Hewett Creek IA 01-VOL-307

mouth (S12 T92N R6W Clayton Co.) to south line of S29 T92N R6W Clayton Co.

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.
Trend: Stable
Created: 10/27/2025 3:09:23 PM
Updated: 1/9/2026 3:31:15 PM

Use Support

Class A1

Presumptive Use

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

5p - Impairment occurs on a waterbody with a presumptive A1 or B(WW1) use.

Cause Magnitude

High

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2014

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Watershed project monitoring

TMDL Priority

Tier III

Class A2

Recreation - Secondary contact

Not Supported

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

2014

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Watershed project monitoring

TMDL Priority

Tier III

Class BCW1

Aquatic Life - Cold Water Type 1

Not Supported

Temperature: Thermal Modifications

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2014

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Watershed project monitoring

TMDL Priority

Tier IV

Biological: low aquatic macroinvertebrate IBI

Support Level

Not Supported

Impairment Code

5b-v - Biological - verified

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2014

Impairment Rationale

Low Biotic Index

Data Source

Biological monitoring: Iowa DNR WQMA

TMDL Priority

Tier IV

Class HH

Human Health -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The presumptive Class A1 and the Class A2 uses remain assessed as “not supported” due to levels of indicator bacteria (E. coli) that exceeded state water quality criteria. The Class BCW1 use was assessed as "not supported" due to violations of the Class BCW1 criterion for temperature and also based on biological sampling data. 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 2022	15220013	Hewett Creek @ Crystal Road (HEW20)
Iowa DNR	6	BIO	See Below	435	Hewett Creek	Volga – REMAP #133

Presumptive Class A1 - Indicator Bacteria:

Site ID	Data Source ID	# Samples per Year (2022)	2022 Geometric Mean	Violates Annual Geometric Mean?	# 2022 SSM violations / SSM Violations needed for impairment	Violates Annual SSM Significantly >10% Rule?	Assessment Type	Support Level
15220013	6	6	962	YES	6 / NA	YES	EVALUATED	NOT

Class A2 - Indicator Bacteria:

Site ID	Data Source ID	# Samples per Year (2022)	2022 Geometric Mean	Violates Annual Geometric Mean?	# 2022 SSM violations / SSM Violations needed for impairment	Violates Annual SSM Significantly >10% Rule?	Assessment Type	Support Level
15220013	6	6	962	YES	1 / NA	NO	EVALUATED	NOT

Presumptive Class A1 & A2 – 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
15220013	6	6 / 1	8.06	8.33	8.18	0	3	NO	EVALUATED	FULL

Class HH - No Data

Class BCW1 - 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
15220013	6	Dissolved Oxygen	6 / 1	8.63	11.41	10.1	0	NA / NA	3	NO	EVALUATED	FULL
15220013	6	pH	6 / 1	8.06	8.33	8.18	0	NA / NA	3	NO	EVALUATED	FULL
15220013	6	Temperature	6 / 1	9.98	20.44	15.73	1	NA / NA	3	NO	EVALUATED	FULL

Class BCW1 - Biological Sampling:

Benthic Macroinvertebrates:

BioNet Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage/ Thermal	BM Sample Gear	CBI score	CBI Class	CBI BIT	CBI Pass/ Fail
435	9/15/14	8	52b	MS/CW	Hess	55	NA	60	Fail
435	8/4/14	8	52b	MS/CW	Hess	53	NA	60	Fail

Overall Assessment:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Fail (0/2)	Not	5bv	Assessment was evaluated because samples used to complete the assessment weren’t collected in the last five years.

Class BCW1 – Continuous Monitoring:

Additional continuous monitoring data collected in 2013 and 2014 for the DNR/SHL coldwater stream sampling project were examined for stream temperature criteria violations. The maximum stream temperature recorded during the sensor deployment period was 23.8 degrees (C). The BCW1 temperature criterion of 20 (C) was exceeded in 40 of 126 days. The percentage exceeding (31.7%) was greater than (failing) the 10% impairment threshold with high statistical confidence (>90%). Based on these data, the BCW1 aquatic life use was assessed as impaired due to elevated stream temperature levels. This assessment agreed with the temperature assessment completed using grab water sampling data from watershed project monitoring in 2012-2014, which also indicated a thermal impairment.

Aquatic Life Use - Trout Natural Reproduction

Based on the latest trout reproduction in Iowa streams summary prepared by the DNR Fisheries Bureau, the aquatic life use was assessed as "fully supported." According to the summary, a portion, or all, of this assessment unit was placed in Category I: streams that exhibit consistent natural reproduction and maintain a wild population of one or more trout species without any stocking.

Monitoring and Methods

Assessment Key Dates

1/1/2025

Biological Monitoring

8/4/2014

Biological Monitoring

9/15/2014

Biological Monitoring

4/21/2022

Monitoring Start

9/13/2022

Monitoring End

Methods

150

Monitoring data more than 5 years old

200

Physical/Chemical Monitoring

320

Benthic macroinvertebrate surveys

330

Fish surveys

420

Indicator bacteria monitoring