Complex sentence: The lower river is more navigable with an experienced boat driver, however the upper river is typically only accessed by rafts which can be dropped off by airplane at Aniak Lake 100 miles upstream from the Kuskokwim confluence.
Split into simpler sentences: The lower river is more navigable with an experienced boat driver. The upper river, however, is typically only accessed by rafts which can be dropped off by airplane at Aniak Lake 100 miles upstream from the Kuskokwim confluence.