Rivers

Rivers are highly influenced by the prevailing climate conditions. The climate determines the amount of precipitation, its seasonality, and its form as rainwater or as ice. Because of the climate and subsequent rainfall patterns, three general types of rivers exist. The first are the perennial or permanent rivers. Normally, these rivers are located in more humid climates where rainfall exceeds evaporation rates. Thus, although these rivers may experience seasonal fluctuations in their levels of water, they have constant streamflow throughout the year. With few exceptions, streamflow in these rivers increases downstream, and these rivers empty into larger bodies of water, such as oceans. In fact, 68% of rivers drain into oceans. All of the world's major rivers are perennial rivers.

The second type of river is the periodic river. These rivers are characterized with predictably intermittent streamflow. Usually appearing in arid climates where evaporation is greater than precipitation, these rivers run dry on occasion, but there are regular intervals of stream-flow. Typically, these rivers have a decrease in stream-flow as they travel due largely to high levels of evaporation. Often, they do not reach the sea, but instead run into an inland drainage basin.

The third type of river is the episodic river. These rivers are actually the run-off channels of very dry regions. In these regions of the world, there are only slight amounts of rainfall and it evaporates quickly. This type of streamflow occurs rarely.

Interestingly, some rivers span two types of climactic regions. These rivers, known as exotic rivers, begin in humid or polar regions and flow into dry areas. The largest of these rivers have enough water at their sources to enable them to reach the sea. The Nile River, for example, gets sufficient water at its humid source to travel over the Nubian and Arabian deserts. While it receives a substantial amount of water from the Blue Nile at Kartoum, it then must travel 1,676 mi (2,700 km) before it reaches the Mediterranean Sea.