Explanation:
Larger networks of streams, lakes, rivers etc describe the river system as there are all part of a river's network.
Rivers, streams, and ground water move across the land and thus they contribute to the river system. Whereas a lake is defined as a large area of water which is surrounded by land and does not really contribute to river system.
Therefore, we can conclude that groundwater, rivers, and streams are the water sources that contribute to a river system.
