Rivers originate from forms of precipitation, springs, groundwater recharge or melted ice and snow gathering at the top of a mountain or hill. This trickling of water starts as a gully, then converges with tributary streams to form a river. A river is then formed by its flowing motion cutting through the surface to allow it to flow.
Gullies gain strength as they merge with several tributary streams along the higher points of elevated surfaces. As the river travels down these hills or mountains, they form depressions called valleys. The river continues to flow downstream, gathering pieces of rock and breaking them down with its flowing movement. Once the stream reaches flatter ground, the broken down pieces of sediment solidify the makeup of the river by depositing themselves to the bottom. The river continues to flow, but at a slower pace in the shape of an S called a meander.
Most rivers form on the surface, but they can also form underground. These types of rivers are called subterranean and subglacial streams. Subterranean rivers are caused by rivers flowing through sinkholes in mountains and flowing through the caves, while subglacial streams are created from melted water and flow under sheets of ice and glaciers.