This chapter describes the variability of rainfall and river discharges in the Ogooué River basin (ORB) in recent decades (since 1940). Due to its location crossing the Equator, the ORB receives abundant precipitation that maintains one of the world’s best-preserved ecosystems. In contrast to neighboring forest basins that have been severely degraded because of deforestation, mining resources extraction, extensions of agricultural areas, and river transport, which is a crucial alternative to the cruel lack of road infrastructures, the ORB is experimenting with an exceptional conservation policy in the region. For example, the rural penetration rate in Gabon is about 1 inhabitant per km² and many studies report a deforestation rate close to 0%, with even full natural regeneration. However, the fluctuations of the standardized anomaly index of rainfall in the ORB show three main phases of variations: the first wet phase was characterized by abundant precipitations from 1940 to 1970, the second phase of the long-term mild drought was extended in the 1970s and 1980s and the final third phase presented a slight return of abundance in precipitation. Even though drought severity in the ORB was mainly weak, its effects in river discharges were very sensitive on seasonal and inter-annual scales. The pure equatorial regime of the ORB characterized by equal maximum floods in spring and autumn changed significantly from the difference between both maximum discharges of 13.5 % during the 1960s to 27.0 %, 38.4 %, 33.9 %, and 26.7 % for the 1970s, 1980s, 2000s and 2010s respectively. A brief comparison between the ORB and the Congo River basin showed that changes in the ORB are part of a regional process that Central Africa is undergoing with some spatial heterogeneities.

The latest work on the main African rivers on the Atlantic coast has made it possible to subdivide the multi-year streamflow records into several homogeneous phases. The year 1970 seems to mark both for West and Central Africa the major hydroclimatic event of the 20th century, heralding its main period of deficit flow. For the first time, this article presents a comparative study of the hydro-rainfall records of five drainage systems (those of the Congo River and its main tributaries Lualaba, Kasai, Sangha, Oubangui) based on field data, obtained on both the left and right banks of the Congo River. A reconstitution of the Cuvette Centrale regime is proposed. The 1970 hydro-rainfall disruption is common in most tributaries of the Congo River basin, with significant reductions in flows depending on various factors (geographical location, vegetation cover, surface conditions and land use, etc.). The Oubangui is the most fragile northern tributary that continues to suffer from flow deficits, with an increase in the duration and intensity of its low flows. Since 1995, flows of the Congo River at its main station in Brazzaville/Kinshasa seem to have returned to the interannual average since 1903. However, from the same year onwards, an increase in seasonal variability and a decrease in spring flood flows can also be observed for its bimodal tributaries. This article explains some of the hydrological paradoxes specific to this basin, which illustrate the complexity of its hydrological functioning. Finally, it shows that the period of excess flow in the 1960s is the major hydrological anomaly of the Congo River over a continuous 116-year history. For the whole basin, hydrological variations are attenuated compared to those of precipitation. Finally, the hydrometric regimes reconstructed by spatial altimetry and modelling are compared with those from in situ data.