Assessment Units

Assessment Units (AUs), segments, and waterbodies are the various terms used in the assessment process to describe the portions of rivers, streams, lakes, and wetlands in Iowa. An Assessment Unit is considered a contiguous entity where the Iowa DNR can reasonably say that the water throughout the Assessment Unit will have similar quality and characteristics as water in any other part.

Some of Iowa’s larger lakes and flood control reservoirs (e.g. West Lake Okoboji and Lake Red Rock) are broken into multiple Assessment Units to maintain portions of similar quality; however, most lakes and wetlands in Iowa are a single Assessment Unit. Similarly, most of Iowa’ small streams are a single Assessment Unit (e.g. headwater and many cold-water streams) whereas medium/larger sized streams and rivers (e.g. Beaver Creek and Iowa River) are broken up into smaller Assessment Units to maintain portions of similar quality.

Each Assessment Unit is assigned two or more Designated Uses depending on how the waterbody is used. Iowa DNR’s Water Quality Standards are the rules that explain the designated uses and specify how the uses are protected. Iowa Water Quality Standards help ensure that all Iowans have surface waters that are fishable and swimmable to the fullest extent practicable, safe drinking water, groundwater that is free from harmful contamination, and water resources that are assigned proper designated uses.

The Surface Water Classification document (SWC) contains the list of Assessment Units in Iowa and their associated use classifications. Iowa DNR’s online assessment database, ADBNet, contains all the Assessment Units found in the SWC but also many Assessment Units not yet added to the SWC. The SWC is a rule-referenced document and it must be updated through annual rule revisions to the Iowa Administrative Code. These newly created Assessment Units, absent a Use Attainability Assessment, are assigned presumptive Class A1 primary contact recreation and Class BWW1 warmwater aquatic life designated uses. According to Iowa Water Quality Standards, all perennial streams and intermittent streams with perennial pools that are not specifically listed in the SWC are designated as Class A1 and Class BWW1 waters.

Retired Assessment Units

• Surface Water Classification (SWC) and/or Use Attainability Assessments may determine that an Assessment Unit should be broken into one or more new AUs with different uses
• An AU may not meet the definition of a Assessment Unit above, or may be found to have no water (more common in the headwaters of small streams and wetlands that have gone dry)
• The Assessment Unit is wholly or mostly within Tribal boundaries. The State of Iowa does not have the authority to assess waters that are on Tribal lands- the tribes themselves have primacy in these cases.
• Error - sometimes an Assessment Unit is placed in the wrong location and needs to be removed.

Retired AUs are documented on the Retired Assessment Units page, along with their metadata and assessment history. These retirements may affect the assessment counts found in assessment cycles that occurred prior to the segment being retired. That is, these numbers may be slightly different than those previously published by the Iowa DNR.