What are the ways a society with medieval or at most middle age technology go about widening and deepening a river?
- Canals have been dug in the classic era. Dig a big canal parallel to the river, then dam the river to redirect it onto the new, deeper bed.
- Some dredging happened in Roman times, too.
- Dam the river, remove major obstacles as the water falls, then re-open the dam.
A technology that could change terrain didn't change until the industrial revolution. Everything had to be done by hand. People would usually dig out a canal and then built a dam. The water was forced to flow away. Then they could widen it, make it deeper, lay down stones on the bottom and sides to prevent it from changing course. (Which happens often with uncontrolled rivers) etc.