Rivers have long transported goods and people in addition to water.
As well as providing food sources like fish, invertebrates and aquatic
plants, rivers can transport floods, toxins and insect pests. The key
river in Sierra Leone (below) is the Rokel because it connects to
Freetown, the capital and largest city. There are challenges due to
gold mining, iron ore extraction and sugar cane production. The
Bumbuna Dam was built near the western edge of the hills (roughly
above the P in Pampana) - this has created a 19 mile long lake. I am
unable to locate anything about mosquito abatement measures.