waters were done primarily by geochemists, environmental engineers, and lake scientists interested in quantifying fluxes of minerals and pollutants from watersheds or into standing bodies of water (lakes and the oceans), and there was little interest in studying the chemical processes of streams and rivers themselves.

Historically, stream science has been identified most strongly with stream biology and stream ecology. Stream biologists often are identified primarily with other parent disciplines such as public health microbiology, fisheries biology, and aquatic entomology. Stream biology was mostly descriptive through the first half of the twentieth century and focused on the distribution and taxonomy of stream organisms. The development of stream ecology or stream limnology as a discipline analogous to lake limnology grew out of initiatives that began in the 1950s and 1960s. Hynes' 1970 book The Ecology of Running Waters is usually regarded as the first book on stream ecology (see Box 2-9).

The use of benthic invertebrates as indicator organisms for organic pollution and the division of streams into zones of pollution and recovery based on the presence of indicator species or groups of organisms have been major driving forces in stream biology since the early twentieth century; the first stream classification system based on the benthic organism species composition was the European "Saprobien" system of Kolkwitz and Marsson (1908, 1909). This paradigm stimulated much research on the structure of stream communities through the middle of this century. It can be considered a precursor of broader and more recent classification schemes and indices of biological integrity and biodiversity, which are currently popular subjects for research in stream ecology (see the background paper "Bringing Biology Back into Water Quality Assessments" at the end of this report).

Stream scientists have developed several organizing principles in recent decades to integrate the separate physical, chemical, and biological disciplines that contribute to studies of streams as ecosystems. The River Continuum Concept (RCC) (Vannote et al., 1980) is the most important of these. The RCC describes river systems as a continuously integrated series of physical changes that cause adjustments in the associated biota (Cummins et al., 1995). Geomorphological and hydrological characteristics of rivers provide a fundamental physical template that changes in a predictable fashion from the headwaters to the river mouth. Biological communities develop in adaptation to the fundamental physical template. The concept thus has a watershed orientation and focuses on terrestrial and aquatic interactions.

Since its development, the RCC has been modified in many ways to accommodate a broad range of factors, such as climate, geology, tributary effects, and local geomorphology, that influence streams. In addition, several organizing concepts developed as alternatives to the RCC have