Hydrology is complicated, and water quality is more than miles of impaired streams; it is with sympathy for the Forest Service that we do it their way. Thus, the map here shows all inventoried and impaired waterbodies by impairment class for some arbitrary wilderness -- the Frank Church–River of No Return. Streams, lakes, and catchment areas are shaded by impairment category from the 2022 assessment cycle.

Data shown is from Assessment, Total Maximum Daily Load (TMDL) Tracking and Implementation System, which has been around a while and has inventories back to 1998 in some places.

Click the map to select a catchment area.

The bar chart here shows catchment counts by impairment category for the available assessment cycles. At most, we have assessments for assessment units, while our map above shows units, which includes some overlapping catchments (and some holes where catchments should be feeding a stream). What's with the gaps?

Grouping the catchment area records together by assessment id and coloring them by NHD reach, we see that each catchment can contain many NHD reaches -- 187 in one case.

EPA's catchment data duplicates the whole catchment record for each NHD reach, so we'll group by assessment unit and dissolve overlapping catchment polygons, and circle back and recreate our clickable map to list all NHD reaches instead of just the first one.