CONTROLS ON LONGITUDINAL PROFILES IN RAPIDLY AGGRADING HEADWATER RIVERS OF THE WAIPAOA BASIN; NORTH ISLAND, NEW ZEALAND

Over 50 years ago, Hack concluded that basins of similar size with comparable geology should exhibit similar long profiles due to the consistent rate at which drainage area is contributed to a river. Although deviations in long profile forms, of same-sized rivers with similar physical settings, are known to exist, few studies have attempted to explain why these profiles vary from Hack's well-established relationship. The adjacent Mangatu and Upper Waipaoa Rivers of the North Island of New Zealand offer a unique setting to examine the controls over long profile development. Similar in basin area, the rivers are underlain by bedrock of comparable resistance. The basins also share a similar climatic regime and rate of uplift over the past ~18 Kyr. Further, the rivers have been transformed from bedrock-dominated to alluvial channels because of a prodigious increase in sediment supply following colonial deforestation. Despite these similarities, the long profiles of the Mangatu and Upper Waipaoa Rivers are quite different. The former assumes a greater degree of concavity than the latter. This research explains how the deviations in the long profiles of the two rivers have arisen and what effect the increased sediment loading has had on their form. To explain the differences in the long profiles similar power-law relationships as used by Hack were utilized in conjunction with the distribution of sediment particle sizes along the rivers. Both basins also possess relatively well-defined fluvial terraces of the late Pleistocene (18 Kyr), which allows the long profiles of the paleo rivers to be reconstructed and compared to their modern-day counterparts. Additionally, because the long profile affects rates of fluvial erosion and landscape development, downstream trends in stream power were calculated for the rivers along with the volume of sediment eroded from each basin since the late Pleistocene. The results indicate that drainage area cannot be assumed to be contributed uniformly along all rivers and that basin geometry exerts a profound control over long-term erosion rates. Lastly, the increase in sediment supply has had negligible effect on the long profiles as supported by the respective rates in sediment fining along the rivers and the similarity of form among the rivers' ancient and modern-day long profiles.