sentences:  This stretch of river in low water seasons was treacherous for commercial tow boats and barges, often requiring them to wait several days for the river to rise.The dam for lock 27 is atypical for the Mississippi, being a weir made of tons of rock laid in the Mississippi to create a small pool elevation upstream from the Chain of Rocks.The drop at Locks 27 can vary from a few feet to over a ten-foot drop depending on the river stage.