Downstream fining in the Waipaoa River : an aggrading, gravel-bed river, East Coast, New Zealand : thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Quaternary Geology at Massey University

Abstract

The Waipaoa River, East Cape, New Zealand, drains a catchment from the Raukumara Ranges into Poverty Bay, near Gisborne. Conversion of the catchment from indigenous forest to pasture, between 1880-1920, initiated a phase of intense erosion in the hill country. The underlying geology consists of crushed and sheared sandstone, siltstone, argillite, and mudstone of Cretaceous and Tertiary age. Channel aggradation occurred in response to the influx of bed material load. Suspended sediment yields in headwater catchments are as high as 7 000 – 17 000 t km. For the period 1948 to 1988, aggradation in the upper reaches was > 5 m, while in the lower reaches it was ~0.5 m. The Waipaoa River is a gravel-bed river. Its morphology changes from a braided to a meandering configuration in the downstream direction. A bed material survey of the Waipaoa River in 1995/6 investigated the fluvial transfer of coarse bed material through the river system. Bed material samples were collected at 1 km intervals along the mainstem, as well as from major tributaries, near their confluence with the Waipaoa River. Surface and subsurface samples were systematically collected between the coast and 104 km upstream. The results of this survey were compared with earlier bed material surveys undertaken in 1950, 1956, and 1960. Results of the 1996 bed material survey indicate that the bed material in the Waipaoa River is polymodal. The gravel-sand transition occurs approximately 8 km upriver from the coast. Over the remaining 96 km reach, the median particle size declined from 5 mm in the headwaters, to 2 mm near the coast. The coarser particle size fractions exhibited a greater rate of downstream fining, and, over the same distance, the coarsest 10% declined from 48 mm to 6 mm. The bed material is dominated by fine sediment, which is illustrated by the fine median particle size over the length of the river, as well as the low fining coefficients for the finer particle size fractions . No downstream change in the proportion of each main pebble lithology was observed, and each pebble lithology exhibited a similar rate of downstream fining. No downstream alteration in particle shape was observed, although particle roundness did increase downstream. Close relationships were observed between the bed slope and particle size. The highest degree of correlation was observed between slope and the coarsest particle size fractions, representing the limiting condition of channel competence. Selective transport is the dominant process that produces downstream fining in the Waipaoa River, however, particle fragmentation, sediment supply or abrasion may be important processes within specific reaches. The rate of downstream fining was consistent for the period 1948 to 1996.