and
Ruvubu rivers, part of the upper
Nile|alt=Photograph of confluence of the Kagera and the Ruvubu, with the Rwanda-Tanzania border post in foreground, taken from a nearby hilltop The
watershed between the major
Congo and
Nile drainage basins runs from north to south through Rwanda, with around 80 percent of the country's area draining into the Nile and 20 percent into the Congo via the Rusizi River. The country's longest river is the
Nyabarongo, which rises in the south-west, flows north, east, and southeast before merging with the
Akanyaru to form the
Kagera; the Kagera then flows due north along the eastern border with Tanzania. The Nyabarongo-Kagera eventually drains into
Lake Victoria, and its source in
Nyungwe Forest is a contender for the as-yet undetermined overall
source of the
Nile. Rwanda has many lakes, the largest being
Lake Kivu. This lake occupies the floor of the
Albertine Rift along most of the length of Rwanda's western border, and with a maximum depth of , it is one of the twenty
deepest lakes in the world. Other sizeable lakes include
Burera,
Ruhondo,
Muhazi,
Rweru, and
Ihema, the last being the largest of a string of lakes in the eastern plains of
Akagera National Park. Mountains dominate central and western Rwanda. These mountains are part of the Albertine Rift Mountains that flank the Albertine branch of the
East African Rift. This branch runs from north to south along Rwanda's western border. The highest peaks are found in the
Virunga volcano chain in the northwest; this includes
Mount Karisimbi, Rwanda's highest point, at . This western section of Rwanda, which lies within the
Albertine Rift montane forests ecoregion, The centre of the country is predominantly rolling hills, while the eastern border region consists of
savanna, plains and swamps. Rwanda has a
temperate tropical highland climate, with lower temperatures than are typical for equatorial countries due to its high elevation. Kigali, in the centre of the country, has a typical daily temperature range between , with little variation through the year. There are some temperature variations across the country; the mountainous west and north are generally cooler than the lower-lying east. There are two rainy seasons in the year. The first runs from February to June and the second from September to December. These are separated by two
dry seasons: the major one from June to September, during which there is often no rain at all, and a shorter and less severe one from December to February. Rainfall varies geographically, with the west and northwest of the country receiving more precipitation annually than the east and southeast. ==Political geography==