The upper reaches of the Min River refers to the reaches above Dujiangyan, bounded by the latitudes 30°45′–33°09′ N and the longitudes 102°35′–103°56′ E, covering an area of about 22,000 km² in the southeast margin of Qinghai-Tibet Plateau. Features include complex geological structures and fault developments (Figure 1). Areas covered by the tributaries include Wenchuan County, Li County, Heishui County and the major part of Songpan County and Mao County, which are all in the Aba Tibetan Autonomous Prefecture, and a small part of Dujiangyan City (the five counties listed are the focus of this paper). The terrain in the area is high in the west and low in the east; the elevation range is 734–6,153 m (Figure 2).

The study area, with an azonal arid valley climate, is characterized by a dry and windy climate, cold in winter and cool in summer, with large temperature differences between day and night, regions, and altitudes. Because of the control of westerly circulation and the monsoon warm current, the climate transitions from the subtropics to a warm temperate zone and then to a cold temperate zone; it demonstrates a climate distribution form of echelonment (Ding et al., 2014). The annual mean temperature of the area is 5.7–13.5°C, and annual precipitation is 400–800 mm for which 80% is concentrated in the period from May to October (Ding et al., 2014). Due to the foehn effect caused by a deep valley and large relative elevation difference, the warm and humid airflow in the southeast and southwest is separated by the mountains, which is not conducive to the formation of precipitation in the valley. Precipitation is relatively abundant in the high mountain areas. The multi-year average precipitation in the northwest of Heishui County is ∼1,200 mm, and coupled with Mao County is the precipitation center, which gradually increases to the southeast and northwest (Ding et al., 2014).

The China Geological Survey from May 2008 to April 2017 was used to classify types of geohazards (Table 1; Figure 3). By 2017, 824 collapses were documented in five counties in the upper reaches of the Min River, with a maximum scale of 300 × 104 m³ and a minimum scale <1 m³. Among them, small collapses account for 79.3%, whereas giant collapses, large-scale collapses, and medium-scale collapses account for 0.24, 2.42, and 17.96%, respectively.

There were 579 unstable slopes, and the maximum number of people threatened is 800. The unstable slopes in this area are divided into three grades: small, medium, and large. The percentages of medium-scale and small-scale unstable slopes were 48.8 and 45.3% respectively, and large-scale accounted for 5.9%.

There were 921 landslides, with a maximum scale of 540 × 104 m³ and a minimum scale <1 m³. Small-scale landslides accounted for the majority (83.3%) of the total number of collapses; large- and medium-scale collapses accounted for 14.7 and 2%.

There were 841 debris flows, with a maximum scale of 200 × 104 m³ and a minimum scale of 0.01 m³. Small-scale debris flows accounted for the vast majority (54.2%) of the total number; medium-scale, large-scale and giant-scale debris flows accounted for 38.6, 5, and 2.1%, respectively.

The grading standard here is slightly modified according to Zhang et al. (2002) in which the unstable slope is graded according to the number of people threatened.

People belong to Tibetan, Qiang, Hui, and Han ethnicities, and the area has a diversified economy and many cultures (Wu et al., 2003). According to the data of the sixth census, there were more than 320,000 people in the five counties in 2010. Because of the high mountains and valleys, people build settlements along the river valley, with the distribution affected by natural and geographical conditions. The settlement locations vary in altitude, forming mountain villages or mountain market towns with a hyper-normal vertical distribution of rural settlements (Ding et al., 2014). The size of villages is small, mainly because of the undulating mountainous terrain, the flat land is confined, and topographic conditions are challenging for building large villages. The residential buildings in this area are mostly stone or Earth rock, with attractive appearances in form and structure and with unique landscape values. The bottom of the valley of the river also is the location of agricultural activity (Ding and Hu, 2020). Affected by the terrain and limited water and soil resources, there is less flat cultivated land, and terracing is common.

The distribution data of point settlements were manually extracted from 91 satellite map software. The location of settlements was represented by the administrative village committee in the local area (Figure 4). The cultivated land data was 1:10,000 in vector form from the land management department.

Valley settlement is the most important settlement form in the upper reaches of the Min River (Ding et al., 2018). Deep “V"-shaped erosion small catchments are well developed in this area (Ding et al., 2014). Unstable slopes, collapses, and landslides change landforms, especially topographic slope, which provides relatively flat space for agricultural production and settlements. The rock and soil on a slope may lose stability, breaking away from the parent body under the action of gravity, collapses, and rolls. They accumulate at the foot of the slope (or valley), or the rock and soil slide downward along the slope caused by natural factors such as rainfall, river scouring, earthquakes, snow melting, and rainstorms. The flood flow with a large number of solid substances (such as mud, sand, and stones), brought by water sources such as ice/snow meltwater or dam releases, forms a relatively deep soil layer attractive for cultivation (Cui, 2011).

In the past, under a small-scale peasant economy in the period of traditional agricultural society, people were self-sufficient, and the land was the most basic and important resource (Zhou et al., 2020), and agricultural production mainly depended on cultivated land in mountainous areas, and the scale of settlements is closely related to the quality and quantity of arable land (Ding et al., 2014).

To facilitate farming and basic life activities, settlements were distributed according to intercepted water and soil flow (Ding et al., 2014). The contact zone between a mountain and plain, or the contact zone between a hillside and alluvial fan, often supported rural settlements. To minimize the occupation of cultivated land, the site selection of Tibetan and Qiang settlements was along the mountain. Houses were built close to small and medium-sized streams with good water quality or with convenient places for water intake (Zhou et al., 2020). Many rural settlements often chose relatively stable diluvial, colluvial, or alluvial fans, and relatively wide and slow ice erosion valley areas, which increases the potential risk from geohazards.

Human production and life are threatened by geohazards. When a settlement is located in a collapse area, the front edge of the landslide with the passing area and accumulation area of debris flow will lead to casualties and damage to buildings (Ding and Hu, 2020). Roads and bridges are vulnerable to collapses, landslides, and debris flow, resulting in road obstruction and damage; arable land also will be damaged.

Unreasonable human activities are one of the main factors inducing geohazards (Ding and Hu, 2020). The reconstruction of the slope in the mountainous area of the upper reaches of the Min River by excavating ore resources, and building roads, houses, and other engineering projects have rendered the slope unstable and the environment is damaged. Even if measures, such as artificial tree planting, are taken to restore the environment, the control of rock and soil stability is low and water and soil loss continue (Ding and Hu, 2020). Further, human activities such as reservoir water storage, channel leakage, stacking of waste, slag filling, and strong mechanical vibrations will lead to slope instability and rock and soil movement. This can result in the destruction of cultivated land and houses, as well as casualties, furthermore, landslides and rock collapses produce loose materials, which become the source of rock debris in the affected areas (Cui et al., 2011). The material in the river valley is easily washed away and transformed into debris flow during a rainstorm. Therefore, the relationship among geohazards, settlements and cultivated land is “hazards appear with human being”.

However, different types of geohazards have different effects on people’s production and life. Collapse occurs suddenly, and can pose a fatal threat to life and property, and the accumulation formed by large slope is poor sorting, which is difficult to cultivate. The formation of landslide is slow, with the rock and soil movement being an integrity, which does not destroy the basic soil structure, and even can effectively slow down the slope, which can increase the farming area. Debris flow mainly occurs at the bottom of river channels or gullies, with relatively concentrated scope and little impact on farming areas, which mainly affects human settlements built along rivers or gullies. However, the debris flow is relatively easier to warn than disasters caused by collapse. Unstable slope is mainly due to its unpredictability, which increases the utilization risk.

The coupling mechanism of geohazards, settlements, and cultivated land is shown in Figure 8. Geohazards change local landforms, provide materials, and open space for the development of cultivated land and settlements. In the traditional self-sufficient subsistence agriculture stage, there is an interdependent relationship between production space and living space. Yet, human activities, such as reservoir creation, irrigation, deforestation, and slope planting, as well as building houses, roads, and bridges, induce geohazards.

The main function of settlements is supporting life activities, whereas the main function of cultivated land is production. According to the different nature of geohazards, settlements can be categorized into production-stressed settlements and life-stressed settlements. A production-stressed settlement means that the cultivated land maintaining the production function of the settlement is damaged or has the potential risk of damage by geohazards. A life-stressed settlement means that the settlement itself is (or can be) affected by geohazards.

The Wenchuan earthquake on 12 May 2008, destroyed the stability of mountains in the fault zone and triggered collapses, landslides, barrier lakes (“earthquake lakes”), debris flow, and unstable slopes (Cui et al., 2011); The Wenchuan earthquake on 12 May 2008, had similar effects and further affected the weathering of regional rock mass via faults and folds in subsequent aftershocks. The earthquakes exacerbated the instability of the slope (Cui et al., 2011; Ding and Hu, 2020), and landslides and rock collapses produce loose materials which are sources of rock debris of debris flow in the affected areas (Cui et al., 2011). The chain effects of geohazards made the stresses on local settlements long-lasting (Cui et al., 2011; Zhang et al., 2011).

China has entered the middle stage of industrialization. The impacts on rural areas have been exacerbated by impacts of urbanization and industrialization, and the social and economic structure has been transformed. China has transformed from a rural society to an industrial and urban society, from a closed and semi-closed society to an open society, and from a single homogeneous society to a diversified society (Lu, 1997). The economy has changed from self-sufficient livelihoods to a market-oriented commodity economy (Liu, 2007). With the development of China’s economy, lifestyles are increasingly urbanized, and there is more demand for unaltered environments. Rural areas are no longer just described as remote, backward places in urgent need of modernization (Woods, 2019). Many functions and values possessed by rural areas can relieve excessive pressure on urban systems.

The upper reaches of the Min River are in the historical ethnic corridor and the channel connecting Qiang and Tibet between Han and Tibet. It is a transitional area of Han, Qiang, and Tibetan culture (Wu et al., 2003). With urbanization, the number and scale of cities and towns in this region have increased rapidly, and the relationship between man and land is symbiotic and fed each other in many directions. Under the background of the construction of ecological civilization (Pan, 2016) and the promotion to building beautiful countryside, the priority development of agriculture and rural areas is not only to make up for the rural shortcomings of building a well-off society in an all-round way but also a major task to realize the great rejuvenation of the Chinese nation (Pan, 2016), and the development of mountain society is the top priority of rural revitalization, facing the opportunity of transformation and development.

With the implementation of the two-wheel-drive strategy of new-type urbanization and rural revitalization, the change of the human social and economic systems will cause changes in the human-land relationship in the upper reaches of the Min River. New-type Urbanization Plan (2014–2020) was put forward by China’s National Development and Reform Commission to refine the existing mode of urbanization and promote the citizenization of transferring agricultural residents (Wang et al., 2015), which means more and more peasants will engage in non-agricultural industries. Rural revitalization strategy aimed to achieve high standards for living, rural civilization, clean and tidy villages, and democratic management (Wang and Zhuo, 2018), which pays more attention to the sustainable development of rural areas and the improvement of farmers’ living standards. Settlement is the spatial form of human survival and residence in mountainous areas, which is the most closely connected space-time unit between human beings and the mountainous environment (Ding et al., 2014). The settlements in the upper reaches of the Min River are the product of local people’s long-term adaption to the mountainous environment and have unique cultural value. With the rapid development of transportation, the economy and society of mountain settlements are undergoing development driven by a series of external factors (Zhou et al., 2013). The unique culture and beautiful mountain scenery have attracted many tourists and promoted the development of mountain tourism. With the increasing development of mountain tourism, the protection of cultural resources has received more attention (Chen et al., 2011). In this context, the effect of geohazards on the transformation and development of settlements is vital to understand. Biodiversity, the sensitivity of ecological environments, the human and environment relationship, and the security and livability of settlements should be considered in the urbanization process.