1 INTRODUCTION

Rivers are complex, multiattribute, and multifunctional systems (Willett et al., 2014). Their circulation, development, and uses are comprehensively influenced by human activities and climate change (Chung et al., 2021; Thorslund et al., 2021). Rivers, as carriers of naturally occurring substances and as sites where energy sources interact, serve as a link between the atmosphere, lithosphere, biosphere, and human sphere. The temporal and spatial distributions of rivers determine the fundamental characteristics of an environment in a region (K. Wang et al., 2019; X. Wang et al., 2019). Moreover, as a raw material used in production and as a commodity, water follows the basic principles of the law of value, the law of supply and demand, and the law of competition in its circulation; consequently, rivers are considerably influenced by human activities supporting economic and social development. Thus, human activities have a profound impact on rivers (Sivapalan et al., 2012; Yoon et al., 2021). In addition, rivers are an important driving force of culture and civilization. From the Hammurabi irrigation project and Dujiangyan more than 2000 years ago to Lake Kariba and China's South-to-North Water Diversion in modern times, rivers not only changed the original natural and cultural landscapes of regions but also created new cultural landscapes. Those projects are also important symbols of human technological progress (Zhao et al., 2012). Recently, with the rapid influence of climate change and human activities, rivers have experienced high-intensity temporal and spatial evolution, leading to many severe water problems and water crises. Some river-related problems, such as water pollution (M. Wang et al., 2022), sediment accumulation (Hoorn et al., 2022), dried-up riverways (D. Li, Yang, et al., 2022; D. Li, Zuo, et al., 2022), ecological environment degradation (Mondal & Tripathy, 2020), and groundwater overdraft (Jasechko & Perrone, 2020) have become critical factors restricting regional economic and social development and ecological environment protection. Therefore, it has become the core issue and research frontier of Earth science to study rivers' evolutionary mechanisms, utilization modes, and management strategies under changing conditions.

As people have gained insight into river systems, in-depth studies have been conducted on the river cycle evolution and its associated processes, green hydraulic project construction, adaptive management, and the sustainable utilization of rivers (L. Zheng et al., 2020). The scientific identification of the water cycle and its associated processes was a prerequisite for river research. Under the dramatic influence of the changing environment, the driving force, structure, and parameters of the water cycle and its associated processes have recently changed significantly (B. Li et al., 2021). Several scholars have realized that quantitatively identifying the changes of various elements of the water cycle using the “natural–artificial” combined mode (H. Wang & Y. Jia, 2016) was a precondition of scientific cognition of river laws. This cognitive change also leads to a change in the thought driving scientific research. Since the 1990s, scientists' research methodologies on the river water cycle and its associated processes have shifted from the “decomposition and refinement” approach to the “integration and crossover” approach. On the one hand, integration and crossover mean the continuous integration of hydroscience with other natural sciences and social sciences; on the other hand, they mean the comprehensive crossover of research orientations in hydroscience. In particular, integrating modern innovative high-tech tools, such as big data, artificial intelligence, cloud computing, digital twin, geographic information systems, remote sensing, and the internet of things with the basic laws of the water cycle and its associated processes is becoming an essential approach to studying the river water cycle (Ren et al., 2018). The water conservancy project was humankind's major means of utilizing rivers. In the past, water conservancy projects were primarily built to develop and use water resources, neglecting to a certain extent the ecological environment protection of rivers. As the concept of respecting nature and protecting natural ecology gained popularity, humankind began to reflect on the adverse effects of water conservancy projects on river systems (Z. Luo, Shao, et al., 2020) and devised various green construction concepts for water conservancy projects (Dong, 2019). Green construction theories generally hold that water conservancy projects need to respect the integrity of river ecosystems, the survival rights of biological populations, the natural landscape and esthetic value of rivers, and the circulation evolution law of rivers (Böck et al., 2015). Effective management was the basis for the high-quality development and utilization of rivers. After decades of development, the river management mode has changed from simply serving water development to managing rivers as limited and vulnerable resources, and has further developed into the comprehensive management of river basins, covering water resources, water ecological environment, and water conservancy projects (Arif et al., 2021; K. Wang et al., 2019; X. Wang, et al., 2019). Presently, the basin-based comprehensive management of rivers has become the consensus of many countries and international organizations, and three management approaches have been formed accordingly: dialog and cooperation approach represented by the Nile river, partly coordinated basin management represented by the Orange River, and comprehensive basin management pursuing the maximum overall benefits represented by the Danube River. Recently, rapid climate change has degraded the consistency and stability of river evolution and posed a severe challenge to the traditional static river management mode. Under these conditions, improving adaptive management based on climate change's impact on rivers has become a key component of river management. Maintaining river health and building a sustainable river utilization mode were the ultimate goals of river research, but the difficulty and complexity of achieving these goals have been increasing (Dugan & Allison, 2010). Numerous practical studies in the past decades have proven that the ecological environment function and water conservancy service function of rivers were in conflict (Ganoulis, 2021): over-exploitation and utilization of the water conservancy service function of rivers will damage the ecological environment of rivers, but overemphasis on the ecological environment function of rivers will affect the normal service functions, which was not conducive to economic and social development. Only by achieving a balance between the two can the sustainable utilization of rivers be maintained (Böck et al., 2018). In this context, the critical threshold determination of river utilization, the ecological value assessment and accounting of rivers, the comprehensive regulation of water and related systems—such as food and energy—and the development of efficient water-saving technologies have gradually become research focuses.

In general, although several achievements have been made in the studies of rivers, and these achievements can assist in the formulation of relevant policies and action plans in different regions, a further in-depth research is required on how to achieve the goal of promoting the balance between human development and the ecosystem as proposed by the United Nations in the 2030 Agenda for Sustainable Development in the context of climate change.

2 FORMATION AND EVOLUTION OF RIVERS

Rivers were formed under the coaction of geological landforms and paleoclimates, which together determine the variation characteristics of their material flow (water volume, sediment, solute), energy flow (gradient, flow velocity, kinetic energy), and dynamics (scouring, siltation) (Do et al., 2014; Ru et al., 2020; Z. Zheng et al., 2019). The basis for river formation was tectonic motion, which shapes the original morphotectonic pattern of the river basin, determines the uplift and subsidence process of rivers, and controls the basic characteristics of river strike and gradient change along the river (S. Ma et al., 2021). For example, the tectonic movement of the Himalayas has shaped an overall topography in China of high and low in the west and east, respectively, forming a three-step terrain tilting down to the east, which results in China's major rivers, such as the Yangtze River and Yellow River, flowing from west to east. The Huaxia rift movement formed several tectonic mega-belts with alternating uplift and subsidence in East Asia. Many basins, such as the Sichuan Basin, Weihe Basin, Hetao Basin, and Jizhong Basin, and a series of lakes between the second and third steps, have formed in China, and these macroscopically determine the erosion and sedimentary characteristics of rivers in China, particularly those in eastern China. After the geological background of rivers was determined, paleoclimatic factors—such as monsoons, the alternation of the glacial and interglacial periods, and sea-level change—have become the controlling factors that determine the development and evolution of rivers (Y. Wang et al., 2021). For example, in a cold and dry climate, sea-level decline makes the river gradient increase, and the river entrenches the river channel and causes headward erosion, forming new geomorphology of the river channel (X. Ma et al., 2019); meanwhile, in a warm and humid climate, sea-level rise reduces the river gradient, causing the headward deposition of gravel and sediment carried by rivers, and accelerates the development of alluvial plains (Macháček, 2020). In the transitional period between a cold and dry climate and a warm and humid climate, the frequency of floods and droughts will substantially increase, and the state of rivers will change accordingly (Parasiewicz et al., 2019).

Landforms are another important factor affecting the water cycle evolution. The long-term action of rivers will affect the morphology of landforms, and landforms will counteract the movement process of river flow and sediment (Capolongo et al., 2019). As an active exogenic force for shaping landforms on land, rivers shape landforms in three ways—erosion, transportation, and accumulation—by controlling velocity, and flow and sediment accumulation (G. Liu, 2011). In mountain river sections, the water flow constantly erodes the valley rocks, thereby disintegrating rocks on both banks, forming various types of valley landforms (Gawrysiak & Kociuba, 2020). When rivers flow from mountainous areas to plains, the wide channel slows the water, and the materials transported by rivers gradually accumulate, thereby forming alluvial fans. When the alluvial fans of several rivers continue to expand and join with each other, a vast piedmont alluvial plain is formed (Ayaz et al., 2018). When a river enters the ocean, if the water-table gradient at the mouth is gentle, the sediment carried by the river deposits at the estuary and forms a delta. As the delta continues to expand into the ocean, it develops into a large delta plain (Caldwell et al., 2019). Although alluvial and delta plains shaped by rivers account for less than 10% of the Earth's land area, these areas are inhabited by approximately 70% of the world's population and have become the areas with the most active economic development globally (Thi et al., 2021). In addition, climate can change the water cycle process and affect the shaping of landforms (Knight & Harrison, 2013). For example, climate changes the velocity, discharge, and sand content of rivers by changing the frequency and intensity of precipitation, yielding changes in the role of rivers in shaping landforms (Tian et al., 2020). Climate will also indirectly affect the shaping of landforms by rivers through vegetation and soil. For example, in dense forests, some river erosion can occur, but once forests are destroyed, river erosion will be amplified (Zhou et al., 2021) (Figure 1).

3 RIVERS BOOST BIODIVERSITY

Rivers support the ecosystems of various life forms on the Earth. These diverse organisms, along with rivers, constitute the vital elements of human society and the natural environment on which humankind depends for its survival (Mashwani, 2020). Rivers contribute to biodiversity in two dimensions: habitat and biology. As for biotopes, there are habitats, material channels, and barriers. As habitats, rivers are the first and foremost important places for organism survival and reproduction. They provide necessary living space, food and water source, and shelter. Moreover, the hydrological rhythm, bed material, and width of rivers will directly affect the habitat function (Jones et al., 2020): wide, interconnected rivers with rich environmental gradients tend to support more abundant life forms than do narrow, scattered rivers with little variation in environmental attributes (Belletti et al., 2020). Furthermore, rivers are channels for material, energy, and biological flow. For example, rivers transport organic matter and nutrients, provide food for invertebrates and fish, and indirectly affect the distribution of aquatic animals. Rivers can also affect plant distribution by transporting plant seeds over long distances. Rivers are vital reproduction and growth channels for migratory fish (Carpenter-Bundhoo et al., 2020). As barriers, rivers dilute and mitigate water pollution, safeguarding river biodiversity downstream. Meanwhile, rivers act as natural boundaries for plant communities and animals that rarely migrate, hindering material, energy, and biological flows between different regions and thus guaranteeing stable biodiversity (Zarfl et al., 2015).

Due to habitat diversity, food chains, and material transport, channels were formed, thereby contributing to the diversity of plants, animals, and microorganisms (Rey Benayas et al., 2009). Consider aquatic plants, for example, over a long period of evolution, aquatic plants gradually developed morphological characteristics and phenological rhythms adapted to the hydrological process of rivers, so that changes in hydrological characteristics, such as water depth, water temperature, and velocity, inevitably affected the evolution of aquatic plants. According to Valk (2005), a rapid change in river water depth will stir the bottom sediments and change the light transmittance of the water body, thereby affecting the photosynthesis of aquatic plants. When the river water depth changes more than 150 cm, some plants will die out and new species will emerge. Even a slight change in water depth can change the biomass of aquatic plants and the predominance of some species. Water temperature is an essential control index for the growth and development of aquatic plants. The change in water temperature will substantially affect the dissolved oxygen content, nutrient load, and pollutant toxicity of water, thereby changing the metabolic processes of aquatic plants. Marčiulionienė et al. (2011) studied the impact of the thermal discharge of Ignalina Nuclear Power Plant in Lithuania on the aquatic plant community and found that the number of species of aquatic plants in this region decreased from 95 before the operation of the nuclear power plant to 69 after the operation (i.e., a 27.4% decrease), and some pollution-resistant large aquatic plants gradually became the dominant species in the region. A change in river flow velocity affects the community composition of aquatic plants. The flow velocity changes the suspension state of river sediments, thereby affecting the photosynthesis of aquatic plants; moreover, a change in flow velocity affects the anchorage and growth of aquatic plants' roots, and some submerged plants with inferior root anchorage ability decrease in number with an increase in flow velocity (Belletti et al., 2020; Jones et al., 2020) (Figure 2).

4 RIVERS CULTIVATE HUMAN CIVILIZATION

The rise and development of human civilization are closely linked to rivers. As revealed by archeological excavations, the Kenyans of East Africa (200–1.75 million bc), Tanzanians of Olduvai Gorge (200–1.75 million bc), Yuanmou Man (1.7 million bc), Lantian Man (1.15 million–700,000 bc), and Shandingdong Man (27–34,000 bc) of China lived in places with interconnected river networks and rolling mountains. These places offered abundant food and caves for habitation, and more importantly regular sources of drinking water (Han, 2019). As the population grew, the traditional way of hunting and gathering could no longer meet the sustenance requirements of primitive humans. At approximately 6000 bc, Neolithic humans made an important decision in their way of life, resulting in a highly settled life dominated by agricultural production (Tao et al., 2022). This new production mode required fertile land and adequate irrigational water. These two requirements promoted the rise of ancient civilizations on river banks (Dai, 2013). Examples include the earliest agricultural civilizations appearing in Egypt along the Nile River, Babylon along the Euphrates and Tigris, India in the Ganges Basin, and China in the Yellow River Basin in 4000–2000 bc (Valipour et al., 2020). Agriculture's advancement and development provided humans with high food security. In addition to meeting the requirements of producers, humans now had surplus food. This surplus food resulted in a higher level of social labor division and laid the foundation for developing advanced civilizations (Fatimah et al., 2020).

In addition to birthing civilization, rivers have considerably influenced the economic, social, political, and religious development of mankind (Argyrou & Hummels, 2019). Consider for example the Nile River: ancient Egypt started systematic irrigation as early as 3100 bc and set up the post of “Irrigation Chief” in 900 bc to take charge of irrigation work on both banks of the Nile River and of the estimation of agricultural yield and tax revenue (Bjornlund et al., 2020). More precisely, the development of irrigation agriculture made the population of ancient Egypt flourish from 100,000 in 5000 bc to 3 million in 1200 bc, a 30-fold increase. The increase in population and the development of agriculture guaranteed a large amount of tax revenue for ancient Egypt, laying the foundation for the governing and expansion of the country (W. Liu, 2000). To further organize manpower and materials to build water conservancy projects nationwide, ancient Egyptians in the early dynastic period (about 3000 bc) regularly conducted a nationwide population census, as well as a land, livestock, and assets inventory (Xie, 2010), and on this basis established a relatively stable ruling class and huge bureaucratic system, strengthening the monarchy of Egypt (El Nabolsy, 2020). The religion of ancient Egypt was also closely related to the Nile River. As water was, for ancient Egyptians, an essential element of farming, the worship of the Nile River became an integral part of people's lives. During each flood event, people on both banks of the strait would perform several sacrificial activities to pray for the blessing of the Nile God. To date, Hapy, the Nile God, is considered one of the greatest gods in Egypt (Chaney, 2013).

Furthermore, river harnessing was also an essential driving force for the development and progress of science and technology (Cao et al., 2021). The periodic flood of the Nile River made it necessary to frequently survey the land, contributing to the development of the earliest geometry (John & Li, 2007). For more accurate agricultural irrigation, the ancient Egyptians developed an advanced solar calendar according to the Nile's flooding patterns, based on which the astronomical calendar commonly used in the world today was developed (Yang & Yuan, 2004). The Chaldaics (of Babylon) feared and worshiped floods, which led them to believe in astrology. Therefore, they performed systematic observation of the relative position and motion of celestial bodies, laying a foundation for the development of early astronomy (Yan & Liu, 2005). Humankind's worship of rivers also contributed to the development of early philosophy, with Thales, the first natural philosopher in ancient Greece, believing that all things arose from water, and assuming that the Earth was a cylinder or disk floating on water (Huang, 2014). Both the Chaldaics and the ancient Egyptians also viewed water as the main constituent element of the world (Sultonmurod, 2021). The ancient Chinese doctrine of the five elements, meanwhile, placed water at the head of the five elements, with water combining with fire, wood, gold, and earth to generate all things (D. Zhang, 2019). With the improvement of water harnessing skills, these thoughts also yielded the further evolution of philosophy and considerably influenced contemporary human value judgments about transforming the natural world (A. He, 2005).

5 INFLUENCES ON AND CHALLENGES FACED BY RIVERS

As a link between the natural world and human beings, rivers were affected by many factors, among which climate change, human activities, and changes in water demand have increasingly become the leading factors of high-intensity spatiotemporal variation of global rivers, thereby causing many problems related to water that pose challenges to human survival and development (Z. Luo, Shao, et al., 2020).

Climate change affects all aspects of ecosystems and human society, with water as the main medium (Ray et al., 2020). In the past decade, more than 90% of major natural disasters in the world were caused by water-related events, such as floods, rainstorms, and droughts (GAR, 2017). The impact of climate change on rivers is multifaceted. First, climate change will affect the safety of river flood control (Q. Zhang et al., 2015). According to Trenberth (2011), rising temperatures will increase surface evaporation as well as enhance the moisture retention capacity of the atmosphere, making the Earth vulnerable to droughts, heavy rains, and floods. Existing research has proven that the frequency of global floods since the 20th century has been much higher than that of any other period (Wan & Yang, 2009); the frequency of large-scale droughts in the Mediterranean Sea, the Sahara Desert, southern Africa, and some regions in southern Asia is increasing (Dai, 2013; Park et al., 2016). Climate change may aggravate the shortage of water resources and endanger the agricultural systems closely related to water systems (H. Wang & J. You, 2016). China's data show that the average drought rate, disaster rate, and grain yield reduction in China since the start of the 21st century are 2.3, 4.3, and 2.6 times greater than those of the 1950s, respectively (J. Zhang, 2009). In addition, in warmer environments, precipitation will occur more in the form of rainfall than snowfall, which will speed up the water cycle and hinder the water regulation role of reservoirs, thereby indirectly leading to water scarcity (Allan et al., 2020; Marques et al., 2019). Climate change will also directly influence the ecosystems of rivers; for example, the sea-level rise owing to increasing temperature will cause the destruction of wetlands in coastal mudflats and increased salinization of coastal lands (Hens et al., 2018); frequent heavy precipitation will increase the rate of soil erosion in ecologically fragile areas (S. Chen et al., 2019); and the degradation or disappearance of permafrost will not only cause changes in vegetation distribution but also accelerate soil erosion in cold areas (B. Li et al., 2021). In addition, the rapid fluctuation of climate deteriorates the service condition of water conservancy projects (Shiru et al., 2020). Meanwhile, changes in river runoff will cause changes in the salinity and conductivity of the water body, which will aggravate the corrosion of hydraulic structures (Kałuża et al., 2018), whereas continuous low or high temperatures will also increase the difficulty of concrete temperature control and reduce the safety factor of dams (B. Li et al., 2018). Moreover, climate change can increase the frequency and intensity of extreme hydrological events and geological disasters, and adversely affect the safety and lifespan of water conservancy projects (Hasan & Wyseure, 2018).

The process of river circulation and exploitation has been changing since the dawn of humankind, but before the industrial revolution, humans' way of life and production had not yet substantially impacted the state of rivers (Ye et al., 2021). With the rapid expansion of the global population in the past 20 decades, the impacts of human activities—such as large-scale agricultural cultivation, urbanization, mineral development, and water conservancy construction—on the structure and function of rivers have been continuously revealed, and many of these impacts have dwarfed the regulatory capacity of river ecosystems themselves (Jayanthi et al., 2020). Agricultural activities were essential factors transforming Earth. Presently, the agricultural land area accounts for 32% of the world's land area (B. Chen et al., 2018). The most direct disturbance of rivers by agricultural activities was the destruction of the natural vegetation on both banks of rivers by reclamation of lakes or reclamation of river beaches (Williams et al., 2021), thereby replacing the river channel's rich biodiversity with homogeneous agricultural vegetation, not only changing the original landform but also changing the hydrological function of the river corridor and aggravating soil erosion and riverbank erosion. In addition, according to the results of the FAO Global Remote Sensing Survey, the global forest land area has decreased by 420 million hectares in the past two decades, 90% of which was used for agricultural land expansion, and this massive reduction of forest land has accelerated the global water system imbalance, thereby disrupting the global energy balance (FAO, 2020). Urbanization was an inevitable trend, but how to mitigate the adverse effects of urbanization on water systems is a critical issue that researchers need to address (Xia et al., 2017). Rivers in urban areas have completely different characteristics from other areas (Zang et al., 2019). Existing research has shown that the ecological environment of rivers deteriorates when the percentage of the impervious area increases by 10% and that with the increase in the impervious area and the construction of drainage networks, the water discharge of cities will increase to 2–16 times more than before (Beach, 2001). The heat island effect of urbanization can also increase rainstorm events. Over the past five decades, the number of extreme rainstorms with more than 80-mm precipitation in a single day in the American Midwest has doubled; the economic damage due to rainstorms in the American Midwest alone was more than $16 billion in 2008 (Swiss Re, 2009). Water conservancy projects, especially the construction of reservoirs, not only promote human development but also considerably influence river systems; segmentation (Belletti et al., 2020) and runoff regulation (Jaramillo & Destouni, 2015) were the crucial factors. Judging by the operation of most dams, dams have changed the movement patterns of runoff and sediment within 100-km downstream of the dam site, with some large-scale water conservancy projects (such as Aswan Dam) impacting downstream up to 1000 km (Biswas & Tortajada, 2012).

Increasing water demand is another critical challenge facing rivers (Carvalho et al., 2019). Water demand is largely influenced by population growth, urbanization, food and energy security policies, and macroeconomic factors, such as trade globalization and changing consumption patterns. Global water use had increased six-fold over the past 10 decades; since the 1980s, global water use has continued to grow at a steady rate of 1% per year. Water scarcity has now become a widespread and increasingly severe problem worldwide (WWAP, 2020). As predicted by the United Nations World Water Development Report 2015, the global population will reach 9.1 billion by 2050, of which 2.4 billion people will live in sub-Saharan Africa (WWAP, 2015), a region with the most severe water shortage. Research in China shows that for every 1% increase in the urbanization rate, the average irrigation water demand in China will increase by 2.7 billion m³, owing to the high consumption of water-intensive foods, such as meat and oil by urban residents. To ensure food security for the global population in 2050, future global agriculture would need to produce 60% more food, and developing countries would need to produce 100% more, and this continued increase in water demand would pose a direct challenge to healthy river cycles (WWAP, 2015). In addition, as water supply and demand conflicts intensify in the future, competition among water uses in different sectors would become increasingly fierce, and some sectors critical to sustainable human development, such as food and energy, may be affected to some degree (Kalair et al., 2019) (Figure 3).

6 THE FUTURE OF RIVERS

From the perspective of supporting the stability of Earth's systems and the development of human civilization, rivers have five attributes: ecological, environmental, resource, social, and economic attributes. Ecological attributes refer to the characteristics of rivers without traces of human activities, such as the randomness and periodicity of river movements; environmental attributes refer to the ability of rivers to serve as a link between the material and energy cycles of nature, supporting the performance of water-related natural functions; resource attributes refer to the regeneration of the water cycle and the limited nature of water resources, and are related to human water demand and the development stage of technologies; social attributes emphasize the equitable distribution of water, including the equitable use of water between human beings and the natural world, the equitable use of water by each individual in the human system, and the intergenerational water balance; economic attributes mainly emphasize efficient water use. Humankind should take as little water as possible from rivers, recycle water as efficiently as possible, and reduce interference with the water cycle and its associated processes (Figure 4).

7 CONCLUSIONS

Rivers form a link between material and energy cycles on Earth. They shape Earth's geological features, foster biodiversity, and promote the development and progress of human civilization. Recently, the ecological function of rivers in many places has been degraded owing to human activities and climate change, resulting in the frequent occurrence of extreme conditions, such as droughts and floods. Dried-up riverways are frequently seen. Balancing the relationship between human development and river conservation in the context of change is an evergreen research theme. Faced with new challenges in water science, humans need to rethink their relationship with rivers and cope with the adverse effects of climate change. At the same time, we also need to use rivers more comprehensively and intelligently and explore a more equitable river management model. Humans are critical factors in the sustainable use of rivers. Faced with the increasing deterioration of rivers, humans need to more clearly recognize their responsibility in the sustainable use of rivers, and must continuously explore new techniques of river development, utilization, and comprehensive protection.

ETHICS STATEMENT

The authors confirm that this article does not contain any studies with animal or human subjects.