Since ancient times, people have established communities in river deltas because it provides water, fertile land, and transportation access, making them an ideal place to live. This pattern has been carried forward to the present. With nearly 6 billion people living in river deltas, they are one of the most densely populated places on Earth (Kuenzer and Renaud, 2011). However, they are facing threats such as climate change, sea level rise, land use changes, and ecosystem degradation. Remote sensing is an excellent choice to obtain information about environmental conditions and their temporal changes. They play a crucial role in finding precursor signals of disasters, predicting the evolution of natural phenomena, among others. In relation to river deltas, remote sensing has been widely applied for coastline change detection, flood monitoring and forecasting, to name just a few (Merkuryeva et al. 2015; Li and Damen, 2010). Monitoring of the river deltas is important to maintain their health, identify problems promptly, and provide the scientific basis for management. Remote sensing is relevant technology to do so.

When a fast-flowing river joins a slow-flowing body of water or standing water (such as a lake, ocean, or reservoir), a river delta is formed (Kuenzer et al. 2019). A river delta is a landform shaped like a triangle with the top pointing upstream and the outer edge facing the injected waterbody, such as lakes and ocean, which can be seen as the "base" of the triangle. Generally, rivers are fed into the ocean. As shown in Figure 1, important large river deltas are located in the coastal area. They are found on every continent and every latitude zone, except the Antarctic and Arctic regions.

Importance of the river delta

As an ideal living location, river deltas have many advantages, such as flat terrain, easily available freshwater and saltwater resources, excellent transportation via waterways and nearby shores, fertile agricultural soil created by sedimentation, a generally abundant biological diversity and recreational values such as wetlands, coastal forests, and beaches (Zhao et al. 2022). It is noteworthy that a significant proportion of the gross domestic product (GDP) of many countries is generated in these important geographical areas (Kuenzer and Renaud, 2011). For example, the Pearl River Delta only occupies 0.5% of China’s territory and accounts for 4.5% of the country’s population, yet contributes 10% of the countries’ GDP while the Yangtze River Delta contributes 20 % of China’s GDP (Syvitski et al. 2009).

Threat to the river delta

River deltas are among the world’s most severely affected ecosystems by social, environmental, and climate change impacts (Kuenzer and Renaud 2011). Figure 2 shows major external and internal threats in the river delta areas (Kuenzer et al. 2019). Major external threats include: 

1. Sea level rise caused by climate change and extreme events such as droughts, upstream-related floods, storm surges, and tsunamis; 
2. Pollution caused by human activities, such as offshore oil spills, external marine pollution, and external air pollution;
3. Impacts caused by oil loading activities, such as upstream diversion/related flood pulse changes/sediment retention/water pollution;
4. Hydraulic action interests, in-migration, extensive tourism.

On top of the external factors contributing to threats for river deltas, numerous internal threats add to the list of risks. These include: 

1. Water, soil, and air pollution related to industry, cities, agriculture, and aquaculture;
2. Land subsidence driven by geological processes and underground mining of resources such as hydrocarbons and groundwater;
3. Loss of natural ecosystems such as coastal protected forests, wetlands, and marshes;
4. Biodiversity loss caused by land use change; and
5. Changes of natural dynamic delta system caused by infrastructure and barrier development.

These natural and anthropogenic threats collectively affect the river delta as well as the people and wildlife inhabiting it.

Principles of river delta monitoring by remote sensing technology

Remote sensing is an important tool for monitoring river deltas. The working principle of remote sensing is that satellite sensors detect the reflection of electromagnetic waves from objects on the ground and the electromagnetic waves emitted by them (Kuenzer et al. 2019). They store the information in remote sensing images. Figure 3 shows an overview of river delta monitoring by remote sensing technology (Kuenzer et al. 2019). By using satellite data, we can obtain information on the deltaic areas, such as the movement of sediments and geomorphic changes, vegetation cover, the health of ecosystems, as well as the climatic changes and the sea-level rise.

Due to the varying reflectivity of different substances, an indicator or model can be constructed based on the characteristics of the objects under study to extract information for analysis (Figure 3). Satellite-based data such as those generated by Moderate Resolution Imaging Spectroradiometer (MODIS), the Landsat and Sentinel fleet, and Advanced Synthetic Aperture Radar (ASAR), provide for a large amount of time-series data sets with global coverage. Satellites represent reliable data sources for long-term monitoring and numerous produced data sets are published in the open domain (Kuenzer et al. 2014a; Wulder et al. 2012).

It is worth noting that a satellite may be equipped with different types of sensors (simultaneously) and applied to different domains. Monitoring river delta via satellites mainly relies on optical, thermal and microwave remote sensors. Table 1 shows commonly used sensors and their applications. For example, time series of optical/thermal satellite data are increasingly being used to track coastlines, coastal erosion and accretion patterns, and disaster-related disturbance effects, particularly flooding; Microwave satellite data is more focused on flood and inundation mapping, water level, biomass mapping, wetland/urban analysis, land subsidence studies. They provide a basis for decisions and policy making on environmental conditions and temporal change, helping policy makers to better respond to environmental changes in river deltas.

Example Application: Monitoring Surface Dynamics of the Yellow River Delta

Kuenzer et al. (2014b) monitored the surface dynamics of the Yellow River Delta. The Yellow River carries a large amount of sediment, most of which is deposited in the lower reaches, resulting in frequent changes in land surface features. Therefore, the authors collected data from the Landsat fleet including the Landsat 1-5 Multispectral Scanner (MSS)/Thematic Mapper (TM) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) to analyzed it. They digitized features manually and developed rate-of-change statistics automatically. The latter were derived from the Digital Shoreline Analyses System (DSAS). Figure 4 shows that the phenomenon of river diversion has occurred in the Yellow River Delta region. The Yellow River used to drain into the Bohai Bay at the northern end of the delta (Figure 4a), and was diverted from its original northern channel, the Diao Kou He Canal, eastward to enter the Quing Shui Gou Canal at the end of 1976. The reason for this human-induced river redirection is that the major floods related to the ice jam in the Yellow River Estuary region have endangered the oil fields in the northern and northeastern deltas. In addition, figure 5 shows that there have been significant changes in the coastline. In the past few decades, the coastline of the Yellow River Delta has been characterized by strong momentum with major changes particularly in the northern and eastern coasts of the delta. There is a significant retreat of the coastline in the northern part of the delta, and extensive land accumulation in the eastern delta rift.

Conclusion

Remote sensing plays an important role in monitoring river delta. It can assess disaster risk, ecosystem health levels, as well as spatiotemporal changes of estuaries, providing an important basis for decision-making and river management. However, monitoring alone is not enough. Achieving the healthy development of the estuarine delta requires joint efforts of the public and the government to eliminate pollution from the source, control greenhouse gas emissions, and create a sustainable ecological environment.