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Introduction Professor Noel Hynes first synthesized the concept of ecological connectivity in the context of river systems in his Baldi Lecture at the 19th Congress of the International Society for Pure and Applied Limnology (Hynes 1975). He eloquently described how rivers are a manifestation of the biogeochemical nature of the valleys they drain and he proposed that understanding the inherent connectivity between terrestrial and lotic biotopes would yield important predictions about factors that control the structure and function of river ecosystems. In the nearly two decades since that seminal lecture, several paradigms (reviewed by Cummins et al. 1984) emerged from scores of studies that examined spatial and temporal aspects of geomorphic, hydrologic, thermal, and riparian influences on biotic attributes (e.g., diversity, zonation, food web associations, bioproduction) of rivers. The river continuum concept (Vannote et al. 1980, Minshall et al. 1985) provided a template for examining how biotic attributes of rivers change within the longitudinal gradient from headwaters to ocean confluence. The serial discontinuity concept (Ward and Stanford 1983a) provided a construct for the propensity of rivers to predictably reset biophysical attributes as distance downstream from on-channel impoundments increases. Comparison of organic matter budgets in streams in different biomes provided the basis for the riparian control concept and demonstrated the extreme importance of allochthonous debris (wood and leaves) on the structure and function of lotic systems (Cummins et al. 1984, Harmon et al. 1986, Webster and Benfield 1986, Ward et al. 1990, Gregory et al. in press). The nutrient spiraling concept (Webster and Patten 1979, Newbold et al. 1983) led to an understanding of how plant growth nutrients are transformed from dissolved to particulate states during translocation from upstream to downstream reaches of stream ecosystems. Lastly, the ecotone concept (Naiman and DeCamp 1990, Holland et al. 1991) has fostered greater understanding of the extreme importance and potential predictive power related to transformations and fluxes of materials that occur within boundaries between functionally interconnected patches that form the riverine landscape. In many ways the ecotone concept integrates the other paradigms by emphasizing the functional connectivitiy inherent in all ecosystems. Studies articulating these paradigms and other syntheses of stream ecology (Lock and Williams 1981, Barnes and Minshall 1983, Dodge 1989, Stanford and Covich 1988, Yount and Nieme 1990), plus a great number of other research projects too numerous to mention here, have largely verified Hynes'. proposition that the 3