Generally, rivers follow a downhill course due to the gravitational pull of the Earth. The majority of the planet's rivers arise in the mountains and are bound in a downstream, southerly direction.
A river usually starts as a small trickle of water that eventually increases in volume as more water is added to it. Torrential rains and melted snow often contribute to a rise in a river's water level, as well as the various tributaries that join with the river along its course. A river that flows over a slope and crisscrosses across land is called a "meandering river," while a river that contains minor channels, which continually split and merge, is called a "braided river." A river empties into its mouth, typically an ocean or a lake.