The source, or beginning, of a river is typically in a high area, such as a hilly or mountainous region. Several different bodies of water are possible sources for rivers, including springs, lakes and bogs.
A spring is formed when water overflows from an aquifer that stores water beneath the water table. Though springs come in many sizes, large ones are sometimes sources for rivers. For instance, England's River Thames is sourced by a spring. The Mississippi is an example of a river that is primarily sourced by a lake: Minnesota's Lake Itasca. Bogs are a less-structured river source, and when saturated ground causes standing water, it occasionally forms a river. In certain cases, rainfall or melting snow runs downhill, forming flowing channels that converge into a single river.