Complex sentence: Steamboats would leave St. Louis early in the spring and try to get over these rapids on the spring rise in mid to late June, and they would then try to get back downstream over the rapids before failing water levels made them more dangerous.
Split into simpler sentences: Steamboats would leave St. Louis early in the spring and try to get above the rapids on the spring rise in mid to late June. They would then try to get back downstream over the rapids before failing water levels made them more dangerous.