Environmental factors used to subdivide the western Lake Michigan drainages into relatively homogeneous units for water-quality site selection

In 1991, the National Water-Quality Assessment (NAWQA) Program was fully implemented by the U.S. Geological Survey (USGS). The goals of the NAWQA program are to (1) provide a nationally consistent description of water-quality conditions for a large part of the Nation’s water resources; (2) define long-term trends (or lack of trends) in water quality; and (3) identify, describe, and explain, as possible, the major factors that affect the observed water-quality conditions and trends (Hirsch and others, 1988).