Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Geography

Supervisor

Ashmore, Peter

Abstract

The aim of this thesis is to understand the morphologic changes to a set of historically braided rivers that have been narrowed. Braided rivers from the agriculturally developed Canterbury Plains, New Zealand, were studied from a period prior to much development (mid-1900s) to the present. Narrowing of channels, decreased braiding intensity, and loss of braided planforms were determined based on aerial imagery, changing the geography of braiding along all rivers. Channel width and count were statistically correlated and show the predictability of braiding change based on narrowing. Reaches with initially wide channels require more narrowing to induce a simplification of braiding, while narrower reaches may be closer to a threshold of change and require less narrowing to transition. The implications of the results can be used in river management to create wide enough river corridors that allow the rivers to maintain their naturally braided planforms while mitigating flood risk.

Summary for Lay Audience

Rivers evolve due to natural changes in the environment, and in their more recent history, they evolve due to human induced pressures. Throughout the world, rare, braided river patterns are recording a loss in areas with human-caused lateral confinement. Lateral confinement restricts the natural mobility of rivers, decreasing river stability and increasing flood risk. This thesis aims to understand the changes to nine braided rivers in the Canterbury Plains, New Zealand, that have been laterally confined. The period of study captures the rivers prior to significant development (mid-1900s) to present. All rivers of interest were recorded to have narrowed over time with increased confinement along their channels. Some sections of the rivers also changed from more complex river patterns (e.g., braided) to less complex patterns (e.g., single channel). The amount of narrowing required to induce a change in river channel pattern was explored and showed that larger channels require more narrowing to see a change in pattern type compared to channels with narrower starting widths. This is useful for future river management plans that want to confine rivers, as a minimum width before channel change can be determined. The Canterbury rivers show changes similar to other rivers around the world that have been affected by human alterations. Parts of the rivers still maintain their braided patterns and there is time to effectively and sustainably manage the rivers to restore them back to their natural functions.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Download

Share

COinS