Spatial and Temporal Changes of Sand Mining in the Yangtze River Basin since the Establishment of the Three Gorges Dam

The global demand for sand and gravel is at 50 billion tons per year, far exceeding global resource capacities. It reached 7.6 billion tons in 2021 in the Yangtze River Basin (YRB), China. However, production is severely limited in the YRB. Therefore, the incongruity between the supply and demand of river sand is prominent. Wise management of decreasing sand resources in the YRB has become critical since the Three Gorges Dam became operational in 2003. This study synthesized spatial and temporal changes in sand mining activities and quantities along the Yangtze River and its major tributaries from 2004 to 2020. Results from the study show that the mining amount during the period reached 76.2 million tons annually. At the same time, riverine suspended sediment discharge (SSD) downstream of the Three Gorges Dam decreased largely. SSD reduction leads to riverbed erosion, further limiting the riverine sand and gravel sources for mining. Thus, alternative sand and gravel resources, as well as optimizing supply/demand balance, are necessary for sustainable development. There is an urgent need to assess the relationship between river sand resources and exploitation in the YRB for creating a sand and gravel data management system in order to cope with the increasing incongruity between their supply and demand.


Introduction
Over the last 20 years, the global demand for sand and gravel has tripled. This demand has been projected to double by 2060, driven by the construction and expansion of urban infrastructure [1,2]. While many coastal areas in the world have been losing land, sand and gravel sources have become crucial for protecting and/or creating land in the cities and towns near the coast. For example, 517 million tons (MT) of sand and gravel have been used in land reclamation over the past two decades in Singapore; most of which was imported [3,4]. Therefore, sand and gravel reserves face multiple challenges worldwide.
As high-quality resources, river sand and gravel are highly beneficial to humans. On the other hand, sand mining activities often have profound negative impacts on the ecological environment of rivers [5]. These negative impacts [6][7][8] include changing the river channel morphology and affecting the stability of the river regime, leading to an increase in flood frequency and intensity and exacerbation in drought occurrences [9][10][11][12]. For example, the Manimala River suffers from groundwater level recession and water resource shortages due to river sand mining in southwestern India [13]. Moreover, massive sand mining in rivers could accelerate beach erosion in their deltaic areas [14][15][16]. Illegal sand mining activities are typically random and ignore environmental impacts, causing more harm to the physical and ecological functioning of river systems. Unfortunately, these activities continue worldwide, especially in Malaysia, India, Cambodia, and South Africa [17][18][19]. 2 of 12 In China, sand and gravel demand was approximately 18 billion tons in 2021 (https: //www.cssglw.com/; accessed on 3 March 2022). The Yangtze River Basin (YRB) is a crucial source and consumption area for high-quality sand. Sand mining along the YRB began in the late 1970s, and illegal sand mining was rampant owing to "profiteering." Mining along the Yangtze River (YR) mainstream was banned during the early 21st century, and illegal sand mining vessels moved to Poyang Lake, which is connected with the YR. Studies have reported that these sand mining activities have caused the water level of Poyang Lake to drop in the dry season [20], the subaqueous topography to change [21], and the fish and bird habitats to deteriorate [22]. Hence, China has recently established strict regulations for sand mining management and regulation in the YRB. Moreover, the government authorities have increased the inspection and supervision of sensitive waters, including unannounced inspections of key river sections [23].
However, as China has been rapidly developing, the demand for sand and gravel exceeds the supply. The demand-supply discrepancy makes it impossible to prohibit sand mining (including illegal sand mining). Therefore, scientifically and rationally exploiting and managing river sand resources is critical. This study focuses on spatial and temporal changes in sand mining activities in the YRB, aiming to (1) estimate the amount of historical sand mining, (2) analyze characteristics of spatial and temporal mining changes, and (3) propose recommendations and countermeasures for sand mining in the YRB, which can be applicable to other alluvial rivers in the world.

Study Area and Methods
The Yangtze River originates in the Qinghai-Tibet Plateau in west China and flows 6300 km eastward before entering the East China Sea (Figure 1) [24]. The YRB serves approximately 40% of China's population and economy. The Yangtze Delta generates almost 25% of China's total economic production, although it encompasses below 4% of the country's land area (https://www.thepaper.cn/; accessed on 4 January 2021). This enormous economic burden requires large amounts of sand and gravel. mining activities are typically random and ignore environmental impacts, causing more harm to the physical and ecological functioning of river systems. Unfortunately, these activities continue worldwide, especially in Malaysia, India, Cambodia, and South Africa [17][18][19].
In China, sand and gravel demand was approximately 18 billion tons in 2021 (https://www.cssglw.com/; accessed on 3 March 2022). The Yangtze River Basin (YRB) is a crucial source and consumption area for high-quality sand. Sand mining along the YRB began in the late 1970s, and illegal sand mining was rampant owing to "profiteering." Mining along the Yangtze River (YR) mainstream was banned during the early 21st century, and illegal sand mining vessels moved to Poyang Lake, which is connected with the YR. Studies have reported that these sand mining activities have caused the water level of Poyang Lake to drop in the dry season [20], the subaqueous topography to change [21], and the fish and bird habitats to deteriorate [22]. Hence, China has recently established strict regulations for sand mining management and regulation in the YRB. Moreover, the government authorities have increased the inspection and supervision of sensitive waters, including unannounced inspections of key river sections [23].
However, as China has been rapidly developing, the demand for sand and gravel exceeds the supply. The demand-supply discrepancy makes it impossible to prohibit sand mining (including illegal sand mining). Therefore, scientifically and rationally exploiting and managing river sand resources is critical. This study focuses on spatial and temporal changes in sand mining activities in the YRB, aiming to (1) estimate the amount of historical sand mining, (2) analyze characteristics of spatial and temporal mining changes, and (3) propose recommendations and countermeasures for sand mining in the YRB, which can be applicable to other alluvial rivers in the world.

Study Area and Methods
The Yangtze River originates in the Qinghai-Tibet Plateau in west China and flows 6300 km eastward before entering the East China Sea (Figure 1) [24]. The YRB serves approximately 40% of China's population and economy. The Yangtze Delta generates almost 25% of China's total economic production, although it encompasses below 4% of the country's land area (https://www.thepaper.cn/; accessed on 4 January 2021). This enormous economic burden requires large amounts of sand and gravel.  During the 1980s, the demand for river sand was low and priced at RMB 2-3 per ton. However, it increased to RMB Table 3 exhibits the average annual runoff and sediment discharge data from Zhutuo, Cuntan, Yichang, Shashi, Hankou, and Datong, along YR. The average annual runoff and sediment discharge data for Poyang Lake is shown in Table 4. Estimation of the total sand mining for the YRB can be as given below: where S total is the total amount of sand mined within the YRB from 2004 to 2020, S 1 is the sand mined in the YR mainstream (Figure 2), and S 2 is the amount of sand mined in Poyang Lake during the same period, including estimated sand mined both illegally and legally during 2004-2010 [25] and sand mining planning amount in Poyang Lake during 2011 and 2020, S 3 is the amount of sand mining planned in Dongting Lake and the other tributaries during 2012-2020 (Table 5), and S 4 is the extra illegal mining data reported by judicial bodies for the period of 2013-2020 ( Table 2).
Estimation of the total sand mining for the YRB can be as given below: where Stotal is the total amount of sand mined within the YRB from 2004 to 2020, S1 is the sand mined in the YR mainstream (Figure 2), and S2 is the amount of sand mined in Poyang Lake during the same period, including estimated sand mined both illegally and legally during 2004-2010 [25] and sand mining planning amount in Poyang Lake during 2011 and 2020, S3 is the amount of sand mining planned in Dongting Lake and the other tributaries during 2012-2020 (Table 5), and S4 is the extra illegal mining data reported by judicial bodies for the period of 2013-2020 (Table 2). Total planned sand mining amount 837.8

Sand Mining from Tributary Rivers and Reservoirs
Over the past 10 years, sand mining from the major tributary rivers and reservoirs of the YR totaled 837.8 MT (Table 5). These include, among others, Han, Jialing, Wu, Fu, Qu, Qing, Han, Xiang, Zi, Yuan, Li River, and Dongting Lake. This mining quantity within a much shorter period of time was higher than that mined from the YR mainstem as well as from Poyang Lake.

Characteristics of Sand Uses
Based on the utilization, sand mining in this review can be divided into three categories: (1) engineering sand mining (used for land formation), (2) construction and operational sand mining (used for building), and (3) river and waterway dredging sand (purpose is to dredge river channel; can be used for land formation and building) (http://slt.hubei.gov.cn/;accessed on 2 December 2019).
The largest amount of sand (584.0 MT) was obtained through engineered sand mining (Table 6). This accounts for 77.22% of the total amount ( Table 6). The maximum

Sand Mining from Tributary Rivers and Reservoirs
Over the past 10 years, sand mining from the major tributary rivers and reservoirs of the YR totaled 837.8 MT (Table 5). These include, among others, Han, Jialing, Wu, Fu, Qu, Qing, Han, Xiang, Zi, Yuan, Li River, and Dongting Lake. This mining quantity within a much shorter period of time was higher than that mined from the YR mainstem as well as from Poyang Lake.

Characteristics of Sand Uses
Based on the utilization, sand mining in this review can be divided into three categories: (1) engineering sand mining (used for land formation), (2) construction and operational sand mining (used for building), and (3) river and waterway dredging sand (purpose is to dredge river channel; can be used for land formation and building) (http://slt.hubei.gov.cn/;accessed on 2 December 2019).
The largest amount of sand (584.0 MT) was obtained through engineered sand mining (Table 6). This accounts for 77.22% of the total amount ( Table 6). The maximum

Sand Mining from Tributary Rivers and Reservoirs
Over the past 10 years, sand mining from the major tributary rivers and reservoirs of the YR totaled 837.8 MT (Table 5). These include, among others, Han, Jialing, Wu, Fu, Qu, Qing, Han, Xiang, Zi, Yuan, Li River, and Dongting Lake. This mining quantity within a much shorter period of time was higher than that mined from the YR mainstem as well as from Poyang Lake.

Characteristics of Sand Uses
Based on the utilization, sand mining in this review can be divided into three categories: (1) engineering sand mining (used for land formation), (2) construction and operational sand mining (used for building), and (3) river and waterway dredging sand (purpose is to dredge river channel; can be used for land formation and building) (http://slt.hubei.gov. cn/; accessed on 2 December 2019).
The largest amount of sand (584.0 MT) was obtained through engineered sand mining (Table 6). This accounts for 77.22% of the total amount ( Table 6). The maximum amount mined was 79.0 MT in 2013 ( Figure 6). However, the minimum amount was 3.7 MT in 2020, which is substantially lower than the annual amounts from 2004-2020 (34.4 MT; Figure 6). amount mined was 79.0 MT in 2013 ( Figure 6). However, the minimum amount was 3.7 MT in 2020, which is substantially lower than the annual amounts from 2004-2020 (34.4 MT; Figure 6).  The amount from construction and operational sand mining was 28.5 MT, accounting for 3.77% of the total ( Table 6). The largest amount mined was during 2015 (8.5 MT), while the annual average was 1.7 MT (Figure 6).
The amount of river and waterway dredged sand was 143.8 MT, accounting for 19.01% of the total ( Table 6). The maximum amount (89.0 MT) was mined in 2018, while the annual average was 8.5 MT (Figure 6).

Estimation of the Actual Sand Mining Amount in the YR
Quantifying the sand mining amount from Poyang Lake is crucial for accurately estimating the total sand mined from the YR. Because illegal sand mining along the YR mainstream was prohibited in the early 21st century, mining activities vessels have moved to Poyang Lake, resulting in excessive illegal mining in the lake over an extended period. Jiang et al. [25] found that the amount of sand mining in Poyang Lake was approximately 2154 MT from 2001-2010, which is 7-8 times the approved amount. According to the shipport affairs department statistics, Chen [26] stated that the actual amount of sand mining in Poyang Lake was approximately 230-290 MT during 2005-2007, which is 4-5 times the planned amount. Therefore, in this study, we referenced the above studies, and the total amount of sand mining was estimated at 3470 MT in the YRB during 2004-2020, yielding an average annual amount of 204.1 MT. The amount from construction and operational sand mining was 28.5 MT, accounting for 3.77% of the total ( Table 6). The largest amount mined was during 2015 (8.5 MT), while the annual average was 1.7 MT (Figure 6).
The amount of river and waterway dredged sand was 143.8 MT, accounting for 19.01% of the total ( Table 6). The maximum amount (89.0 MT) was mined in 2018, while the annual average was 8.5 MT (Figure 6).

Estimation of the Actual Sand Mining Amount in the YR
Quantifying the sand mining amount from Poyang Lake is crucial for accurately estimating the total sand mined from the YR. Because illegal sand mining along the YR mainstream was prohibited in the early 21st century, mining activities vessels have moved to Poyang Lake, resulting in excessive illegal mining in the lake over an extended period. Jiang et al. [25] found that the amount of sand mining in Poyang Lake was approximately 2154 MT from 2001-2010, which is 7-8 times the approved amount. According to the shipport affairs department statistics, Chen [26] stated that the actual amount of sand mining in Poyang Lake was approximately 230-290 MT during 2005-2007, which is 4-5 times the planned amount. Therefore, in this study, we referenced the above studies, and the total amount of sand mining was estimated at 3470 MT in the YRB during 2004-2020, yielding an average annual amount of 204.1 MT.

Discrepancy between Supply and Demand of Sand Resources in the YR
Development of the YR Economic Belt resulted in reserve land resources driving the demand for sand and gravel. The reclamation project expanded from reinforcing embankments, filling ponds and foundations, and farmland improvement to coastal protection, island reclamation, and even national defense construction. For example, dredged sand from the Huangpu River was used to reclaim Fuxing Island in Shanghai. Therefore, sand and gravel resources in the YRB have been in short supply.
Additionally, studies show that the middle and lower reaches of YR, including delta regions, will likely continue to erode in the future [27]. However, the sand discharge in the YRB continuously decreased (Figure 7). Sediment discharge has been limited, particularly since the Three Gorges Dam (TGD) became operational in 2003 [28,29]. During 2004-2018, the TGD trapped 77% of riverine sediment inflow, largely reducing sediment load downstream [30].  Figure 7). Therefore, there is a prominent incongruity between the sand and gravel supply and the demand in the YRB.

Discrepancy between Supply and Demand of Sand Resources in the YR
Development of the YR Economic Belt resulted in reserve land resources driving the demand for sand and gravel. The reclamation project expanded from reinforcing embankments, filling ponds and foundations, and farmland improvement to coastal protection, island reclamation, and even national defense construction. For example, dredged sand from the Huangpu River was used to reclaim Fuxing Island in Shanghai. Therefore, sand and gravel resources in the YRB have been in short supply.
Additionally, studies show that the middle and lower reaches of YR, including delta regions, will likely continue to erode in the future [27]. However, the sand discharge in the YRB continuously decreased (Figure 7). Sediment discharge has been limited, particularly since the Three Gorges Dam (TGD) became operational in 2003 [28,29]. During 2004-2018, the TGD trapped 77% of riverine sediment inflow, largely reducing sediment load downstream [30].

Coping Strategies and Enlightenment
Based on the comprehensive review of sand mining in the Yangtze River Basin, we propose the following strategies that address the discrepancy between sand and gravel supply and demand. These include 1. Gain more insight into the YR channel evolution and adjust sand mining areas scientifically and dynamically.
The YR has experienced inconsistent channel scouring and deposition over time. Therefore, more studies on the river channel dynamics need to be conducted to identify spatiotemporal erosion-deposition equilibrium. The recoverable area should be dynamically adjusted according to runoff and sediment discharge characteristics. For example, the river section from Yichang to Chenglingji is currently in a state of erosion. Therefore, sand mining should be prohibited along this reach.
2. Reduce demand for natural sand.

Coping Strategies and Enlightenment
Based on the comprehensive review of sand mining in the Yangtze River Basin, we propose the following strategies that address the discrepancy between sand and gravel supply and demand. These include 1.
Gain more insight into the YR channel evolution and adjust sand mining areas scientifically and dynamically.
The YR has experienced inconsistent channel scouring and deposition over time. Therefore, more studies on the river channel dynamics need to be conducted to identify spatiotemporal erosion-deposition equilibrium. The recoverable area should be dynamically adjusted according to runoff and sediment discharge characteristics. For example, the river section from Yichang to Chenglingji is currently in a state of erosion. Therefore, sand mining should be prohibited along this reach.

2.
Reduce demand for natural sand.
Reducing demand for natural sand can be achieved through two approaches. First, mining amount and demand are often not equal on a yearly basis. Therefore, a modeling approach to optimize supply/demand balancing can be used, such as the model developed by Zhai et al. [31]. For instance, if the quantity of sand mining this year exceeds the demand, mining can be adjusted accordingly next year.
Second, research should focus on the use of recycled and alternative materials, such as "green concrete" with lower carbon emissions, ultra-high-performance, fly ash, lightweight, and geopolymer concretes, as well as recycled construction materials [32][33][34][35][36][37]. Alternatively, manufactured sand can replace natural sand. Manufactured sand with a particle size below 4.75 mm is formed by removing soil from rocks, pebbles, and mine tailings and crushing them to size.

3.
Strengthen the geological survey of sand and gravel resources in the YR and its tributaries and establish a sand and gravel data management system.
Geological surveys should include location and reserve information [38]. These surveys can play an essential role in sand mining planning, alleviating discrepancies between supply and demand, tracing sand mining sources, and reducing the negative environmental and social impacts.

4.
Establish scientific standards for sustainable sand mining.
Application of the quality management method from the plan-do-check-act cycle can aid sand and gravel mining management. Thus, the total amount of sand mining, recoverable area and period, and mining elevation can be planned and monitored. First, the on-site supervision should be strengthened following the plan implementation. Topography, actual sediment supply, and flow velocity variation should then be evaluated to gradually establish technical standards and institutionalize the best practices for sustainable sand mining.
Additionally, natural sand consumption must be reduced during construction [39,40]. Relevant departments can review and verify technical standards for construction projects to avoid an engineering "overdesign" [41,42]. We should reduce the construction of nonessential infrastructure and expand the use of green infrastructures, such as permeable pavements [43][44][45][46].
The middle and lower reaches of the YR are typical alluvial rivers with good-quality river sand, being an important sand resource in China. The recent decadal rapid development of the economy and building industry in the country has led to urbanization along the river. Conflicts between supply and demand have become and will remain challenging. Scientific assessment of the sand and gravel resources and planning reasonable sand mining behavior are prerequisites for maintaining the sustainable development of the YR sand. Establishing a scientific management system and strengthening management and control of sand mining processes are important in the sustainable development of sand resources. Additionally, research on renewable and alternative materials is important in solving the incongruity between the supply and demand of sand in China's Yangtze River Basin. Furthermore, the assessment and review conducted in this study can also serve as a reference for the mining and management of river sand for other rivers in the world, especially those where illegal sand mining is widespread, such as the Indus, Narmada, Nile, and Congo Rivers.

Conclusions
Currently, the global demand for sand and gravel is approximately 50 billion tons per year. With this gigantic demand, sand and gravel resources in countries worldwide are facing multiple challenges. Taking the YRB as an example, this study analyzed spatial and temporal changes in sand mining activities. The main conclusions are as follows.

1.
After 2015, sand mining areas in the YR expanded to their upper and lower reaches (Chongqing City and Shanghai City, respectively). Currently, sand mining is mainly distributed in the reaches of Chongqing, Hubei, Jiangxi, Anhui, Jiangsu, and Shanghai.

2.
There were 756.3 MT and 539.6 MT of sand mined in the YR mainstream and Poyang Lake during 2004-2020, respectively. The demand for sand and gravel increased to 7.6 billion tons within the YRB during 2021. There is a prominent incongruity between the sand and gravel supply and the demand in the YRB.

3.
Based on the current state of sand mining in the YR, reducing natural sand consumption for city construction is necessary. Optimizing supply/demand balance and replacing natural sand with recycled (including recycled construction materials) and alternative materials are essential. Furthermore, the recoverable area should be dynamically adjusted according to the YR evolution. Moreover, a geological survey of sand and gravel resources should be strengthened, and a sand and gravel data management system established. Institutional Review Board Statement: Not applicable.