This layer was developed to classify sites for the Water Resources Division Procedure 51 scoring process. The classifications were partly derived from EPA's StreamCat Dataset catchments and Level 3 Ecoregions, where ecoregion, catchment slope, and wetland composition, are components of the site class determination.

The north region is comprised of catchments within level 3 Ecoregion 50: Northern Lakes and Forests and Ecoregion 51: North Central Hardwood Forests. Monitoring sites within the north region are classified according to stream width and percent coverage of wetland and open water land cover types according to the table below.

The south region is comprised of catchments with level 3 Ecoregion 55: Eastern Corn Belt Plains, Ecoregion 56: Southern Michigan/Northern Indiana Drift Plains and Ecoregion 57: Huron/Erie Lake Plains. Longitude -83.72 is used to divide the East classification from both WestFlat and WestSteep classifications which are differentiated by percent slope of catchments according to the table below.

Please note that catchments whose boundaries intersected the north/south ecoregion boundary and the -83.72 longitude were split accordingly. These fragmented catchments retained their original % Wetland and % slope values from the StreamCat dataset for classification purposes . These values are not representative of the actual composition within those areas , they correspond to the original catchment boundaries.