Coralville Reservoir IA 02-IOW-630

Johnson County S22T80NR6W (dam) 3 mi N of Iowa City.

This is a DRAFT assessment

Draft assessments may change without notice

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 2022 - 2024
Stream/River Water Quality Data	CY 2022 - 2024
Lake Water Quality Data	CY 2020 - 2024
Biological Sampling Data	CY 2020 - 2024*
Fish Kill Data	CY 2020 - 2024
Fish Tissue Sampling Data	CY 2020 - 2024
Continuous Dissolved Oxygen Sampling Data	CY 2020 - 2024
*Biological sites sampled in both 2019 and 2023 were also assessed.

Open Segment Map
View 2026 assessment summary page

Cycle: 2026
Current Phase: Public Comment
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Eutrophic
Trend: Stable
Created: 10/27/2025 3:09:32 PM
Updated: 1/12/2026 11:03:51 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

2006

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Corps of Engineers/ISU/UI

TMDL Priority

Tier II

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2020

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: Corps of Engineers/ISU/UI

TMDL Priority

Tier II

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

Continuing

Source

Agriculture

Source Confidence

Moderate

Cycle Added

2024

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Corps of Engineers/ISU/UI

TMDL Priority

Tier II

Class BWW1

Aquatic Life - Warm Water Type 1

WINOFI

Fish Kill: Due To Natural Causes

Support Level

Further Investigation Needed

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Slight

Status

New

Source

Natural

Source Confidence

High

Cycle Added

2026

Impairment Rationale

Potential Impairment

Data Source

Fish kill investigation: Iowa DNR

Class HH

Human Health -

Fully Supported

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed as “not supported” due to aesthetically objectionable conditions caused by poor water transparency due to inorganic turbidity, algal blooms, and due to high levels of indicator bacteria. The Class BWW1 use was also assessed (evaluated) as “WINOFI” due to a reported fish kill. The Class HH (fish consumption) use was assessed as "fully supported" based on fist tissue sampling data.

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
USACE	12	WQ	CY 2020-2024	3333333	Coralville Reservoir	Deep Point
USACE	12	WQ	CY 2022-2024	99990027	Sugar Bottom Beach (Coralville)	Beach
USACE	12	WQ	CY 2022-2024	99990028	West Overlook Beach (Coralville)	Beach
USACE	12	WQ	CY 2022-2024	99990121	Sugar Bottom Beach SC(Coralville)	Beach
Iowa DNR	6	FT	See Below	1288	Coralville Reservoir

Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples per Year (2022/ 2023/2024)

2022 Geometric Mean

2023 Geometric Mean

2024 Geometric Mean

Violates Annual Geometric Mean?

# 2022 SSM violations / SSM Violations needed for impairment

# 2023 SSM violations / SSM Violations needed for impairment

# 2024 SSM violations / SSM Violations needed for impairment

Violates Annual SSM Significantly >10% Rule?

Assessment Type

Support Level

3333333

9/8/10

0 / 3

MONITORED

FULL

99990027

16/20/18

2 / 4

1 / 5

2 / 4

MONITORED

FULL

99990028

18/18/18

3 / 4

0 / 4

4 / 4

YES

MONITORED

NOT

99990121

10/21/20

191

104

YES

4 / 3

4 / 5

7 / 5

YES

MONITORED

NOT

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
3333333	12	92 / 5	7.54	8.99	8.23	0	14	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
3333333	12	Chlorophyll a TSI	49 / 5	8	81	66	YES	MONITORED	NOT
3333333	12	Secchi TSI	78 / 5	62	93	74	YES	MONITORED	NOT
3333333	12	Total Phosphorus TSI	35 / 2	69	93	80	NA	NA	NA

Class BWW1 - Binomial 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
3333333	12	Dissolved Oxygen	92 / 5	1.07	20.26	9.34	10	NA / NA	14	NO	MONITORED	FULL
3333333	12	pH	92 / 5	7.54	8.99	8.23	0	NA / NA	14	NO	MONITORED	FULL
3333333	12	Temperature	92 / 5	0.2	29.8	17.1	0	NA / NA	14	NO	MONITORED	FULL

Class BWW1 - Toxic Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Acute / Chronic Violations	>1 Acute / Chronic Violation	Assessment Type	Support Level
3333333	12	Ammonia	89 / 5	0.79	0.2	0.14	0 / 0	NO	MONITORED	FULL

Class BLW - Fish Kill:

The Class BLW use was assessed (evaluated) as “WINOFI” due to fish kill that occurred in July of 2024. Because the cause of the fish kill was not identified, or the fish kill was attributed to a non-pollutant cause (e.g., winterkill), the assessment type is “evaluated.” Segments assessed using “evaluated” assessments are placed in Category 3b and are added to the state list of waters in need of further investigation. If no additional fish kills are reported in the segment for a 5 year period, the Category will be changed from 3b to 3a (water not assessed).

Details about the 2024 fish kill can be found here: https://programs.iowadnr.gov/fishkill/events/1062

Class HH - Fish Tissue Sampling:

Site ID	Biopart	Year	Species	# in sample / # of samples	Avg Length (cm)	(Avg) Hg (ppm)	Chlordane (ppm)	Sum PCBs (ppm)
1288	tissue plug	2021	walleye	2		0.071
1288	tissue plug	2020	crappie spp.	10	27.9	0.087
1288	tissue plug	2020	flathead catfish	4	50.3	0.093
1288	tissue plug	2019	walleye	2	49.9	0.081
Support Level	Full

Monitoring and Methods

Assessment Key Dates

1/6/2020

Monitoring Start

12/2/2024

Monitoring End

7/1/2020

Fish Tissue Monitoring

7/1/2019

Fish Tissue Monitoring

6/3/2021

Fish Tissue Monitoring

7/8/2024

Fish Kill

Methods

140

Incidence of spills and/or fish kills

200

Physical/Chemical Monitoring

260

Fish tissue analysis

420

Indicator bacteria monitoring