A catchment basin is an area of land where all surface water from rain, ice or melting snow converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another body of water.
The catchment of the Ōpāwaho-Heathcote River drains an area of approximately 100 square kilometres of land within the Christchurch city boundary. It includes nearly one-third hill catchment and an area of rural land in the south-western part of Christchurch that is decreasing over time due to residential development. It is slow flowing and meandering and have a number of tributaries that include both natural streams and human-made drains.
The river begins in the area of Templetons Road, and it also receives wet weather flows from as far west as Pound Road. It flows through Hoon Hay and Spreydon, then it meets the Cashmere Stream before flowing through Cashmere, Beckenham, St Martins, Opawa and Woolston. It drains into the Avon-Heathcote Estuary/Ihutai at Ferrymead. Seawater intrusion extends to a point just above the Mackenzie Avenue footbridge.
Water in the Ōpāwaho-Heathcote River comes from two different sources; springheads which are connected to confined aquifers, and rainwater running off the surface of the land into tributaries.
Confined aquifers are stores of water trapped in layers of gravel by layers of mud or clay that seal the water in. If the water in a confined aquifer runs into a dead end because of barriers such as the rocks of Banks Peninsula or a wall of clay, the pressure builds up, a bit like putting your finger on the end of a running hose. This pressure forces the water up to the surface. When the water erupts through the surface it is called a spring. Springs are recharged by water travelling very slowly underground from the Alps to Christchurch plains, and by rain soaking into the ground.
Springs contributing to the Ōpāwaho-Heathcote river are located mostly in the Halswell, Oaklands, Wigram, and Hillmorton. However two lovely springs can be found in Beckenham Park and Ernle Clark Reserve.
Prior to the development of Christchurch, there are more than 400 named waterways. Now there are only 356 km of open waterways (less than 1 km per named waterway!), and 500 km of underground pipes which are used to divert and drain the surface water away.
The major natural tributary into the Ōpāwaho-Heathcote River is the Cashmere Stream. Other tributaries were often turned into open drains. Examples are Haytons and Curletts Road streams, which feed into the upper reaches of the Ōpāwaho-Heathcote. For most of their length, these tributories are straight, timber-lined drains vulnerable to industrial pollution. Other tributories include Steamwharf Stream and Coulings Stream. Dozens of small creeks running off the Port Hills are also diverted into storm water network or drains.
The biggest difference between Ōpāwaho-Heathcote River and the Otakaro-Avon River is that the Ōpāwaho-Heathcote River receives water running off the Port Hills, which tends to be loaded with fine soil particles (sediment).