# River Discharge and Saltwater Intrusion Level Study of Yangtze River Estuary, China
## 1 Introduction  
As intersections of the ocean and rivers, estuarine regions contain rich freshwater and ecological resources, as well as geographical advantages. Therefore, they have become major areas for rapid economic development and progress. At the same time, as the impacts of human activities intensify, the utilization of water resources continues to increase, and the inflow of many rivers into the seahave been declining [1]. In particular, the durations of small discharges during dry seasons have been increasing, with some rivers even having no current flow. The balances between fresh water and saltwater which have formed over long periods of time have been broken [2]. The saltwater intrusion disasters not only have negative impacts on the human populations of the affected areas, but also endanger aquatic animals and plants [3] and destroy local fragile ecological environments. For quite some time, the estuaries of countries throughout the world have been affected by saltwater intrusion, particularly in the United States, Germany, Italy, Britain, China, and other countries with abundant estuary areas. Saltwater intrusions into these estuaries has caused enormous losses each year [4]. River discharges are restricted by many factors such as climate change, human activities, and forest coverage rates. [PERSON] et al. (2009) examined the impacts of climate change and human activities on the river discharge of the Gushanchuan Basin, with the proportions of 72.7% and 27.3%, respectively [5]. With the exceptions of arid and semi-arid regions, increases in forested areas generally result in increased river discharge. Changes in the discharges of rivers may also cause changes in the geography and stability of river channels [6]. Furthermore, changes in river discharges impact the sea levels in estuaries. The sea levels tend to rise as river discharges decrease, and its amplification will decrease from upstream to offshore [7]. Increases in river discharge push the isohaline (0.45 psu) down-estuary, and decreases in river discharge tend to drag the isohaline up-estuary. The effects of river discharge on saltwater intrusion appear on both the horizontal and vertical of the horizontal plane, where both the saltwater intrusion distance and river flow rates are low. In the vertical direction, the river flow has greater influence on the surface layers than the bottom layers. As the largest estuary in China, the YRE has been the focus of many saltwater intrusion research studies. The saltwater intrusions of the YRE have been studied in depth since the 1980s, and more prominently in recent years, as a result of increases in the saltwater intrusions' levels and frequency. For example, numerical simulations of the Yangtze River Estuary salinity have been performed [8; 9; 10; 11]. The impacts of freshwater resources have also been studied [2]. Also, previous studies have been conducted regarding saltwater intrusion and the utilization of freshwater resources in the Yangtze Estuary [12]. Research was completed regarding the effect of extremely low level runoff on the saltwater intrusion at YRE was completed during extreme drought years [13]. It has been determined that the tide and river discharge have the greatest effects and are the major influencing factors in the YRE. [PERSON] (1995) showed that when a station's total capacity was less than 10,000 m\({}^{3}\)/s, saltwater intrusions would have significant impacts on the freshwater resources in the southern branch of the Yangtze River [14]. Also, using a mathematical model, [PERSON] (2014) proposed that the saltwater intrusions on the surface and bottom of the Qingcaosha Reservoir intake mainly originated from the intrusion of saltwater in the North Branch (NB) of the river [15]. [PERSON] (2013) established a model to predict the saltwater intrusion in the Chenhang Reservoir of the Yangtze River estuary using statistical laws [16].