Multi-scalar controls on channel geometry of headwater streams in New Zealand hill country

Abstract

Catchment-, reach-, and local-scale controls upon channel geometry are examined for small headwater streams in adjacent sub-catchments with variable land use histories in a typical hill country area of the North Island of New Zealand. Although systematic downstream increase in channel cross-sectional area is evident in one sub-catchment (a result of catchment-scale controls on geometry, including discharge and slope), marked local variability in channel geometry in the two other sub-catchments reflects non-systematic patterns of reach-scale controls including the distribution of floodplain pockets and associated bank materials, and local-scale controls such as bedrock outcrops and wood. Land use change exerts a primary control upon these relationships. For a given catchment area, streams in native forest are wider and shallower than streams that flow within pasture or Pinus plantations. Increased hillslope erosion following land clearance has instigated the accumulation of post-settlement alluvium (PSA), which has altered the boundary conditions and bank materials within which streams flow. Significant PSA accumulation following native forest clearance in one of the sub-catchments has allowed the stream to flow entirely within its own alluvial deposits. In this instance, catchment-scale controls on channel geometry are dominant. In contrast, local-scale controls on channel geometry are more important in the other sub-catchments where lesser accumulation of PSA reflects differences in land clearance. These findings indicate that place-based understandings which consider the spatial and historical contexts of landscape settings are required to interpret patterns of channel geometry, and to inform river rehabilitation initiatives.