>Safety First: Rivers can be dangerous. Stay away from the edge as it is easy to slip in and get swept away. Bring an adult or stand on a bridge above the water.
Each person drops a stick in the river. See which stick moves the fastest. If you are on a bridge, drop your stick upstream and race to the other side to see whose stick comes out from under the bridge first.
This activity promotes judgment based on visual clues (where the water seems to run the fastest and will result in your stick moving the fastest and therefore winning).