Hooray! Attack the aquatic weeds. They are a central part of most of Africa's problems. Which is the worst varies by location, but all of them are harming you. But you cannot control these plants as simply weeds. They will keep coming back until they exhaust the funding to control them. You will need to find a profit in controlling them so that you can support the crews and equipment to keep them in check. There has been some success in brewing water hyacinth into ethanol. Great success has been achieved with another weed - Typha (bulrush, cattail, kachalla). The same equipment and human resources are needed to control it as are needed for water hyacinth. Small plantations of Typha could help feed and fuel your nations, while keeping available the workers and equipment to keep your rivers and lakes clear. If you use one of the natural hybrids, you will have less trouble controlling it because it will not spread by seed. Just harvesting the existing wild Typha infestations would provide enough food to feed your whole continent, and remove millions of mosquito nurseries as a side effect.