Dynamics of sediment and woody debris in headwater streams, southeast Alaska.

摘要

Headwater streams are the most important sources of water, sediment, nutrients, and organic matter for downstream systems. Timber harvesting and mass movement alter hydrologic, geomorphic and biological processes in stream channels and riparian zones of headwater systems. In particular, changes in abundance of woody debris and sediment related to timber harvesting and mass movement and the recovery processes for such disturbances affect the material dynamics and habitat conditions. Therefore, the amount and distribution of sediment and woody debris as well as bedload and suspended sediment transport for different management and disturbance regimes were examined in headwater streams of southeast Alaska. External influences (mass movement and timber harvesting) modified channel morphology and sediment transport from undisturbed old-growth conditions in different ways. In recent clear-cut channels, inputs of logging slash significantly increased the abundance of in-channel woody debris. In the absence of landslides and debris flows, woody materials remained in the channels 50 years after logging where young-growth confers (logged in 1950's) dominated the riparian zone. Woody debris related to logging activates initially stored sediment, created channel steps, and reduced sediment movement. When landslides and debris flows in 1962 (7 years after logging), woody debris pieces were transported from upper reaches of headwater streams and deposited in downstream reaches in recent landslide channels and in channels with young alder riparian stands. Because of the high sediment production from bank slopes, more bedload and suspended sediment was transported in recent landslide and debris flow channels. Once red alder actively re-colonized riparian zones 20 to 50 years after mass movement and then recruited woody debris and organic matter, greater amounts of woody debris and sediment storage behind woody debris were observed. The recovery processes related to vegetation regeneration on disturbed soil and woody debris recruitment into channels significantly decreased sediment transport. Temporal and spatial variations of availability of sediment and woody debris characterize processes and morphology in headwater streams. Such spatial and temporal variations in headwater systems are important for understanding organic and inorganic material dynamics through channel networks and evaluating the influence of timber harvesting on downstream ecosystems.