Complex sentence: The falls were historically important as they were the furthest point up the Sénégal River from Saint Louis that could be reached by boat but because of the seasonal variation in the water level, navigation was only possible for a few months after the rainy season.
Split into simpler sentences: The falls were historically important as they were the furthest point up the Sénégal River from Saint Louis that could be reached by boat. Because of the seasonal variation in the water level, navigation up to the falls was only possible for a few months after the rainy season.