Imagine trying to live in a stream instead of on land. It would be like living in a constant hurricane. Not only would you have to fight for your life to stay put or find shelter, you would also have to gather and eat food, extract oxygen from water, and reproduce in the maelstrom.

Most of the food items available to you are whizzing by you and being swept down stream.

Then consider the difference in the stream flow from one season to the next. In March and July, the stream bed may be almost completely dry. In September and October, it may be in full flood with rocks tumbling down stream. It takes very special adaptations to live in this environment.