Some rivers have several meanders that leave ponds or pools in their former beds. There are also reservoir lakes (Rwegura, Dogodogo and Kavuruga) considered to be ponds. At the level of the Rusizi plain, on the edge of the ponds, we observe here and there Ludwigia leptocarpa and Polygonum pulchrum.

The floating carpets are composed of Nymphaea lotus, Nymphaea nouchalii, Utricularia inflexa, Pistia stratiotes, etc., and a large carpet of Azolla pinnata fern. Algae are still found especially Cyanophyceae such as Spirulina major, Oscillatoria tenuis and Anabaenopsis arnoldii, Euglenophyceae and a large quantity of Euglena div. sp. which, depending on the lighting conditions, forms sheets whose color varies from red to green and is reported as well as Bacillariophyceae. With the addition of water, the nature of algae changes. Colonial algae are taking over: there are a lot of Volvox, Pandorina, Pediastrum and other Scenedesmus, in association with Cyanophyceae. In this plain of Rusizi, Cyanophyceae, Euglenophyceae, Bacillariophyceae and Green Algae are the best represented. The brackish marshes of the Rusizi Delta have shown the existence of about 245 species of algae: 32 species of Cyanophyta, 3 species of Dinophyta, 2 species of Xanthophyta, 33 species of Chlorophyta, 33 species of Chlorophyta and 135 species of Bacillariophyta.

At Lake Dogodogo, border vegetation is observed mainly dominated by Typha domingensis and Phragmites mauritianus. The most remarkable floating plants are: Chara zeylanica, Potamogeton sp. and Najas marina.

At the level of Lake Rwegura, floating plants are observed, mainly composed of Nympheaceae. On certain impermeable soils in the plain, pools form during rainy periods, which have vegetation known for its spectacular phenophases: Vossia cuspidata pond, Polygonum senegalense pond, Oryza longistaminata pond, Oryza longistaminata pond.