Figure 13.4.1 (left) & Figure 13.4.2. (right) Helen Hunt Falls in Colorado Springs, CO. Note the large cobbles and boulders that make up the stream bed, characteristic of juvenile streams on the left. Also note the step-pool form and deep channel as shown on the right.

Youthful streams that are actively down-cutting their channels tend to be relatively straight and are typically ungraded (meaning that rapids and falls are common). As shown in Figures 13.0.1 and 13.4.1, youthful streams commonly have a step-pool morphology, meaning that the stream consists of a series of pools connected by rapids and waterfalls. They also have steep gradients and steep and narrow V-shaped valleys—in some cases steep enough to be called canyons.

In mountainous terrain, such as that in Colorado Spring and west of Boulder, steep youthful streams typically flow into wide and relatively low-gradient U-shaped glaciated valleys. The youthful streams have high sediment loads, and when they flow into the lower-gradient glacial valleys where the velocity isn’t high enough to carry all of the sediment braided patterns develop, characterized by a series of narrow channels separated by gravel bars (Figure 13.4.3).

Figure 13.4.3 A braided channel near the Katmai Coast of Alaska.

Braided streams can develop anywhere there is more sediment than a stream is able to transport. One such environment is in volcanic regions, where explosive eruptions produce large amounts of unconsolidated material that gets washed into streams.

A stream that occupies a wide, flat flood plain with a low gradient typically carries only sand-sized and finer sediments and develops a sinuous flow pattern. As you saw in Figure 13.3.1, when a stream flows around a corner, the water on the outside has farther to go and tends to flow faster. This leads to erosion of the banks on the outside of the curve, deposition on the inside, and formation of a point bar (Figure 13.4.4). Over time, the sinuosity of the stream becomes increasingly exaggerated, and the channel migrates around within its flood plain, forming a meandering pattern.

Figure 13.4.4 A point bar developing along the Crystal River, Pitkin County, CO (the white area between the river and the trees on the right). The sand and gravel point bar must have formed when the creek was higher and the flow faster than it was when the photo was taken.

A well-developed meandering river is shown in Figure 13.4.5. The meander in the middle of the photo has reached the point where the thin neck of land between two parts of the channel is about to be eroded through. When this happens, another oxbow lake will form like the others in the photo.

Figure 13.4.5 The meandering channel of the Nowitna River, Alaska. Numerous oxbow lakes are present and another meander cutoff will soon take place. [Image Description]

At the point where a stream enters a still body of water—a lake or the ocean—sediment is deposited and a delta forms. The Mississippi River emptying into the Gulf of Mexico in Louisiana is perhaps the most famous such example in the country (Figures 13.4.6-7). Deltas are known for having lush vegetation due to the nutrients also carried and subsequently deposited by the rivers. Despite this benefit, since deltas are near the ultimate base level, they are prime grounds for frequent flooding and ground subsidence events.

Figure 13.4.6 (left) The Mississippi River delta as seen from space, and from an aerial view (Figure 13.4.7., right).

Media Attributions

• Figures 13.4.1, 13.4.2, 13.4.3., 13.4.4., 13.4.6., 13.4.7.: Wikimedia Commons
• Figure 13.4.5: “Nowitna river” by Oliver Kumis. CC BY-SA