Impaired Streams

Stream segments are categorized with designated uses, for example a stream used for kayaking and swimming would be designated for “recreation”. Streams may have multiple designated uses. Different water quality standards are set for each designated use. State agencies determine whether stream segments meet the water quality standards for any or all of their designated uses, classifying them in the five categories described below.

Category 1 – Attaining all designated uses
Category 2 – Attaining some designated uses, and insufficient or no data information to determine if remaining uses are attained
Category 3 – Insufficient or no data and information to determine if any use is attained
Category 4- Impaired or threatened for one or more uses but not needing a TMDL because: TMDL has been completed, or Expected to meet standards, or Not impaired by a pollutant
Category 5 – Impaired or threatened by pollutant(s) for one or more designated uses and requiring a TMDL. These are the waters entered onto a state’s 303(d) list.

Rio Grande, by Paul Tashjian.

How is it measured?

For all regions except Lower Rio Grande, the total length of non-impaired waters in the region (segments in Category 1 and Category 2) was divided by the total length of waters streams with available data (Category 1+ Category 2+ Category 4 + Category 5), then multiplied by 100. This percent of unimpaired stream length was used as the score for the indicator. For Lower Rio Grande, data were available separately for the New Mexico and Texas portions of the basin. The score was calculated separately for each, and the average of the two scores was used as the score for the Lower Rio Grande score.