A natural source of running water, like a small stream, offers you and your child endless opportunities to explore and learn. Here's an idea for an exploration. Next time you are by a small stream or river, pile up some sticks, rocks, or leaves and try to stop the flow of the water. Which dam designs work? Which don't?
If a nature outing isn't possible, you can make a stream using a hose or a watering can. To dam your "stream," rather than using sticks or rocks, you could use toys or materials from a recycling bin. Can you make the water collect in one spot like a little pond? A sand box is also great for making dams.