Conceptualization of Sediment Flux in the Tongariro Catchment

To quantify sediment flux in river systems requires not only measurements (or estimates) of how much sediment moves through channel cross-sections or reaches over a given time period, but also consideration of how representative those measurements are and how long those rates of sediment movement will be sustained. Geomorphic analysis must accompany application of engineering principles to develop this understanding, explaining controls on sediment availability for a particular system. Geomorphic controls on sediment flux are discussed here in terms of four principles: landscape setting, landscape connectivity, reach sensitivity and sediment organization. These principles are applied in the Tongariro catchment. The landscape setting of this catchment is fashioned primarily by the volcanic history of the area, with abundant sediment supply. Landscape connectivity is limited, with many sediment stores in upland areas disconnected from the lower course of the river. Reach sensitivity downstream of the gorge is limited by the area of active channel inset within terraces. Significant channel adjustments have occurred in the braided reach beyond the terrace-confined reaches. This reach has acted as an efficient trap for gravel-sized materials, such that further downstream, the meandering and multi-channeled delta reaches comprise sand-sized materials. These latter reaches have shown negligible channel adjustments over the last 80 years. Collectively, these inter-related controls determine variability in sediment availability in the Tongariro catchment over time, thereby exerting a dominant influence upon sediment flux. Human disturbance is concluded to have had a negligible impact on sediment flux in this resilient system.

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