6.2 Streams and River Systems

Merry Wilson


Streams are bodies of water that are in constant motion. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers might all be lumped together as streams. In streams, water always flows downhill, but the form varies with rock type, topography, and many other factors (1).

Stream Landscape
Figure 6.2.1 Stream Landscape. Wikimedia Commons, Public Domain.

Path of a Stream

A stream originates at its sources of water, which may be from precipitation, such as high mountains where snows collect in winter and melt in summer, or a spring. It is common for a stream to have more than one source.

Figure 6.2.2. Water starts collecting on the top of mountains from small streams. The stream grows by receiving the flow of more streams and it turns into a river. Rivers collect water from other rivers as they flow towards the ocean. “River Source” by Tony Ferrie CC-BY.
Figure 6.2.3. The Ohio river meets the Mississippi River. The Ohio is considered the tributary of the Mississippi River. NASA Landsat Science, Public Domain.

Where multiple streams combine, the smaller stream is called a tributary. In Figure 6.2.3, the Ohio and Mississippi Rivers meet, and the Ohio River is considered a tributary at this junction.  Streams will eventually run down the landscape and the water molecules could infiltrate into the ground, evaporate into the atmosphere, or continue to run-off until it intersects with a large body of water, like an ocean or a lake.

Figure 6.2.4. A river delta is the place where the river encounters a larger body of water, such as a lake or, more typically, the ocean. “Mouths of the Amazon River” by NASA, Public Domain.

Divides and Drainage Basins

A drainage basin is all of the land that is drained by one stream, including its tributaries.  A divide is a topographically high area that separates a landscape into different water basins. The rain that falls on the north side of a divide flows into the northern drainage basin, and rain that falls on the south side flows into the southern drainage basin. On a much grander scale, entire continents have divides, known as continental divides.(1).

Figure 6.2.5. North America Water Divides Pfly, CC-BY-SA

According to Fig. 6.2.5, the State of Arizona is bracketed by the Great Divide to the east, and the Great Basin to the west.

Discharge and Stream Stage

Along the path of the stream, the water volume and velocity may change, depending on the amount of water input and the overall shape and gradient of the stream as it winds through the landscape. This is referred to as stream discharge. Discharge measures stream flow at a given time and location, and specifically is a measure of the volume of water passing a particular point in a period of time. It is found by multiplying the area (width multiplied by depth) of the stream channel by the velocity of the water, and is often in units of cubic feet per second (cfs). Discharge increases downstream in most rivers, as tributaries join the main channel and add water.

Figure 6.2.6. The most common method used by the USGS for measuring velocity is with a current meter. However, a variety of advanced equipment can also be used to sense stage and measure streamflow. In the simplest method, a current meter turns with the flow of the river or stream. The current meter is used to measure water velocity at predetermined points (subsections) along a marked line, suspended cableway, or bridge across a river or stream. The depth of the water is also measured at each point. These velocity and depth measurements are used to compute the total volume of water flowing past the line during a specific interval of time. Usually a river or stream will be measured at 25 to 30 regularly spaced locations across the river or stream. USGS, Public Domain

Stream stage, sometimes called gage height, is the height of the water at the surface of the stream.  Stage zero would be the dry streambed, and stage 1 would be 1 feet above zero.  Every stream in every part of the world has a unique calculation of stage and discharge that is related to its unique features.

A stream gage showing water depth in the Colorado River.  USGS.  Public Domain
Figure 6.2.7. A stream gage showing water depth in the Colorado River.  USGS, Public Domain
Backyard Geology:  The Colorado River Drainage Basin
Figure 6.2.7. Colorado River Drainage Basin, USGS. Public Domain

Path of the Stream: The Colorado River is nearly 1,450 miles long, beginning with snowmelt, rainwater, and springs in Colorado and Wyoming.

Drainage Basin: The entire basin covers approximately 246,000 square miles and includes all of Arizona (Fig. 6.2.7).

Discharge: The average discharge of the river is highly variable, because it is regulated through a series of dams along the river, as well as extensive use for agriculture. In it’s natural state, the discharge would still be highly variable due to climate but would average about 22,500 cfs, but very little of it typically makes it to the Gulf of California.



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