Segments, waterbodies, and assessment units are the various terms used in the assessment process to describe the portions of rivers, streams, lakes, and wetlands in Iowa.
The key concept for a segment is: A segment is considered a contiguous entity where the Iowa DNR can reasonably say that the water in any given part of the segment 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 segments to maintain units of similar quality; however, most lakes and wetlands in Iowa are a single segment. Similarly, most of Iowa’ small streams are a single segment (e.g. headwater and many coldwater streams) whereas medium/larger sized streams and rivers (e.g. Keg Creek and Iowa River) are broken up into smaller segment lengths to maintain units of similar quality.
Each segment is assigned two or more Designated Uses depending on how the segment 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 segments in Iowa and their associated use classifications. Iowa DNR’s online assessment database, ADBNet, contains all the segments found in the SWC but also many segments 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 segments, 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.
Segments and their designated uses are independent of neighboring segments and uses. For example, the Des Moines River is a large river running across the state. It is made up of many individual segments (preserving the key concept of a segment). An impairment on one segment does not mean the whole river is impaired- just that particular portion and that particular use.
Many existing segments in Iowa are not assessed every cycle because water quality monitoring has not been conducted in the segments; therefore, we do not have the information to determine whether or not they are impaired. Additionally, due to the lack of sampling data, there are also many segments, especially the headwaters of streams, small ponds and wetlands, that are not delineated as assessment segments nor assigned designated uses.