human features. This assessment is designed to fill crucial gaps in existing data for each study unit. The design of water-quality investigations conducted during the occurrence and distribution assessment represents a balance between maximum flexibility of study units to target issues of local importance, and national consistency in constituents measured, sampling approaches, and spatial and temporal resolution to allow for comparisons among study units. The occurrence and distribution assessment serves as a basis for designing field activities to evaluate long-term changes in water-quality conditions and studies of source, transport, fate, and effects.
Assessment of long-term trends and changes in selected water-quality characteristics will be designed on the basis of results of the retrospective analyses, reconnaissance, occurrence and distribution assessment, and the concurrent development of information on the environmental framework. Temporal (for example, decadal) changes in the relations among physical, chemical, and biological factors will be interpreted in the context of changes in landscape features and human activities.
Source, transport, fate, and effect studies are conducted to test hypotheses and examine specific issues about characteristics and causes of water-quality alteration. These studies are targeted for high-priority water-quality issues for individual study units and the Nation. The accumulation of results from these studies among study units enables the linking of broad assessments of status and trends to specific causes and processes by example and inference. Source, transport, fate, and effect studies a redesigned by and unique to individual study units and are conducted at a wide range of spatial and temporal scales.
The stream habitat sampling design provides a framework for characterizing stream habitat at multiple spatial scales. The challenge of integrating stream habitat data into an assessment of physical, chemical, and biological factors relates to the problem of spatial scale when studying habitat. Hynes (1975) supported the view that relations among physical, chemical, and biological components of streams are determined not just within the context of a stream, but also within the broader context of the surrounding watershed. Therefore, to adequately examine the relations among physical, chemical, and biological attributes of streams, evaluating stream habitat must be accomplished within a systematic framework that accounts for multiple spatial scales.
A framework for evaluating stream habitat must be based on a conceptual understanding of how stream systems are organized in space and how they change through time (Lotspeich and Platts, 1982; Frissell and others, 1986). Among physiographic regions, or among streams within a region, different geomorphic processes control the form and development of basins and streams (Wolman and Gerson, 1978). Therefore, researchers have