Connectivity in forested upland catchments and associated channel dynamics: The eastern Ruahine Range

摘要

This study relates connectivity of sediment sources to the channel to the high variability in short-term channel dynamics and stream bed stability in mountain stream reaches of the eastern Ruahine Range in New Zealand's North Island. Despite similar geographical and geological characteristics of the catchments upstream of these reaches, the morphological response to flood events ranges from almost none to frequent migration of the channel in a wide active channel zone. These dynamics are mainly driven by sediment supply from slopes and gullies over the last decade and from in-channel storage. The latter is particularly important where recent erosion on hill slopes has decreased. In the upper Waipawa catchment 22% of the area, dominantly gullies, could potentially supply sediment to the studied reach whereas in the Tamaki and Mangapuaka catchments, 3.7% and 1.9% of the area was affected by erosion and coupled to the respective study reaches. In contrast, in the Manawatu and Coppermine catchments this figure was less than 0.3%. In these potentially connected catchments where sediment supply is scarce, coupling between slopes and channel exerts an important control upon channel dynamics. In the connected catchments such as Tamaki,Mangapuaka and Waipawa, reach-to-reach connectivity is effective, with transport barriers such as large woody debris having a relatively short-term modulating effect on coarse sediment conveyance.