Date of Award

Spring 8-1-2008

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geography, Geology, and Anthropology

First Advisor

Basil Gomez

Second Advisor

Jay Gatrell

Third Advisor

Ryan Jensen

Abstract

The longitudinal profiles of the adjacent Mangatu and Upper Waipaoa Rivers, of the Waipaoa basin of the North Island of New Zealand, differ in their form despite their similarity in size and that they flow over comparable bedrock. AdditionaJly, both basins have experienced an increase in sediment supply following deforestation during the 19th and 20th centuries. This research addresses how the deviations in the long profiles of the Mangatu and Upper Waipaoa Rivers have arisen and what effect the increased sediment loading has had on their form. Using methods similar to those of Hack (1957), who found that in same sized basins rivers flowing over equivalent bedrock supported similar shaped longitudinal profiles due to the consistent rate at which drainage area was contributed in the downstream direction, power-law relationships between both area and length and the downstream reduction in channel slope with the contribution of drainage area in conjunction with the distribution of sediment particle sizes along the rivers were derived. Additionally, because the long profile influences rates offluvial erosion and landscape development, downstream trends in stream power were also calculated for both rivers along with the volume of sediment eroded from each basin since the late Pleistocene. The :findings indicate that drainage area is not contributed at equal rates along the rivers. In particular, the proportion of drainage area added in the headwaters of the Upper Waipaoa River is much greater than that contributed to reaches further downstream. Accordingly, channel slope declines at a faster rate downstream in the Upper Waipaoa River as compared to the Mangatu River. In the Mangatu, drainage area increases in the downstream direction on accoW1t of geologic structures foW1d along its lower reaches. Because of the manner in which drainage area is contributed to the rivers, stream power values were found to be initially greater in the upper portions of the Upper Waipaoa allowing the river to incise further into the landscape and thus reducing its channel slopes. Further, the Upper Waipaoa has eroded a greater volume of material from its basin than the Mangatu, presumably because of its higher initial stream power. These results suggest that drainage area cannot be assumed to be contributed in a uniform manner along all rivers and that the rate in which it is contributed may influence the longterm erosion rate. Finally, the recent increase in sediment supply did not affect either long profile, which can be regarded as inherited forms.

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