A study of 16 streams in eastern North America shows that riparian deforestation causes channel narrowing, which reduces the total amount of stream habitat and ecosystem per unit channel length and compromises in-stream processing of pollutants. Wide forest reaches had more macroinvertebrates, total ecosystem processing of organic matter, and nitrogen uptake per unit channel length than contiguous narrow deforested reaches. Stream narrowing nullified any potential advantages of deforestation regarding abundance of fish, quality of dissolved organic matter, and pesticide degradation. These findings show that forested stream channels have a wider and more natural configuration, which significantly affects the total in-stream amount and activity of the ecosystem, including the processing of pollutants. The results reinforce both current policy of the United States that endorses riparian forest buffers as best management practice and federal and state programs that subsidize riparian reforestation for stream restoration and water quality. Not only do forest buffers prevent nonpoint source pollutants from entering small streams, they also enhance the in-stream processing of both nonpoint and point source pollutants, thereby reducing their impact on downstream rivers and estuaries.

The coefficients of variation of daily discharge and the logarithm of daily discharge were calculated from long term records in 15 river systems, including runoff—fed prairie rivers in Illinois and Missouri, groundwater— and spring—fed upland rivers in Southern Ohio, Southern Illinois and the Missouri Ozarks, and the snowmelt—fed Powder River in Wyoming. In almost all cases variability was lower in downstream sections. The patterns of temporal variability were consistent with the hydrology of the basins, with source of flow and size important determinants. Diversity patterns of fishes were tabulated for the same rivers from literature records. In all rivers, diversity increased rapidly from upstream to downstream sections, almost entirely the result of addition of new species with little replacement of the upstream fauna. Maximal species richness was usually in the lowest sections (order VI or greater). Headwater diversity was lowest in the rivers with the most variable headwaters, the increase in diversity downstream steepest for those rivers with the steepest decrease in variability, and the number of species in downstream sections was greater in rivers with more constant downstream sections. Longitudinal replacement rates were very low in all rivers studied, but were generally larger in the most temporally constant rivers. Literal replacement was moderate between small tributaries, but was low between major tributaries. The diversity gradients of individual feeding groups were similar to the gradient for all species, with an increase in richness from headwaters downstream in all groups with intermediate maxima sometimes seen. Piscivores showed consistently steeper diversity increases. These observations were related to hypotheses of factors controlling stream fish distributions. They are most consistent with hypotheses relating control to extermination—recolonization dynamics or the interspecific competition mediated by temporal variability and habitat structure.

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