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In recent decades substantial progress has been made in improving the quality of surface waters in the United States (Hawkins et al., 2000; EPA, 2000; EPA, 2001); nevertheless, much work remains to be done in assessing the state of impairment of lake waters. Impairment implies that the existing water quality of a lake, as measured by selected criteria (e.g., nitrogen, phosphorus, chlorophyll-a, Secchi depth), exceeds a threshold value or standard that presumably reflects optimal attainable lake water quality conditions (or "reference" conditions) (Hughes, 1995; EPA, 2000; EPA, 2001). Such impaired waters are not suitable for designated uses such as drinking, irrigation, recreation or fishery (Carpenter et al., 1998). The management of lake water quality requires an effective means to establish which lakes are most impaired (and, hence, may require restoration) and which lakes are least impaired.
It is estimated that about 43 percent of the 16.4 million hectares comprising the United States' lake area have been adequately assessed for water quality (EPA, 2000). Of the lakes that have been assessed, 45 percent are "impaired" and 9 percent of the impaired lakes are listed as threatened. Nutrients exported from agricultural lands contribute about 50 percent of water quality problems in impaired lakes (Figure 1.1) (EPA, 2000). Water quality standards are particularly difficult to establish for lakes located in areas highly modified by humans, such as agricultural landscapes of the Midwest. In these areas (a) few, if any, lakes may represent pre-settlement "reference" conditions, and (b) many lakes are human constructed (e.g., reservoirs). The principal objective of this research is to develop and evaluate an approach for establishing lake water quality standards using watershed-based classification of lakes.