Why do some rivers have natural whitewater rapids while others do not?

Whitewater rapids appear when a river loses elevation quickly over boulders, bedrock steps, or narrow channels. The steep slope and obstacles break the current into churning eddies, standing waves, and spray. Rivers flowing over soft, even sediment are less likely to become turbulent rapids. Rapids often occur in mountainous regions or where glacial deposits and bedrock create a rough riverbed. They are a normal stage in river systems and can also signal areas of rapid erosion.