeSoto Bend Lake

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
22430001	6	10 / 4	6.9	8.4	8	0	3	NO	Monitored	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
22430001	6	Chlorophyll a TSI	10 / 4	32	72	62	NO	Monitored	Full
22430001	6	Secchi TSI	10 / 4	70	77	73	YES	Monitored	Not
22430001	6	Total Phosphorus TSI	10 / 4	72	82	80	NA	NA	NA
22430001	6	Inorganic suspended solids	10 / 4	4	20.3	8.5	NA	NA	NA

Although the index value for Chlorophyll a was below the impairment trigger of 65 for this assessment cycle, the Class A1 use of DeSoto Bend Lake remains "not supported" due to aesthetically objectionable conditions. Based on DNR's methodology, the median TSI value for Chlorophyll a must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). Therefore, DeSoto Bend Lake will remain listed as "not supported" for the 2022 assessment/listing cycle.

Class BLW - 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
22430001	6	Ammonia	10 / 4	0.0003	0.328	0.04	NA	0 / 0	3	NO	Monitored	Full
22430001	6	Dissolved Oxygen	10 / 4	5.3	10.7	7.6	0	NA / NA	3	NO	Monitored	Full
22430001	6	pH	10 / 4	6.9	8.4	8	0	NA / NA	3	NO	Monitored	Full
22430001	6	Temperature	10 / 4	16.5	28.3	23.6	0	NA / NA	3	NO	Monitored	Full

Class BLW - Lake Planktonic Cyanobacteria:

Site ID	Data Source ID	# Samples	Maximum Cyanobacteria Wet Mass	Maximum Cyanobacteria % Composition	Median Cyanobacteria Wet Mass	Median Cyanobacteria % Composition	Median Cyanobacteria Concentration = 75th Percentile	Assessment Type	Support Level
22430001	6	5	16.45	44.6%	2.01	23.5%	NO	Evaluated	NA

Class HH: No Data

The Class HH "full support" based on fish tissue monitoring was removed based on the age of the data.

Additional Notes: According to Iowa DNR Fisheries Biologists, DeSoto Bend was impacted by record flooding in 2011 on the Missouri River include the introduction of silver and bighead carp. The channel catfish population benefited the most from the 2011 flooding. Quality sized white bass have returned to the fishery in 2014. Walleye stocking continues on an annual basis in an effort to restore their numbers. Common carp are common in DeSoto Bend and have a negative impact on the aquatic plant community and water quality.

Monitoring and Methods

Assessment Key Dates

5/18/2016

Non-fixed Monitoring Start Date

9/10/2019

Non-fixed Monitoring End Date

Methods

120

Surveys of fish and game biologists/other professionals

222

Non-fixed-station monitoring (conventional during key seasons and flows)

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)