Rivers

Rivers can have different origins and, as they travel, often merge with other bodies of water. Thus, the complete river system consists of not only the river itself but also of all the converging tributaries. Every river has a point of origin. Because gravity plays a key role in the direction that rivers take, rivers almost always follow a down hill gradient. Thus, the point of origin for rivers tends to be the highest point in the watercourse. Some rivers start from springs, which are the most common type of river source in humid climates. Springs occur as groundwater rises to the earth's surface and flows away. Other rivers are initiated by run-off from melting glaciers located high in the mountains. Often, rivers having their origins in huge glaciers are quite large by the time they emerge from openings in the ice.

Lakes and marshes are the sources for other rivers. As river sources, lakes can be classified in three ways. They can be true sources for rivers; they can be an accumulation of water from small feeder streams; or they can hide a spring that is actually the true source of the river. The Great Lakes are prime examples of source lakes. Although there are a few springs that feed them, the majority of the water coming into the lakes arises from precipitation falling onto their surfaces. Therefore, they, not their tributaries, are the source of surrounding rivers.

As rivers make the trip from their source to their eventual destination, the larger ones tend to meet and merge with other rivers. Resembling the trunk and branches of a tree, the water flowing in the main stream often meets the water from its tributaries at sharp angles, combining to form the river system. As long as there are no major areas of seepage and as long as the evaporation level remains reasonable, the volume of water carried by rivers increases from its source to its mouth with every tributary.

When two bodies of water converge, it is clearly evident as their shorelines merge. However, the water from the two bodies often continues to flow separately, like two streams flowing in a common river bed. This occurrence is especially clear when two rivers meet that contain different amounts and types of suspended sediment. For example, when the Ohio and the Mississippi rivers meet, a clear difference in the color of water in the Mississippi river can be seen. Specifically, there is a strip of clear water one quarter of a mile wide on the river's eastern side that runs for miles. To the west of this strip, however, the water color is a cloudy yellow.

Along its path, a single river obtains water from surface run-off from different sections of land. The area from which a particular section of a river obtains its water is defined as a catchment area (sometimes called a drainage area). The lines that divide different catchment areas are called watersheds. A watershed is usually the line that joins the highest point around a particular river basin. Therefore, at every point along the line of a watershed, there is a downward slope going into the middle of the catchment area.