Identify the different geographic patterns of stream systems.

Water is one of the most important compounds on Earth. In the hydrologic cycle, water evaporates from the oceans, enters the atmosphere, condenses into clouds, and precipitates over land (see Section 3.1). The resulting surface runoff flows back to the oceans in streams. A stream is a channel in which water flows downhill by the force of gravity. The term stream includes flows from the smallest rivulet of water to the mightiest rivers on Earth. Depending on the geographic region, streams are also called rivers, brooks, runs, creeks, washes, wadis, and many other names.

Streams that are connected together form a drainage basin. A drainage basin (or watershed) is a geographic region drained by a single trunk stream and its tributaries. A trunk stream is a single large stream into which smaller tributaries merge. A tributary is a stream that joins with other streams to form a larger stream. Figure 16.2 illustrates these concepts using the Amazon River.

Topography determines the boundaries of drainage basins. There is no set limit to their size: The size of a drainage basin depends on the geographic scale used to define it, as illustrated in Figure 16.3. A single drainage basin can cover a small hillside or occupy much of a continent.

512

A ridge or highland that separates drainage basins and defines their boundaries is called a drainage divide. A ridge or highland that separates drainage systems that empty into different ocean basins is called a continental divide (Figure 16.4).

Most streams drain into the oceans. But some do not. Some drainage basins have no outlet to the oceans. Instead, streams flow into a low-lying basin on land. Such regions are said to have an internal drainage system. As Figure 16.5 shows, every continent has internal drainage systems, and a large portion of Eurasia has internal drainage.

513

Internal drainage systems are located in arid regions where evaporation rates exceed precipitation. The water that collects in a large terrestrial basin can leave only through evaporation or infiltration into the ground. During evaporation, salts and other dissolved minerals remain behind in the standing water. For this reason, many internal drainage basins contain saline lakes (Figure 16.6).

514

Trunk streams and their tributaries form different drainage patterns within a drainage basin depending on the topography and the type of surface over which they are flowing. Although there are many different kinds of drainage patterns, six common ones are illustrated in Figure 16.7. The most common drainage pattern is dendritic drainage, in which the pattern of the streams resembles the branches of a tree.

Arthur Strahler (1918–2002) created a stream order system that is used to rank streams based on the number of tributaries flowing into them. The higher the stream order number, the larger the stream. The smallest streams are first-order streams, which have no tributaries, as is likely to be the case in the headwaters of a drainage basin, where streams originate. A second-order stream is formed at the confluence (merging point) of two first-order streams. Stream order increases only when two streams of the same order join (Figure 16.8).

About 80% of all streams are third-order streams or less. Very few streams are above the eighth order. The Mississippi River is a tenth-order stream, and the largest is the Amazon River, which is a twelfth-order stream.

Stream Permanence

Stream order is related to stream permanence. A stream that runs dry during part or most of the year is an intermittent stream. Many first-order streams are intermittent streams. In contrast, an ephemeral stream flows briefly only after heavy rainstorms. Permanent streams flow year-round. Intermittent and permanent streams flow where and when the water table (see Section 9.3) is at or above the level of the ground, as illustrated in Figure 16.9.

515

Ephemeral streams are most common in arid regions, where thunderstorms bring sudden downpours. Ephemeral streams go by the informal names of arroyos, dry washes, or wadis, depending on the country in which they occur. Ephemeral streams are short-lived because their water evaporates and infiltrates the soil.

An exotic stream is a permanent stream that originates in a humid region and flows through an arid region where there is little precipitation. The Nile River, for example—which is the longest river in the world—begins in equatorial Africa, where there is ample rainfall. It then flows northward through the arid eastern Sahara in Sudan and Egypt as an exotic stream. By the time it reaches the Mediterranean Sea, its flow is much reduced because of evaporation, infiltration into the ground, and water diversions by people.

Anthropogenic Intermittent Streams

Diversions of water from streams by people have made many formerly permanent streams run low or run dry. When this happens, human conflict may ensue, particularly where a stream crosses state or international borders. In the Middle East, for example, Turkey diverts water from the Tigris and Euphrates rivers near their headwaters, causing political friction with downstream neighbors such as Iraq.

516

The Colorado River and many of its tributaries in the southwestern United States provide water for 30 million people in seven western states. Water diversions are so extensive in this area that during dry periods, the Colorado River does not have enough water flowing in its channel to reach the Gulf of California in Mexico, where it emptied historically. Picture This highlights the Gila River, a tributary of the Colorado River, in this context.