Abstract

In New Zealand’s East Cape, gully complexes are a major component of catchment sediment budgets, and they drive persistent river dynamism, including catastrophic aggradation and braid-channel avulsion. Despite their overwhelming influence on the sediment cascade, the genesis of these complexes is poorly understood. In this paper, we reconstructed the initiation of one such gully complex and the associated debris train, which fills a second-order valley to more than 40 m and significantly impacts the trunk channel. Cut-and-fill analyses (1957–2012–2015) using light detection and ranging (LiDAR) and structure-from-motion (SfM) data suggest the valley-fill deposit totals 6.87 × 106 m3, with historic aggradation rates of almost 1 m yr–1. This long-term perspective was made possible through the combination of archival aerial photos and SfM techniques. The sediment delivery ratio between the scar zone and the main-stem Raukokore River had a long-term average of 20% over the first 55 yr, as the valley was filled and the deposit steepened. In the last 3 yr, the ratio was 74%, suggesting improved efficiency in delivery. This considerable increase in connectivity resulted from the rapid aggradation of the debris train, which achieved a quasi-linear profile with mean slope of 0.077 m m–1 by 2015. The major implication of this work is that the peak effects of these long-lived, valley-filling features arrive decades after the initial disturbance. Digital elevation models of difference from SfM techniques provide numerous insights into the changing configuration of gully–deposit–trunk channel linkages, which govern this temporal change in sediment delivery. Moreover, the presence of many such complexes in the soft-rock, steepland terrain of the Waikura, Whangaparaoa, and Raukokore Rivers, and notable absence from the Motu River, suggests their distribution is primarily controlled by lithology rather than land use. Given the vast stores of unconsolidated sediment in valley fills such as this, the regime of river aggradation is likely to continue for decades.

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