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Bienvenue au Congo! The mighty Congo River and the vast Congo Basin that feeds it have attracted the attention of travelers, writers, and scientists for centuries. Henry Morton Stanley (1885) likely published the first water‐related findings in the Congo Basin. He and his team found the plunge pools on the mainstem Congo below Kinshasa with “a strong under‐current of water flowing up stream” and estimated its discharge at 40,700 m³/s.

During the first half of the twentieth century, hundreds of monitoring stations were established and maintained on the river, providing streamflow and rainfall measurements (Figure 1.1). There were also research institutes that coalesced these measurements into collections, notably the four‐volume Atlas Climatique du Bassin Congolais by Franz Bultot (e.g., Bultot, 1971). From this research effort, we now have a time capsule stretching back a century upon which to build today’s understanding of changing climate. These historic archives are unique among the world’s tropical watersheds. Likewise, the Congo itself stands alone, belying easy characterizations. For example, the vast majority of the basin is above 300 m elevation, which is much higher than the world’s other major rivers such as the Amazon or Mississippi. How does this simple difference in elevation factor into the Congo’s atmospheric and fluvial processes? What governs the meteorology and hydrology of the Congo? Our monograph brings these basic questions and many others to the fore. Despite the opportunity to address such key science questions, over the past half century scientific knowledge about the Congo has been overshadowed by its cousin, the Amazon. In 1973, Marlier noted that “we know less about the physical characteristics of the Amazon” than the Congo Basin, but this was soon reversed. Amazon research has grown considerably since then, both within its basin and internationally, while the Congo Basin has experienced relatively little interest from outside scientists.

Figure 1.1 Map of the Congo Basin. See Alsdorf et al. (2016) for details. Created by Brian Alsdorf for the AGU Chapman Conference on “Hydrologic Research in the Congo Basin” (Beighley et al., 2019).

Alsdorf et al. (2016) compared the number of published papers related to hydrology in the Congo with those of the Amazon and found about ten times fewer Congo papers. This comparison is appropriate because both basins lie under tropical rain belts, constitute the two largest river water flows in the world, and have an abundance of tropical forests and wetlands. Note that the comparison by Alsdorf et al. was not intended to be a comprehensive review of all hydrological literature (indeed, their focus is on English‐language journals available internationally) but rather illustrated the relative comparison between the two basins in the peer‐reviewed English language scientific literature of an international audience.

Although our contemporary understanding of the Congo Basin and its hydrology is an order of magnitude smaller than that of the Amazon, important advances have been made regarding Congo research, but these have been limited by language and economic barriers. This monograph aims to address some of these issues by presenting new science on the Congo Basin, with chapters written by experts from and in the region, and presented in both English and French to enable greater access and discoverability.

This monograph is a foundation for new scientific discoveries. A key for making a new discovery is to study a location that has not been well explored and hence not well documented (e.g., Oliver, 1991). The 27 chapters in this monograph document new in‐situ and remotely sensed measurements, describe recent model results, and provide insights toward exploring the Congo and surrounding sub‐Saharan basins (Photo 1.1). Another key for making a new discovery is to rework historic data using new methods and to combine it with contemporary measurements (e.g., Oliver, 1991). Historically, there were more than 400 stream gauges and rainfall stations operating throughout the Congo Basin at various times during the first half of the 1900s. Some of the data from these gauges are available via the Global Runoff Data Center (GRDC, 2021) but much of it is available thanks to collaborations with those who contributed to this book. Adding to these in‐situ measurements, satellites from the past two decades have provided measurements of rainfall, changes in total storage, water surface elevation, flooded areas throughout the entire basin, and land‐use land‐cover change. And in the past year, new streamflow and sediment flux measurements have been recorded by chapter authors (Photo 1.2). Therefore, a foundation of hydrologic research and measurements upon which to build opportunities for scientific discovery exist in the Congo Basin.

Photo 1.1 View of the Congo River and research boat in the Cuvette Centrale.

Credit: CRREBaC/CRuHM.

Photo 1.2 Several chapter authors have collected new measurements on Congo Basin rivers. Photo taken on Lomami River at Isangi, where the Lomami River joins the Congo River.

Credit: CRREBaC/CRuHM.

Photo 1.3 International collaborations are welcome in the Congo. Photo taken at Kisangani where the research team is traveling downstream from Kisangani to Kinshasa, collecting river measurements over 1700 km of flow distance.

Credit: CRREBaC/CRuHM.

This monograph is a foundation for new collaborations. The 106 people who have contributed to the chapters in this book are open to building new research ventures (Photo 1.3). We anticipate that many readers will have studied the Amazon Basin and other tropical watersheds. They will recognize the similarities between the world’s two largest rivers and their basins and, importantly, find that the differences are significant and will likely lead to new discoveries. For example, the Amazon wetlands along the mainstem Solimoes River are marked by numerous floodplain channels and by water depths approaching ten meters, whereas the Congo’s Cuvette Centrale has no similar channelization and thus appears to have diffuse, shallow flows of only about a meter in depth (e.g., Alsdorf et al., 2016). What do these differences imply about water and sediment exchange between the mainstem river and its wetland, or about the related biogeochemical processes that include some of the world’s largest peatlands (Dargie et al., 2017), or about the fluvial geomorphic history of these two large river complexes? This monograph will hopefully inspire each reader to hypothesize testable ideas that they will use to forge new collaborations and hence new discoveries.

This monograph is a foundation for resource management. The Congo River is the lifeblood of more than one hundred and twenty million people who live in the basin. The river and its tributaries are the main and sometimes only transportation corridor (Photos 1.4, 1.5, 1.6). Challenges to maintaining river traffic include knowing water depths given that these change from shifting bathymetric dunes and change from seasonal to climatic variations in rainfall. Moreover, specific rivers will experience changes in flow if engineering projects are implemented, such as ideas expressed to channel flow away from the Ubangui River to Lake Chad. Perhaps, too, small hydroelectric generators such as those of the Kibali gold mine on the Nzoro River or larger generators such as the Inga Dam (Photo 1.7) need to know the long‐term expected rainfall and hence capability to provide peak electricity demand. Chapters in this book provide science foundations toward addressing resource management throughout the Congo Basin.