Anastomosing river system along the subsiding middle Yangtze River basin, southern China

Abstract

The study focuses on the formation processes of the uprivermost (Jingjiang reach) of three anastomosing river reaches along the middle Yangtze River, downstream of the Three Gorges Dam site and 1700 km upriver from the seacoast at Shanghai in southern China. The Jingjiang reach consists of large anastomosing channels: the Songzi, Hudu, Anxiang and Ouchi rivers, which depart from the Yangtze trunk channel and flow southward and southeastward up to 200 km into the Dongting basin and lake; the lake rejoins the Yangtze at Chenglingji. The 7000 km 2 anastomosing reach is characterized by multiple, stable, low gradient channels (0.00004 m/m, 4 cm/km), with low width/depth ratios (<40), low stream powers (<7 W/m 2) with very high sedimentation rates (33.5–123.3 mm/year) in some channels. Tectonic subsidence, caused by the Himalayan Orogeny, is thought to be the primary forcing mechanism in causing the anastomosing channel processes and deposition. Historical documents have shown that the Jingjiang channels were caused by repetitive avulsions southward from the Yangtze channel. The earliest recorded avulsion occurred in a high flood period before 1644, and subsequent avulsions have continued to modify the topography. Three major avulsion channels have diverted 14–33% of the discharge and 17–47% of suspended sediment load from the Yangtze channel. These channels have remained laterally stable, a characteristic of anastomosing rivers due to their low gradient, silt-clay banks, densely vegetated natural levees and stable interchannel islands consisting of clayey-silt that supports dense riparian vegetation. Data derived from hydrological gauging stations over the last 45 years demonstrate that total discharge and sediment load of anastomosing channels has decreased by two thirds, largely due to sediment aggradation on channel beds. Reduced sediment load and discharge, and human-caused channel modification were more pronounced in the eastern channels. With the completion of the 3-Gorges Dam in 2009, the anastomosing channels will cease to carry significant quantities of water and sediment to maintain fluvial vigor. In the next 100 years, the Yangtze main channel may become the only active channel as the anastomosing channels slowly will be infilled.