Decatur Lake IA 06-WEM-1727

Monona County S17T83NR46W 6 mi. WSW of Onawa.

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 2014 - 2016
Stream/River Water Quality Data	CY 2014 - 2016
Lake Water Quality Data	CY 2012 - 2016
Biological Sampling Data	CY 2012 - 2016*
Fish Kill Data	CY 2012 - 2016
Fish Tissue Sampling Data	CY 2012 - 2016
Continuous Dissolved Oxygen Sampling Data	CY 2012 - 2016
*Biological sites sampled in both 2011 and 2015 were also assessed.

Open Segment Map
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Cycle: 2018
Release Status: Final
Overall IR: 3 - Insufficient data exist to determine whether any designated uses are met.
Trend: Unknown
Created: 6/13/2019 12:56:16 PM
Updated: 6/13/2019 12:56:29 PM

Use Support

Class A1

Recreation - Primary contact

Not Assessed

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Flow Alteration

Support Level

Further Investigation Needed

Impairment Code

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

Cause Magnitude

Slight

Status

Continuing

Source

Multiple Sources

Source Confidence

Low

Cycle Added

2002

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Biological monitoring: Iowa DNR Wildlife

Class HH

Human Health -

Not Assessed

General Use

General Use water -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 (primary contact recreation) uses remain "not assessed" due to the lack of water quality information upon which to base an assessment. The Class B(LW) aquatic life uses remain assessed (evaluated) as "not supported" (IR Category 3b) based on information from the IDNR Wildlife Bureau. This assessment was also based on results of watershed modeling conducted by IDNR staff in 2002. Fish consumption uses remain "not assessed." This assessment is the same as that developed for previous assessment/listing cycles. Note: this assessment has not been updated following the severe flooding of the Missouri River during summer 2011 and thus does not reflect flood-related changes that may have occurred at this floodplain wetland.

Assessment Explanation

The Class A1 (primary contact recreation) uses remain "not assessed" due to a lack of water quality information upon which to base an assessment. In general, Iowa wetlands are not managed to support primary contact recreation.

The Class B(LW) aquatic life uses remain assessed (evaluated) as "not supported" based on information from the Iowa DNR Wildlife Bureau (see assessments developed for the 1998 and 2000 reporting cycles). Decatur Lake is part of an Army Corps of Engineers mitigation project along the Missouri River; this project will result in water being pumped into the wetland thus correcting, or at least mitigating, the flow alteration impairment. Watershed modeling completed in 2002 for Decatur Lake indicates that sediment delivery to this wetland from the watershed is very low. The local DNR biologist indicates that the Missouri River does not breach the original high bank, but occasionally (10 -15 years) flood waters do enter the wetlands directly adjacent to the river. An updated assessment for this wetland will be needed following completion of the mitigation project.

The assessment category for this wetland of "evaluated" indicates that the assessment is based entirely on "best professional judgment."In terms of Section 305(b) reporting, "monitored assessments" are based primarily on recent, site-specific ambient monitoring data and thus have relatively high confidence. DNR considers waterbodies identified as "impaired" based on "monitored assessments" as candidates for the state's Section 303(d) list. "Evaluated assessments" are those based on data older than five years or other than site-specific ambient monitoring data (e.g., questionnaire surveys of fish and game biologists [=best professional judgment]) and thus have relatively low confidence. DNR does not consider waterbodies identified as "impaired" based on "evaluated assessments" as candidates for the state's Section 303(d) list.

Fish consumption uses remain "not assessed."In general, Iowa wetlands are not managed for recreational fisheries; thus, fish contaminant monitoring is not typically conducted at these waterbodies.

Note: Flow alteration multiple sources are listed as;Bank or Shoreline Modification/Destabilization, Hydromodification and Natural Sources.

Monitoring and Methods

Assessment Key Dates

None listed

Methods

120

Surveys of fish and game biologists/other professionals

180

Screening models (desktop models/models not calibrated or verified)