Evolution of a Channel/Floodplain System: Waipaoa River, New Zealand

Indiana State University, Terre Haute IN

Investigators

Abstract

Although rivers and the flood plains they occupy have long been the focus of scholarly attention, many aspects of their dynamic evolution are not fully understood. Vertical accretion and channel narrowing due to suspended sediment deposition have been documented by geomorphologists over long time scales in qualitative or semi-quantitative terms, but complementary data on the contemporary rates at which these interrelated processes operate are lacking. This research project will assemble a comprehensive, quantitative geomorphic history for a channel-floodplain system along the Waipaoa River on the eastern side of the North Island of New Zealand. Previous research has shown that the floodplain is characterized by rapid growth through vertical accretion, which has a measurable lateral component at the decadal time scale. This will be achieved by analyzing cross-section survey data, obtained since 1948 at 0.8-km (0.5-mile) intervals within a 40-km long reach. These analyses will ascertain the controls on in-channel sedimentation and will determine the extent to which the observed changes in cross-section geometry compliment the previously documented process of vertical accretion on the Waipaoa River floodplain. By considering complementary data for both the channel and floodplain, this research will shed light on the manner in which a river adjusts it hydraulics and morphology following a well-documented disturbance to the catchment environment (clearance of the native forest in the nineteenth and early twentieth centuries). By correlating patterns of overbank deposition and the complimentary process of in-channel deposition that promotes channel narrowing, the project will show how the coupled channel-floodplain system has responded to human-induced change in the historic period. The project therefore will afford a unique perspective on a suite of processes that has long been surmised from stratigraphic studies and is recognized to represent a fundamental adjustment of the fluvial system to human-related disturbance. This research also will provide the direct observational experience necessary to test relevant width adjustment and floodplain growth models which provide a mechanistic explanation for the tendency of a gravel-bed channel, like the Waipaoa River, that is subject to an increase in the supply of fine sediment load, to evolve into a narrower, deeper river with fine banks and a coarse bed.