Deciphering historic change in tributary-junction fans in response to catchment-wide sedimentary disturbance

Abstract

Alluvial fans at tributary junctions are important moderators of sediment flux through river networks, particularly where the main stem and tributary valleys are responding to disturbance at different rates. In this work, I examine the factors that govern the decadal-scale rate and morphologic expression of tributary-junction fan development, following major sedimentary disturbance. In New Zealand’s East Cape region, the combination of recent deforestation with sandstone and mudstone lithology, high rainfall, uplift, and steep topography produce a landscape where rates of hillslope erosion and sediment transfer through rivers are some of the highest measured on earth. This study investigates the response of tributary-junction alluvial fans to sediment generation following deforestation and during extreme storms, such as Cyclone Bola in 1988. The dynamics of five tributary-junction fans along the Tapuaeroa River are examined for the period 1939-2015, using historic aerial photography, channel cross-sectional surveys and structure-frommotion photogrammetry. In response to major sediment loading, fan morphologic change included areal increases of up to 130,000 m², aggradation of up to 12 m, and fan-front progradation of up to 170 m. The maximum volume of fan sediment storage ranged from 15,000 to 1,500,000 m³. Change in fan morphology, sediment storage and coupling was related to temporal and spatial variation in a range of influences, both in the tributary catchments and the main stem channel. The major influences were climate, change in land cover, tributary-catchment connectivity, and most importantly, the nature of fan interaction with the main stem channel, at both seasonal and decadal scales. The fans’ ability to buffer the Tapuaeroa River from change in the tributaries was strongly conditioned by their interaction with the main stem channel. Their buffering capacity decreased through the study period, when the main stem aggraded and widened. Although previous studies have examined relationships between fan morphometry and tributary catchment characteristics, this study has shown that fan morphometry can vary considerably at decadal, annual or even monthly timescales. Such studies could benefit by examining regional histories of sedimentary disturbance.