## 1 Introduction  
As river systems have beneficial natural and social functions and provide many services and goods, such as shelter, food sources, and ecological refuge for an array of biological species, fresh water, transport mediums, recreation and tourism opportunities, aesthetic and cultural values and aid in flood management to protect society, they have been widely regarded as the most essential of natural resources and as places to experience the serenity of nature [1]. However, many river systems are globally threatened by various human activities such as water extraction, channelization, logging, mining, and pollution from domestic, agricultural, and farming activities [1, 2]. For instance, since the late 1980s, algal blooms have frequently occurred, and approximately 41 fish, 65 zooplankton,and 16 macrophytes species have disappeared from the Taihu Lake located in the Yangtze River Delta in China [3]. Nitrogen and phosphorus levels in surface water from the Yangtze River are higher than those in surface water from other areas of China [4]. The United States Environmental Protection Agency (USEPA) has reported that 44% of surveyed streams and rivers in the United States have been impaired for at least one designated use [5]. Evidently, only a strikingly small fraction of the world's river systems have remained unaffected [1; 5; 6], and river systems have become one of the most endangered global systems at an alarming rate [7]. Therefore, the increasingly urgent needs to maintain healthy river systems have become important objectives of river management [8]. Philosophically, the term \"river health\" is useful because it is readily interpreted by the general public and evokes societal concern about human impacts on river systems [9]. For the past few decades, countries across the globe have been in a constant battle to monitor and evaluate the healthy status and variation of a river system and of identifying anthropogenic impacts. Many assessment indices, mathematic methods, and digital tools have been developed in multiple countries such as Australia [6; 10], China [8; 11; 12], the United Kingdom [13], Sweden [14] and the United States [15]. For instance, in Australia, AUSRIVAS (Australian River Assessment System), FARWH (National Framework for the Assessment of River and Wetland Health), IRC (Tasmanian Index of River Condition), and SRA (Sustainable River Audit, systematic river health assessment for the Murray Darling Basin) have been established to assess the health status of the river systems at state or national levels [16]. In China, [PERSON] et al. (2013) established a river health diagnosis index system, which consisted of natural structure index, ecological service index, and social services index [11]. They also designed a GIS-based intelligent river health diagnosis and decision support system (RHD-DSS) to assess the health status and diagnose the causal factors that influenced a river's conditions or health status [12]. In Europe, the EU Water Framework Directive (WFD), brought into effect in 2000, aims to defend vulnerable water bodies and bring the vast majority of Europe ground and surface waters back to good health by 2027 [17; 18]. RIVPACS (River Invertebrate Prediction and Classification System), which is an aquatic biomonitoring system based on macroinvertebrate species for assessing water quality in freshwater rivers, is used across the United Kingdom [19]. In practical terms, river managers hope to obtain answers to the questions: \"Are there any risks in a river system and what is the extent of the risks and changing trends? How could managers deal with an emergency even?\" Only with a foundational understanding of the future trends of a river system, could river managers be alerted to make timely, precise decisions or policies during extreme events.