Complex sentence: But most importantly, Hoover only controlled the lower portion of the river, and the upper basin, whose rivers flowed wild and free, had no way to ensure it could utilize its water allotment in dry years because of the lack of sufficient storage.
Split into simpler sentences: But most importantly, Hoover only controlled the lower portion of the river. The upper basin states, whose rivers flowed wild and free, had no way to ensure they could utilize their water allotment in dry years because of the lack of sufficient storage.