Preliminary Analysis of Hydrodynamics and Water Chemistry at the Confluence of Yangtze River and Poyang Lake, China

Poyang Lake and Yangtze River are the largest freshwater lake and the largest river in China, respectively. They are interacting at the confluence located in the Hukou reach, where the waters from the Poyang Lake enter into the Yangtze River. This interaction affects the control of basin flood and shipping security and it was modified from the Three Gorges Project. Some previous studies demonstrated that the operation of Three Gorges can weaken the blocking effects of the Yangtze River to Poyang Lake, which, in turn, decrease the backwater discharge and increase the outflow discharge in Hukou, hence determining an advanced dry season in the Poyang Lake.

A field study was carried out in August 2018 at the confluence of Yangtze River and Poyang Lake to analyze hydrodynamics and water chemistry there and to assess the effects of that large-scale upstream Project on these processes. The filed study was performed at high flow conditions (August 2018–Q_P=3820 m³/s, Q_Y=17180 m³/s, Q_R=0.22, U_R=0.35, M_R=0.07). During this field campaign, acoustic Doppler velocity profiling (ADCP) was used as well as water sampling and Multi-Parameter Water Quality detecting for the measurement of several water chemistry parameters (temperature, conductivity, pH, dissolved oxygen).

Preliminary results showed that: (1) at the confluence there is a large difference in velocity between the Yangtze River (up to 1.5 m/s at maximum) and the Poyang Lake (up to 0.6 m/s at maximum); (2) downstream of the junction the Yangtze River has to change its direction of about 50°; (3) at the confluence the average water depth was about 18 m, while no bed discordance was found between the Yangtze River and the Poyang Lake at the confluence entrance; (4) large difference in terms of conductivity and small difference in terms of pH and dissolved oxygen were observed between the Yangtze River and Poyang Lake waters.