In current research, there is rich content related to coastal cities, such as port cities, seaside cities, coastal cities, etc. However, there is no clear definition for cities at the mouth of rivers. Port cities are located along rivers, lakes, oceans, etc., have ports, and are the hub of water and land transportation. The most important driving force for their growth comes from the port, including the initial contact between the port and city, mutual relationships, the agglomeration effect, and the urban self-growth effect (Guo et al., 2013). Port cities are not only the center of economic activities but also an important hub of the transportation network, becoming a gathering point for import and export trade, industry, and tourism (Wang et al., 2015; Zheng et al., 2020). However, port cities rely on ports as their driving force and can be divided into estuary port cities, coastal port cities, river port cities, and lake port cities, depending on their geographical location. Estuary port cities and coastal port cities have a certain degree of overlap with estuarine cities, but not completely, mainly because the construction of port cities is mainly influenced by the conditions for building ports in bays (coasts) and is not necessarily located in the area of the mouth of rivers. Seaside cities are cities built along the coast, known for their beautiful environment, suitable climate, and excellent conditions for developing marine tourism (Han et al., 2022). Seaside tourism originated in England in the mid-18th century and entered the leisure and holiday development stage in the mid-20th century. China has 18,000 km of mainland coastline and abundant excellent bays and port cities in coastal areas, providing a unique advantage for developing seaside tourism. Cities at the mouth of rivers are similar to seaside cities in terms of natural conditions for developing bays or seaside tourism. In addition, cities at the mouth of rivers also have the scenic beauty of rivers, making them important for the development of water-based tourism. In addition, estuarine cities also have river landscapes, while coastal cities are not necessarily located along rivers. Coastal cities are located in belt-shaped areas extending from land to sea, including coastal plains, estuary deltas, and underwater slopes. They are in a unique environmental system where coastal dynamics interact with coastal land and have location advantages such as convenient transportation and rich resources. A coast is a broad area in which cities are not necessarily near the sea or along a river. Estuarine cities are a special type of coastal and riverside cities.

In conclusion, the estuarine cities, port cities, coastal cities, and coastal strip cities have both similarities and differences (as shown in Table 1). Estuarine cities are a new concept in the study of urbanization, urban development, and sustainability. Estuarine cities are characterized by the convergence of a river and ocean, the integration of land and ocean, as well as the close relationship between marine diverse ecosystems and human social and economic activities in the estuary area.

An estuarine city is a specific type or form of city that takes into account both the natural attributes of an estuary and the social and economic attributes of a city. As a node in the multi-phase interface process where rivers and land meet the ocean, an estuarine city has accumulative effects of material and hydrodynamic forces and interwoven impacts of water and land ecosystems, often in areas where the local ecological environment is particularly fragile and complex. As a confluence of river basins and near-shore areas, the development of the city’s economy and society must also consider the protection and security of the regional river, ocean water systems, and the local ecological environment. Therefore, the development positioning, city image creation, industry selection, and cultural connotation enhancement of the city are of great significance in promoting the sustainable development of the city and the region. An estuarine city is an important carrier for implementing new development concepts such as ecological civilization construction, sustainable development, and people-oriented livable cities. Its connotations are mainly reflected in the following four aspects: confluence of river and ocean water systems. Estuarine cities are located in transitional areas where river and ocean water systems are frequently impacted, with the characteristics of rivers dominant in the sections of the river near the estuary and ocean characteristics dominant in the coastal areas outside the port. The rivers and ocean interact alternately with each other, with unique properties such as unique sediment movements and interactions between freshwater and saltwater.

Integrated development of land and sea. The estuarine city is located in a transition zone where the land and ocean systems frequently interact and has a unique environment that combines different attributes of land and sea. It is a zone where the Earth’s hydrosphere, lithosphere, atmosphere, and biosphere interact most frequently and actively and requires a systematic approach to the development and protection of both land and sea.

The estuarine city has the unique geographical advantage of being a comprehensive hub that connects upstream, midstream, and downstream cities along the river and also serves as an important hub for establishing international connections with cities in other parts of the world through the ocean. This location creates a distinct advantage that cannot be compared to other cities. The ecological environment is important and fragile. Due to the complex processes and mechanisms of interaction between land and sea in the coastal city region, complex ecological systems such as mixing of fresh and salt water, tidal flat wetlands, river estuary islands, sandbanks, and wetlands are formed. These areas are important convergence and dispersion zones for energy and material flow, are greatly disturbed by human activities, and are highly sensitive to the impact of climate change.

The global river vector data was obtained from the HydroSHEDS database, the Esri World Major Rivers layer, and the World River Data Set provided by the “World-wide Hydrogeological Mapping and Assessment Programme” (BGR-UNESCO). City location data was obtained from the high-precision Global Administrative Areas Database (GADM) and administrative boundary data from Open Street Map. River statistical data, including river length, river flow, sea into which river flows, etc., came from various international organizations or government statistical yearbooks, the National Geographic Education Resources Network in the US, etc. City average temperature and precipitation data came from the ERA5-Land meteorological data product, and city vegetation productivity data was calculated based on the MOD17A3HGF product’s annual net primary productivity. Population and economic data came from city or regional statistics, and road network data came from the GRIP Global Road Data Database (https://www.globio.info/download-grip-dataset).

This study focuses on representative cities at the mouth of a great river in the world and analyzes and summarizes their geographical characteristics and development patterns. First, great rivers that flow into the ocean are defined based on both their natural attributes and their socioeconomic significance. Then, great river estuarine cities are selected based on their location and distribution (Figure 1).

From a natural attribute perspective, big rivers refer to rivers that have significant main natural geographical attributes such as length, flow rate, watershed area, etc., and natural geographical attributes such as average annual sediment discharge, delta area, and number of countries through which the river flows, such as the Mississippi River, Yangtze River, etc., that have developed into famous estuary cities such as New Orleans and Shanghai. From a social and economic perspective, big rivers should include rivers that have played a significant role in human social and economic activities and development processes, although some rivers may not be prominent in natural attributes such as length and flow rate but have played a major role in the region, country, and even in the history of human civilization worldwide, such as the Thames River (346 km) and Rhine River (1233 km), which have given birth to famous estuary cities such as London and Rotterdam and have been defined as estuary cities of big rivers.

Based on the extraction of rivers with estuaries based on basic geographical data, rivers are ranked according to natural attributes such as river length, flow rate, etc. At the same time, based on river social attributes, major rivers with significant regional social, economic, and cultural development and historical significance are selected from each continent, forming a list of big rivers and their estuarine city spatial point data. Secondly, using spatial overlay analysis, the list of 50 big rivers and cities at estuaries is determined (the detailed list is in Appendix 1).

These 50 rivers are distributed across six continents, and they converge extensively with oceanic water systems (Figure 2). The rivers in Asia mainly flow into the Pacific Ocean, followed by the Indian Ocean and the Arctic Ocean. The rivers in North America primarily flow into the Atlantic Ocean, as well as the Pacific Ocean and the Arctic Ocean. In Europe, besides flowing into the Atlantic Ocean, some rivers also flow into the Black Sea and the Mediterranean Sea. African rivers mainly flow into the Atlantic Ocean and the Mediterranean Sea. The rivers in South America all flow into the Atlantic Ocean, and the rivers in Oceania all flow into the Indian Ocean. Nearly one-third of the rivers entering the sea have an average annual runoff of more than 10,000 m³/s, of which the Amazon River in South America has the largest runoff at 210,000 m³/s. The average runoff of the Congo River in Africa, the Orinoco River in South America, and the Yangtze River in Asia exceeds 30,000 m³/s.

According to the data, the 50 cities where large rivers empty into the ocean are distributed across the six continents (Figure 3), with the highest number of cities being in Asia (19 cities) located along the Pacific coast. In North America and Europe, there are six and eight cities, respectively, located along the Atlantic coast. In South America, there are five cities located along the northern coastal region near the equator. There are three cities in Africa and one in Oceania, respectively.

The elevation of the 50 estuarine cities ranges from 0 to 400 m, with an average elevation of 30.59 m, belonging to low-altitude areas. Among them, 12 estuarine cities have an elevation lower than 10 m, mainly located in Southeast Asia and South America. There are more estuarine cities with elevations between 10 m and 50 m—21 in total, accounting for 42%—concentrated in Asia, North America, and Europe. The terrain of the estuarine cities is mainly river delta plains, with flat terrain and low elevations.

The climate in the 50 estuarine cities is generally favorable, with an average temperature range of 10-25°C. According to ERA5-Land weather data analyzed from 1981 to 2020 (Figure 4), 24 of the 50 estuarine cities have an average temperature of 10-25°C, accounting for 48%. A total of 14 cities have an average temperature of 25-30°C, mainly including cities in Asia such as Bangkok, Yangon, and Ho Chi Minh City, accounting for 28%. Five cities have an average temperature of 0-10°C, mainly Quebec, Saint Petersburg, Sapporo, and Vancouver, accounting for 10%. Seven cities have an average temperature below 0°C, mainly including cities in Russia such as Tiksi and Dudinka and in Canada such as Inuvik, accounting for 14%.

Most of the estuarine cities have abundant rainfall. Among the 50 estuarine cities, 28 have an average rainfall of more than 800mm during 1981-2020, located in the humid region and accounting for 56%. During the same period, 16 estuarine cities were located in the semi-humid region with a rainfall between 400-800 mm, accounting for 32% of the total. The other six estuarine cities are located in semi-arid and arid regions, accounting for 12%.

The advantage of biological resource endowment is obvious. In 2020, the mean of net primary productivity (NPP) in 50 coastal cities was 3958 kgC•m^-2 a^-1, and cities with higher NPP values (Figure 5) were mainly concentrated in South America, mainly Giudad Guayana, Brejo Grande, Belém, and Macapa cities with NPP values exceeding 9000 kgC•m^-2 a^-1. Among these 50 cities, 32 cities had high NPP levels, with 10 cities distributed in Asia, eight in Europe, eight in North America, and six in South America.

In terms of social and economic characteristics, there are significant differences in the development of cities around the world, with huge variations in the population size and economic level of different cities. There are six cities with populations exceeding 10 million, including Shanghai, Dhaka, Guangzhou, Karachi, Tianjin, Bangkok, and Cairo. There are five cities with populations between 5 million and 10 million, including London, Ho Chi Minh City, New York, Yangon, and Saint Petersburg. There are 13 cities with populations between 1 million and 5 million, including Busan, Buenos Aires, Vancouver, Belém, Niigata, Dongying, Sapporo, Manila, Hamburg, etc. Some cities along the coast of the Arctic Ocean, such as Gillham, Tiksi, Salekhard, and Inuvik, have populations of less than 100,000. In terms of the total city economy, cities such as New York, London, Shanghai, Philadelphia, Cairo, and Guangzhou, which are more developed, have GDPs exceeding 300 billion US dollars, which are 10 to hundreds of times higher than those of cities in less developed regions. In terms of per capita GDP, cities in North America and Europe, such as Washington, D.C., New York, Portland, and Hamburg, have per capita GDPs exceeding 50,000 US dollars, while cities in Africa, such as Cairo, and in Asia, such as Ho Chi Minh City, Manila, and Dhaka, have lower per capita GDPs, all less than 10,000 US dollars.

The urban infrastructure construction at the estuary is relatively good. In 2015, the average transportation network density of 50 estuarine cities was 3997 m/km². The transportation density of seven cities is in the range of 10,000-20,000 m/km², including Buenos Aires, New York, Philadelphia, Manila, Washington, D.C., Lisbon, and Portland. The densities of nine cities range from 5000 m/km² to 10,000 m/km², including Cairo, Porto, London, Bangkok, Yangon, and Shanghai. Ten cities had a network density in the range of 1000-5000 m/km², with an average density of 2491 m/km², mainly in some cities in Europe and Asia, such as Hamburg, Saint Petersburg, and Rotterdam in Europe and Busan, Sapporo, Guangzhou, Dhaka, and Tianjin in Asia. Seven cities had a network density less than 100 m/km². According to the statistics of the World Shipping Council, among the 50 seaport cities, five cities rank among the top ten in terms of port throughput in the world, namely Shanghai, Guangzhou, Busan, Tianjin, and Rotterdam. The port throughput of these cities exceeded 14 million TEU in 2020.