Just having a quick browse on this page and thought this may help:
"Many waterways are seasonal and pieces of satellite imagery are captured in different seasons. It is therefore very difficult to decide what waterways to trace from satellite imagery and how to trace them. Local knowledge of the Senegalese waterways is needed !" - I usually tag these with "waterway=*"+"intermittent=yes". Though this currently renders the same as a normal waterway in Mapnik. Hope this helps!
- Adding "intermittent=yes" is my current practice, but the detection problem remains: you can't know about intermittence if you don't observe the waterway outside of the wet season... Therein lies the limits of armchair mappers like me.