RIVER RECOVERY IN AN URBAN CATCHMENT: TWIN STREAMS CATCHMENT, AUCKLAND, NEW ZEALAND

This study examines the evolution of a small urbanizing catchment in Waitakere City, New Zealand over the period since European settlement. Geomorphic changes are interpreted from the use of historical documents complemented by sediment analysis at eight sites throughout the catchment. Differing forms and rates of geomorphic adjustment are characterized for five different types of stream. Reach sensitivity to change and trajectory of change are related to the capacity for adjustment of differing stream types and their position within the catchment. These relationships are analyzed using the recov- ery diagram developed by Fryirs and Brierley (2000). Profound disturbance has been expe- rienced, and streams have responded to a series of land use changes over a relatively short time fr ame (around 160 years). The system responded quickly to forest clearance and sub- sequent phases of pasture, horticulture, viticulture, and urbanization. Significant geomor- phic recovery is under way along most stream courses. While substantial increases in sediment yield are inferred, the fine-grained nature of these deposits has not significantly altered geomorphic features of headwater streams. In mid-catchment, benches have developed along overwidened channels. Prospects for geomorphic recovery are much more limited along lowland reaches, where cumulative impacts have brought about sig- nificant changes to river morphology, such that reco very to predisturbance conditions is no longer a realistic prospect over management time frames (50-100 years). The relative geomorphic resilience of this urbanizing catchment is considered to reflect the geologic imprint upon this landscape (a dissected volcano), the reforestation of headwater reaches within a short interval of initial clearance, and the location of urban impacts that are con- centrated in the lower part of the river system. (K ey words: river evolution, human distur- bance, river change, river recovery, river sensitivity.)