Rivers are much smaller or lacking altogether in deserts and polar regions. They may also be largely invisible in areas of high rainfall that are underlain by limestone, such as Florida and portions of the southern Appalachians and Ozarks. Rivers always begin at their source , or headwaters , and flow downhill. One walks upriver when following a river towards its source, downriver when following it downhill. Many rivers begin in mountains, and all eventually empty into the sea, unless they are stopped by impenetrable mountains or arid deserts.
Rivers follow different patterns depending on the type of ground they flow over.
As they flow across relatively level land, merging with other rivers, they resemble a branching tree or spreading plant roots from the air. This is called a dendritic pattern . Where the ground is made of folded bedrock, rivers may follow a straighter course along the softer bedrock, with hard rock on either side. This is called a trellis pattern . Rivers that flow down the slopes of volcanoes tend to follow a radial pattern , similar to the spokes of a bicycle tire.