to show patterns or trends in the state of the environment;
to show patterns or trends in the human activities that affect, or are affected by, the state of the environment;
to show relationships among environmental variables; or
to show relationships between human activities and the state of the environment.
This definition of an environmental indicator is purposely very broad to reflect the diversity of assessment and reporting contexts in the term as used. Thus the definition includes both measured or observed variables and composite indicators that aggregate a number of variables into a single quantity.
An example of a possible indicator of environmental quality is suspended sediment concentrations in the nation's rivers and streams. Figure 2 is a graphical display showing this measure as obtained from the USGS's NASQAN data base.
About 10 percent of NASQAN stations showed decreased suspended sediment concentrations over the sampling period 1980–1989. The quantity of suspended sediment transported to coastal waters decreased or remained the same in all but the North Atlantic region.
Assessments of environmental quality can utilize either primary data collected specifically for the purpose for which they are used or secondary data that