Three large rivers, the Waimakariri, Rakaia and Rangitātā, rise in the mountains and have glaciers at their heads. The Ashburton springs from smaller glaciers in the outlying Arrowsmith Range. The Waiau and Hurunui rise on the Main Divide but lack glaciers.
Most major Canterbury rivers have braided shingle beds and flow bank to bank only in high flood. Smaller, rain-fed rivers rise in the foothills and flow down between the fans of the major rivers.
In North Canterbury there are extensive areas of downlands – rolling country between the foothills and the plains. The underlying rocks are limestones and marine sands, with some coal and older volcanic rocks. These were laid down between 100 and 13 million years ago and eroded when mountain building raised the Southern Alps.
The Canterbury Plains, about 180 kilometres long and of varying width, are New Zealand’s largest area of flat land. They are not strictly flat but slope at an average 1 in 132 from the base of the foothills (at 365 metres or more above sea level) to the coast. They have been formed by the overlapping fans of glacier-fed rivers issuing from the Southern Alps.
The plains are often described as fertile, but the soils are variable. Most are derived from the greywacke of the mountains or from loess (fine sediment blown from riverbeds). Some hold little moisture. The best were formed from mud and peat accumulating in the hollows between the fans of rivers.
Unlike most urban water supplies, Christchurch’s water comes from aquifers (water-filled bands of gravel) beneath the city. According to some, it’s the best drinking water in the world.
Canterbury has abundant water, in the rivers which carry mountain rainfall to the coast, and in aquifers (underground gravels holding water). Beneath the plains, layers of porous gravels are interspersed with impermeable finer sediments. Near Ashburton, bedrock is at a depth of 1,600 metres.
Groundwater flows towards the coast through these porous layers. The aquifers are recharged by rainfall and by river seepage. They have been tapped to irrigate farmland and for town water supplies.
Most of Canterbury’s coastline is open beach – sandy north of Banks Peninsula and shingly to the south. All Canterbury’s beaches are composed of material eroded from the Southern Alps and carried down the rivers. Varied habitats include cliffs and coves along the coastline north of Waipara in North Canterbury, lagoons at the river mouths, and the estuary of the Heathcote and Avon rivers.
The most significant wetland is Te Waihora (Lake Ellesmere), about 20,000 hectares in extent, near the coast south of Christchurch. Some swamplands, for instance those at Longbeach, were drained after European settlement, to create excellent soil for farmland. One of the reasons for the damaging effect of the February 2011 earthquake was that much of Christchurch was built on land that had once been a swamp.