Tributary-junction fans of Chinaʹs Yangtze Three-Gorges valley: Morphological implications

The tributary junctions to main rivers within incised canyons reflect the relationships between stream power and sediment fed to the main river by tributary. Tributary junctions within the Three-Gorges valley of the Yangtze River, China, were classified into 7 types on the basis of the presence or absence of tributary-junction fans, and the size and style of the fans. Three reaches, totaling 180 km in length, were mapped in the field, and for a sample of 120 tributary junctions, morphometric properties of the feeder catchments were derived. Small steep tributaries do not generate tributary-junction fans, nor do the largest tributaries. For intermediate-sized tributary catchments (of approximately 1–30 km2) threshold conditions can be identified between the relatively steep, more deeply dissected catchments (mean basin slope > approximately 0.1; relief range approximately 250–800 m) that generate large tributary-junction fans, and less steep, less deeply dissected catchments (mean basin slope: < approximately 0.1; relief range approximately 100–700 m) that generate only small ephemeral (?) fans. There are differences related to geology and overall geomorphic style, with few fans generated from terrain in interbedded sandstone, shale and limestone sequences, within which the main river and main tributaries are strike-parallel. Transverse terrain through similar geology generates the largest fans, and limestone gorge terrain generates important but relatively small fans. With the closure of the Three-Gorges Dam these sites will all be flooded, but this approach could be used to predict where major new fan-deltas will form at the margins of the new high lake.