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At 3.7M km² in size and 41,000 m³/s in average discharge, the Congo River Basin is second only to the Amazon in watershed area and flow rate (Figure 1.1; Alsdorf et al., 2016; Laraque et al., 2013, 2020; Tshimanga & Hughes, 2014). Goudie (2006) has suggested that sometime in the past 30 Myr stream capture by a small coastal river permanently linked the Congo River to the Atlantic Ocean, resulting in plunge pools with depths greater than 100 m. For example, just above Matadi approximately 150 km from the river’s mouth, Stanley (1885) reported depths of ninety fathoms (i.e., 165 m). Oberg et al. (2009) verified plunge pool depths of about 100 m in the reach below Kinshasa with one pool at 220 m deep. Given the land surface elevations, some of these pools bottom out below sea level. At the other end of the mainstem, where the Congo River is known as the Lualaba River, a number of cataracts and narrows likely indicate a variable channel bathymetry controlled by geology and tectonics. In between, the middle reaches of the mainstem have shallows of only a few meters depth (see Alsdorf et al., 2016). In summary, the hydraulics of the Congo River are variable and bely simple characterizations, especially for a massive river.

Photo 1.4 Waterways are major transportation routes in the Congo. Photo is taken in the Cuvette Centrale.

Credit: CRREBaC/CRuHM.

Photo 1.5 Rivers serve commerce at small scales such as this boat. See Photo 1.6 for larger scales of commerce.

Credit: CRREBaC/CRuHM.

Photo 1.6 Rivers serve commerce at large scales such as this boat moving logs. See Photo 1.5 for smaller scales of commerce.

Credit: CRREBaC/CRuHM.

Photo 1.7 The Inga Dam. The Congo River is in the background. The canal in the foreground feeds penstocks just downstream of the photo.

Credit: CICOS.

Precipitation in the Congo ranges from an average of about 1900 mm/yr in the central basin to about 1100 mm/yr at its northern and southern boundaries (Bultot, 1971). Rainfall generally follows a south to north migration with the southern parts of the basin experiencing their annual maximum precipitation in December to March, whereas rainfall peaks in the northern basin areas in July to October. The central basin areas experience two precipitation maxima with a smaller peak in March to May and a greater peak in September to November. Traditionally, the driver of this seasonally migrating precipitation was assigned to the InterTropical Convergence Zone (ITCZ), suggesting that it enhanced local convection. Recently, however, meteorologists have recognized mesoscale convective systems bounded by migrating jets that are a more likely driver of the migrating rainfall (Nicholson, 2009).

The seasonally varying rainfall produces a bimodal river discharge on the mainstem Congo (see Alsdorf et al., 2016, for details). Because it takes between two weeks and two months, depending on flow distance, for local flood waves to migrate downstream, the timing of the flood peaks occur later than the associated rainfall maximums. At Kinshasa‐Brazzaville, the smaller flood peak occurs in about May, associated with rainfall over the southern basin, and the larger peak occurs in about December, associated with rainfall over the northern basin. Historically, discharge has varied at Kinshasa‐Brazzaville between about 1960 and 1995, with a notable significant increase in the decade from 1960 to 1970, and is only now returning to its long‐term average (Laraque et al., 2001, 2020).

Remote sensing is important for measuring wetland and lake areas and for measuring water elevations. Bwangoy et al. (2010) used visible band satellite imagery, synthetic aperture radar (SAR) mosaics, and a digital elevation model to find that wetlands in the Congo Basin cover 360,000 km². Lee et al. (2011) used radar altimetry to measure water surface elevations of the Congo River and the immediately adjacent waters in the Cuvette Centrale. They found that the wetland water levels were consistently higher in elevation, suggesting that locally the water exchange was always from the wetland to the river. Jung et al. (2010) used interferometric SAR to measure changes in water levels in the Cuvette Centrale and suggested that flows were diffusive, not channelized at the 100 m resolutions of the measurements.

Since Richey et al. (2002) first published their findings regarding CO₂ evasion in the Amazon Basin, understanding gaseous carbon evasion from tropical wetlands into the atmosphere has been recognized as a key part of the global carbon cycle. However, while the Amazon Basin has experienced numerous carbon evasion studies, the Congo Basin has relatively fewer. Notably, Borges et al. (2015) measured pCO₂ to estimate a carbon flux of 0.5 PgCO₂ equivalents/yr from the Cuvette Centrale. But this is only a beginning, and more studies are needed to better understand the carbon evasion from the world’s second largest basin.

In the review of the Congo Basin by Alsdorf et al. (2016), the findings of many previous researchers were summarized to suggest seven testable hypotheses. These hypotheses became the foundation for an AGU Chapman Conference held in Washington D.C. in September 2018 (Beighley et al., 2019) and helped to frame the content of this monograph. These hypotheses are:

1 The water in the Cuvette Centrale is supplied mostly by rainfall.

2 The water empties from the Cuvette Centrale mostly by evapotranspiration.

3 Despite known variations in the discharges of the Congo and Oubangui Rivers, previous rainfall amounts have varied comparatively less across the basin.

4 Because of its location beneath the “tropical rainbelt,” the Congo Basin will experience significant changes in both rainfall amounts and geographic locations from climate change.

5 Deforestation of 30% of the headwater sub‐basins will significantly increase headwater flows and hence increase downstream discharge.

6 Future hydroelectric power generation will not impact waters flowing in rivers.

7 The annual average amount of CO2 and CH4 evasion from all Congo Basin waters is more than 480 Tg C/yr, i.e., more than a value comparable to that of the Amazon per unit area.

Note that these are not statements of fact; rather, these are testable ideas that will be proven true or proven false. Indeed, many of the chapters in the monograph address these statements and provide insights regarding their true or false nature. Authors met at the Chapman Conference to discuss the hypotheses and to build new collaborations while strengthening existing research programs. These interactions became the launching point for this monograph. Contributions to the monograph were broadly invited, including authors who participated in the Chapman as well as researchers who were not able to attend. As a guideline, but not a requirement, authors were encouraged to address any of the hypotheses.