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Abstract

Wilson, D.D. Erosional and depositional trends in rivers of the Canterbury Plains, New Zealand

The major rivers of the Canterbury Plains the Waimakariri, the Rakaia, the Rangitata and the Ashburton, on emerging from the Southern Alps and foothills have built out fluvioglacial fans, and with progressive downstream aggradation formed a series of surfaces over 50 km wide and 160 km long. Each river has aggraded across the entire width of the plain during glacial maxima, and is inferred to have incised a seaward-developing trench, typically 2 km wide, during each interglacial period. How regimes and gradients throughout the late Quaternary were controlled by sediment load, sea level, and coastline position. Waimakariri River terrace sequences indicate that the dominant control of river gradient during glacial periods was sediment load; but during interglacials, sea level and sediment load were of equal importance.
Bed-level measurements for the Waimakariri River show a change from river entrenchment upstream and fan building downstream at about 18 km to 19 km from the coast, and a downstream migration of the intersection point. Relatively recent coastal erosion of the Canterbury coast to the south of the Rakaia River mouth, has caused the development of trenches extending inland from the coast.

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