Enormous meandering rivers are the most remarkable fluvial feature of the Amazon landscape, but these rivers are only the largest component of a much denser network of streams which finely dissects and drains the basin. In terms of combined length and total amount of lotic habitat, streams dominate over their more visible downstream counterparts; this dominance is especially dramatic for first- and second-order streams which alone may account for greater than 80% of total channel length in meso-scale Amazon drainag basins. The flow of Amazon streams emerges directly from the extensive forests and savannas that compose the basin. Biogeochemical cycles in streams are thus intricately associated with processes operating in adjoining riparian and upland ecosystems. Terrestrial processes regulate the input of organic and inorganic species to stream systems, and the chemistry of inflowing waters determines, to some extent, the nature of subsequent reactions and even the composition of the stream’s biological community (Fittkau 1971). Undisturbed Amazon streams are thought to experience virtually no primary production (Walker 1995), thus most inputs of energy, as well as nutrients, must ultimately derive from terrestrial sources. This connection is particularly acute in first-order streams where there is no upstream input and all water, particulates, and solutes derive from immediately adjacent to the stream. Pathways linking the two systems include groundwater runoff, surface and subsurface storm runoff, wetland seepage, direct litterfall, and litter blow-in. These pathways are active across the entire Amazon basin, but their relative importance may vary regionally (McClain and Richey 1996, Elsenbeer and Lack 1996). Riparian ecosystems continue to influence the biogeochemistry of downstream reaches, but as streams become rivers upstream and in-channel influences become increasingly dominant. Streams and the corridors through which they flow also play a crucial role in regional-scale biogeochemical cycles. Greater than 90% of all terrestrial to lotic transfers in the Amazon basin occur in streams of order 6 and less. Thus, organic and inorganic species moving from terrestrial systems to large rivers and ultimately to the ocean must first pass through streams, where rates of material cycling and processing are rapid.