Complex sentence: The section between Ust-Ilimsk Dam and Boguchany Dam has not been navigable due to rapids; however, with the completion of the Boguchany Dam, and filling of its reservoir, at least part of this section of the river will become navigable as well.
Split into simpler sentences: The section between the Ust-Ilimsk Dam and the Boguchany Dam is not been navigable due to rapids. However, with the completion of the Boguchany Dam, and filling of its reservoir, at least part of this section of the river will become navigable as well.