Confluences are important elements in braided rivers because they are points of channel convergence where sediment transport rates and bed scouring are maximized. Confluences control the formation of downstream bars and therefore may determine the overall morphology of braided rivers. Confluence migration and significant bed scour, confluences have a substantial influence on the geometry of braided river deposits. Prior research on confluences has focused on their equilibrium morphology and flow structure and has not assessed the amount of geometric change at a confluence nor addressed the link between confluence morpho-dynamics and sedimentology. The purpose of the thesis is to further our understanding of confluence behaviour in braided rivers and to link this directly to the geometry and sediment sorting of braided river deposits. This is done by focusing on the morpho-dynamic development and sediment sorting at 24 confluences observed in a physical-scale model of a gravelly braided river.

The analysis uses high resolution, photogrammetrically-derived ortho-images, Digital Elevation Models (DEMs) and surface grain size maps using techniques developed and tested as part of the thesis. The analysis of a time series of DEMs, combined with overlays of grain size is a novel aspect of the research and allows direct, quantitative analysis of both confluence morpho-dynamics and the resulting deposits. While the static area of confluences in a river is instantaneously small (approximately 3%), their spatial influence is much greater as a result of confluence migration, accounting for approximately 20% of the active river area. Confluences tend to migrate about the same distance as the typical confluence-bifurcation spacing in braided rivers along a flow-parallel direction. Confluence mobility is strongly correlated with confluence depth, area, and length, though not with confluence width, suggesting that deeper and longer confluences may migrate further than small, shallow confluences.

The shape of confluence deposits indicates that their static dimensions are roughly equivalent to those dimensions reported in the sedimentology literature. However, migration tends to increase the spatial area of confluence deposits by greater than 2 times their static area and therefore their depositional aspect ratios are similar to gravel sheets and bars. Most confluences erode to the minimum surface of the deposit, and the thickness of the deposit is defined by multiple migrating confluences. The incidence of confluence deposits is approximately 15% of the total river deposit.

The sedimentology of the boundary surfaces of a braided river and at confluences suggests there is little difference in grain size populations between the lower and upper boundary surfaces. This finding challenges the conventional view of braided river deposits where a coarse lag is often deemed diagnostic of the basal scour surface of braided river deposits. Grain sorting patterns at a confluence are highly variable with no strong trend evident, indicating that current models of diagnostic grain sorting patterns at confluences are too simplified to fit reality.