Abstract:

Recently, reverse density flow was found in many tributary bays of Three Gorges Reservoir (TGR), such as Xiangxi River and Daning River. Nutrients of high concentration carried by intruding water from TGR are likely to be the main nutrient source of these tributary bays. Due to the multifaceted importance of water quality in the Shennong Bay (SNB), the eutrophication in SNB increasingly draws extensive attention of local government. In addition, the water ecenvironment change of SNB are strongly affected by the regulation of TGR. Therefore, continuous monitoring has been carried out in SNB to study the hydrodynamics and their impacts on nutrient distribution during preflood water supply period. Results of the hydrology, water quality and aquatic ecology of SNB were presented in this paper. According to the water regime information of TGR during the period of monitoring, on the basis of the daily change extent and variation trend of water level, the preflood water supply period of TGR in 2013 can be divided into three stages. The first was February 3 to April 5, which lasted 62 days, the water level changed slowly from 170.07 m to 161.33 m with a total decrease of 8.74 m and by an average daily decline of 0.14 m. The second stage was April 6 to May 10, water level changed from 161.36 m to 160.21 m with an overall decline of 1.15 m. The third stage was May 11 to June 21, water level changed rapidly from 160.15 m to 145.06 m, creating a total decline of 15.09 m with an average daily decline of 0.36 m. It can be seen from the spatial and temporal distribution of flow rate in SNB that flow velocity ranged from -0.249 m/s to 0.246 m/s in the bay, and bidirectional flow phenomena persisted during the period of monitoring as the mainstream water of TGR was intruding into SNB in a form of reverse density flow from varied depths. It again provided a piece of evidence for the universal presence of reverse density flow in tributary bays of TGR. This special flow pattern in SNB provided a hydrodynamic basis for nutrient transport, and nutrient distribution of tributary bays was obviously affected by the mainstream of TGR. Affected by the intrusions from mainstream of TGR, nutrients decreased as the distance from estuary of SNB increased until the end of the bay, the mainstream water of Yangtze River supplied a large number of nutrients to SNB. Form and range of intrusions can be affected by water level management which was proven to have interplays between the algal blooms. At the same time, sustained and steady change of the water level can make a specific type of intrusion so that stable water stratification can form which was proved to be related to algal blooms in tributary bays of TGR. Therefore, algal blooms and eutrophication in tributary bays of TGR can be resolved through reservoir operation.