Haast River flowing over rocks, Haast Valley, Mt Aspiring National Park

The Haast River is one of several major rivers that drain the central Southern Alps, flowing seaward via the glacial Haast Valley to meet the South Westland coast at Haast township. It is a river with plenty of character, thundering through precipitous gorges in the upper catchment and eventually spreading out over wide, gravelly plains towards the coast. It is also markedly seasonal in temperament and is often at its quietest in midwinter when much of the upper catchment lies blanketed in snow and ice, awaiting the thaw of spring. Spring and early summer, on the other hand, bring warmer rain-bearing storms in from the west and this is when some of the most violent floods can charge down the Haast River, bringing down tens of thousands of tonnes of rock and silt in a single event.

Most of the Haast catchment is underlain by schist, a hard, quartz-laden rock that has been pushed up from great depths over millenia by movements along numerous faults along the Southern Alps. The erosive power of glaciers and flowing water has gradually worn away at this basement rock to form extensive deposits of gravel, making up the present river bed and also underlying the extensive river flats in its lower reaches. Schist gravel is often rich in mica, giving it a distinctive silvery sheen, and individual rocks may also sport striking bands of pure white quartz. Ranging from pebbles to boulders larger than houses, schist can be found throughout the Haast catchment and, during low flows, the sight of crystal clear mountain water flowing over these colourful rocks and gravels is impressive.