Influence of Headwater Streams on Downstream Reaches in Forested Areas

The source areas of headwater streams typically compose 60% to 80% of a catchment. This, plus the typical increase in precipitation with elevation, means that headwater streams generate most of the streamflow in downstream areas. Headwater streams also provide other important constituents to downstream reaches, including coarse and fine sediment, large woody debris, coarse and fine organic matter, and nutrients. The relative importance of headwater streams as a source of these other constituents is highly variable because the amount and quality of each constituent can be modified by in-channel storage, dilution, biological uptake, diminution, and chemical transformations. Headwater sources of water, fine sediment, and fine particulate organic matter are more likely to be delivered to downstream reaches than coarse sediment, woody debris, nutrients, or an increase in water temperatures. The complexity and temporal variability of channel-hillslope interactions, in-channel processes, and downstream conditions makes it difficult to rigorously link upstream inputs and anthropogenic activities to the condition of downstream resources. These issues may preclude the use of adaptive management, particularly in larger basins, as adaptive management implicitly assumes that (1) downstream changes can rapidly be detected, (2) management will change rapidly in response to any adverse change, and (3) a management change will rapidly improve the affected resource. Since these assumptions may be difficult to satisfy—particularly in larger basins—the use of adaptive management must be carefully examined before it can be applied at the watershed scale. FOR. SCI. 53(2):148–168.