Response to urbanization of the avon-heathcote estuary, christchurch, New Zealand

Abstract

The Avon-Heathcote is a small, microtidal, predominantly intertidal, weather-dominated estuary. It has experienced large alterations to its physical environment as a result of the establishment and growth the adjacent Christchurch City, on what was previously a swampy, dune-bordered coastal plain. During the first 25 years of settlement the volume of the tidal compartment of the estuary decreased from 7.7 × 106 m3 to about 5 × 106 m3. A combination of rapid drain laying in the 1880s and 1890s and the conversion of much of the catchment of the estuary to urban uses altered the sediment yields and runoff characteristics of the area and resulted in a reversal of the early trend. By 1925 the tidal compartment had returned to its original volume, and by 1975 it had reached a volume of 10.93 × 106 m3. The early decrease was accompanied by widespread deposition of a 30–60 cm thick layer of muddy sediment, some of which remains. The estuary inlet underwent major adjustments to accomodate the increasing tidal flow and now appears to be approaching a new, posturban equilibrium.