Moderate relief
From coastal plains to medium mountains, sandy plateaus and fertile valleys, the Republic of the Congo is a country with little relief and very low altitudes. The mountain ranges are all located in the southwest of the country, including the Mayombe massif, some 50 km wide, which separates the coastal zone from the interior. This low mountain range (less than 1,000 m), covered by a beautiful primary tropical forest, extends into the Niari region with the Chaillu massif and further north with the chain dominated by Mount Nabemba, the highest point in the Congo, at 1,020 m altitude. Numerous iron ore deposits can be found in the surrounding area. The Chaillu massif, nestled between the Téké plateaux and the Niari valley, part of which lies in south-east Gabon, is an imposing mountain range of granite and gneiss, culminating at Mont Milondo (1,020 m) in Gabon. Vast sandy plateaus (Batéké and Konkouya) separate the Congo Basin from the Ogooué Basin in Gabon. Peaking at 689 m (Mount Leketi), these plateaus border the Cuvette region and are covered in savannah, partitioned by gallery forests or swamps that accompany the navigable tributaries of the Congo River. Three different plains cover a large part of the country: the coastal plain, the Niari valley and the Congolese Cuvette. In the Pointe-Noire region, this narrow, sandy coastal plain stretches between the Atlantic Ocean and the Mayombe, fluctuating between 200 and 300 meters in altitude and bordered in places by lagoons and mangroves. Between the Mayombe and the Chaillu massif, the Niari valley occupies almost 8% of the country's surface area. Formed by the middle part of the Niari river basin, this depression is around 320 meters above sea level. Its vegetation consists mainly of grassy savannahs, interspersed with groves of dense rainforest. In the north of the country, the Congolese Cuvette is a large depression in the western part of the Congo Basin, extending into the Democratic Republic of Congo. It is a semi-aquatic alluvial plain with altitudes of no more than 400 meters.
A watered country
It's impossible to talk about the Republic of Congo without mentioning the river that bears its name. At 4,700 km long, it is the second longest river in Africa after the Nile, and the second longest in the world in terms of flow, which can exceed 70,000m3 /s. Its basin covers a total area of some 3,800,000 km², and extends over the territory of Congo-Kinshasa, Angola, the Central African Republic and, of course, Congo-Brazzaville. Supplied in all seasons, the river is navigable and would be an incomparable penetration route if the section from Brazzaville to the mouth were navigable. Upstream of the capital, it remains the "Congolese freeway", a vital link in national and international communications, since upstream on the Ubangi, you reach Bangui, the Central African capital. The river flows through the country along some 700 km of its course, which can be divided into four sections. The Cuvette section stretches from the Oubangui to Mpouya, with a width varying from 10 to 20 km. Here, the river receives water from its two main tributaries, the Oubangui and the Sangha. The Couloir, which is the Mpouya-Pool section, is only 1.5 km wide and often steep. Between the two capital cities facing each other, Brazzaville and Kinshasa, the river widens disproportionately to form a basin measuring 20 by 30 km, with the island of M'bamou at its center, whose morphology changes with the seasons. Downstream of Brazzaville, as it crosses the Cataract Plateau, the river bed is blocked by falls and rapids that have always prevented navigation. Far behind the giant Congo, the Kouilou-Niari basin is the country's second-largest watershed. Above all, it is the basin of a river that has changed its name three times. The Niari is called Ndouo in its upper reaches and Kouilou after it enters the Mayombe. The river is 700 km long, and its flow varies considerably according to the season, oscillating between 300 and 1,000m3/s. Frequently watered by tropical rains, Congo boasts some thirty navigable rivers that form a vast network throughout the country. These include the Kouilou, Niari, Bouenza, Alima, Ngoko, Sangha, Likouala-Mossaka and Likouala aux Herbes rivers in northeastern Congo.
Impoverished ferral soils
Although the Congo's bedrock is made up of various geological formations (granite, limestone, clastic and metamorphic rocks), its soils are poor. Nearly 90% of the country's surface area is covered by ferrallitic soils, typical of humid tropical zones, of which almost a third contains impoverished soils. They are found mainly on the coastal plain, the Batékés plateaux, part of the Cataractes plateau and in the Cuvette region. On the coast, the soils are sandy and therefore difficult to cultivate, in contrast to the Batéké and Cataractes plateaus where they are sandy-clay and therefore better supplied with organic matter. Covering half the territory, the reworked ferrallitic soils are home to dense natural vegetation, but deteriorate rapidly after a few years of cultivation. These poor soils need chemical fertilizers to be productive. In addition to these various ferrallitic soils, the rest of the country is covered by hydromorphic soils, which are partially or completely submerged throughout the year. They are mainly located in the Congolese Cuvette, along rivers, and are made up of grassy vegetation, particularly in the Niari valley, which has long been chosen as a grazing area by farmers. It is estimated that agricultural land in the Republic of Congo covers around 10 million hectares of the country, of which around 2% is cultivated every year using a variety of traditional techniques such as slash-and-burn farming.
