The West Coast has been described as a region of mountains, rainfall and rivers. These features combined with the processes of uplift and erosion - have resulted in a landscape of unique character, two thirds of which is mountainous.
The Alpine Fault runs most of the length of the region. East of the fault are deeply dissected mountain ranges. To the west, rivers and streams are steeply graded - the distance from source to sea seldom exceeding 50 km. Towards the coast alluvial and beach deposits occupy a 10-15 km wide strip which extends inland along river systems. Plains areas are, with some exceptions, generally localised and composed of outwash silts and gravels. They are subject to frequent flooding.