Explain how streams evolve through time as they erode Earth’s surface.

Flowing water is the most important and widespread agent of erosion on Earth. Without the Sun’s energy, fluvial erosion would not exist. As we have seen, the Sun provides heat energy to evaporate water and lift it into the atmosphere. The force of gravity is important as well: Water that condenses in the atmosphere falls to Earth as precipitation, and Earth’s gravitational field pulls that water downslope. The kinetic energy of the stream’s flowing water works to cut downward into Earth’s surface (a process called downcutting) and transport rock fragments downslope toward the oceans.

517

As water flows back to the oceans, it forms stream channels. The force of water tumbles rocks in the channel and breaks them apart physically. At the same time, chemical weathering dissolves minerals in the rocks. Streams transport this broken and dissolved material downstream in the process of fluvial erosion (erosion by running water; from the Latin word fluvius, which means “river”).

Fluvial erosion begins as rain falls on the ground. Some of the rainwater flows into and through the ground, where it contributes to soil moisture and groundwater. Where the ground surface is sloped, rainwater flows downslope in thin sheets called sheet wash.

As rainwater collects in low-lying areas, it begins to form shallow channels. Newly forming stream channels are called rills. Rills develop into gullies as they deepen. The flat areas between stream channels, where sheet wash occurs, are called interfluves. Stream channels are always lower than the interfluves.

As the amount of water and the speed of flow in a stream channel increase, the stream gains more energy to do the work of carrying soil particles and rock fragments downslope. Figure 16.10 illustrates the process of headward erosion, by which a stream channel lengthens upslope through time, forming new rills and gullies by fluvial erosion. The spatial scale of headward erosion ranges from a few meters to hundreds of kilometers or more, and its time scale varies from weeks to thousands of years or longer, depending on the geographic setting, geology, and climate.

Stream discharge is the volume of water flowing past a fixed point within a stream channel. It is expressed in cubic meters per second (m³/s) or cubic feet per second (ft³/s). Discharge can be visualized as the number of 1 m (or 1 ft) cubes passing beneath a bridge every second. Discharge is often recorded on a stream hydrograph (see Figure 16.11).

Discharge is important because the more water is flowing through a stream channel, and the faster that water is flowing, the more energy the stream has to erode Earth’s surface. The discharge of a stream is influenced mainly by climate, stream order, season, and surface permeability (Table 16.1).

Urbanization changes the permeability of the ground surface and therefore the discharge of streams. Paved surfaces are impermeable, and water flows from them quickly into nearby streams, rather than infiltrating the ground. As a result, streams in urbanized areas experience sudden spikes in discharge during heavy precipitation events, and discharge drops quickly after storms. Figure 16.11 compares the discharge of an urbanized stream with that of a stream in a rural location.

518

519

In calculations, stream discharge is symbolized by Q. Discharge is measured by multiplying the stream channel area (A) by the speed of water flow, or velocity (v): Q = A · v. Stream channel area is calculated by multiplying channel width by channel depth. The calculation of stream discharge takes into account the variations in flow velocity, stream channel area, and river height that are unique to each stream (Figure 16.12).

The Amazon River has the highest average discharge in the world, about 200,000 m³/s, or 7 million ft³/s. The Amazon’s flow constitutes nearly 20% of the world’s total discharge by rivers into the oceans. Crunch the Numbers compares the flow of North America’s largest river (in terms of discharge), the Mississippi River, with that of the Amazon River.

The force of flowing water, called hydraulic action, frees rock fragments and sets them in motion. The size of those fragments may range from tiny grains of sand and silt to large boulders.

Stream abrasion rounds and polishes rocks in a stream. Abrasion is the process by which movement of one material wears away another material. The continual tumbling of rocks in a streambed wears away other rocks and the channel bedrock (Figure 16.13). Heavy rocks and boulders in a stream channel move only with high flows, however, such as those that follow heavy rains or snowmelt.

The material that moves within a stream channel is called the stream load. The size of the particles that move within a stream is directly related to stream discharge and flow velocity. There are three kinds of stream loads:

520

Geomorphologists refer to the size of fragments a stream can transport as stream competence. Streams that can transport large fragments, such as boulders, have high stream competence. Stream capacity refers to the total amount of material (of all sizes) that a stream can transport. During periods of peak discharge, when the stream has more kinetic energy, stream capacity and competence increase.

Streams are referred to as graded if sediment transport within the stream equals the rate of sediment input from around the stream. In other words, in a graded stream, the channel is neither accruing nor losing sediments. Along the length of a graded stream, there is a dynamic equilibrium (or balance) between erosion, transportation, and deposition of sediments within the stream channel. A fully graded stream would have a smooth longitudinal profile like the one shown in Figure 16.16. Few streams, however, have such smoothly graded profiles because of the different rock types, geologic activity, and climates within any given drainage basin.

521

Similarly, stream gradient refers to the drop in elevation of the stream channel in the downstream direction. High-gradient streams drop quickly in elevation, and low-gradient streams are almost flat.

Base level is the lowest level a stream can reach, usually sea level. A stream can erode Earth’s surface only where it is above base level. The higher a stream is above base level, the more energy it will have to erode the land surface. At base level, a stream becomes standing water, loses its capacity and competence, and drops the sediments it is carrying.

The rate of stream downcutting and the resulting topography depend on the stream’s elevation above base level, stream discharge, flow velocity, and the hardness and structure of the bedrock the stream is cutting through. Harder rocks such as metamorphic rocks are more resistant to downcutting than softer rocks such as sandstone. Most streams cut V-shaped valleys in their headwaters, but this is not always the case, as shown in Figure 16.17.

522

523

Knickpoints are locations where there is an abrupt increase in stream gradient over a short distance. All waterfalls, for example, form at knickpoints. They often form where a layer of rock that is resistant to erosion overlies a relatively weak and soft layer of rock. The resistant layer of rock creates an escarpment, a long cliff face or steep slope, over which the water flows. As the stream spills over the lip of the resistant rock layer, it may form a bowl at the base of the waterfall, called a plunge pool. Plunge pools are created by abrasion caused by rocks circulating at the base of the waterfall. Eventually, a plunge pool can undercut the support of the resistant layer above, causing it to collapse. As a result, the knickpoint migrates upstream. This process is illustrated in Figure 16.18.

Stream rejuvenation is a process in which a stream gains downcutting energy as its base level is lowered relative to its drainage basin. Stream rejuvenation is usually a consequence of geologic uplift, but lowered global sea level can also cause stream rejuvenation.

The higher above its base level a stream lies, the more it will cut into the sediments and bedrock beneath it. Eventually, barring further geologic uplift, the stream will return to its original gradient through weathering, mass wasting, erosion, and deposition (Figure 16.19).

524

525

As we will see in Section 16.3, a stream that flows across a flat floodplain may form looping bends called meanders. If geologic uplift creates a level plateau, rather than a slope, stream meanders may be preserved as entrenched meanders as the stream erodes the plateau (Figure 16.20). Entrenched meanders are found throughout the southwestern United States, where geologic uplift raised what was a large, flat floodplain and shallow sea to form today’s Colorado Plateau. The uplift took place gradually and was slightly inclined toward the southwest.

Stream rejuvenation may also result in stream terraces where rivers flow through floodplains. A rejuvenated stream can cut down into the floodplain sediments through headward erosion, creating stepped terraces (Figure 16.21).

Antecedent and Superimposed Streams

Antecedent streams are streams that cut through bedrock ridges rather than flowing around them. Geologic uplift may allow a preexisting stream to cut through resistant ridges of rock as they are slowly lifted. If the rate of uplift and erosion is gradual, the stream can cut into the underlying ridge of bedrock while maintaining its original drainage pattern, resulting in a water gap (Figure 16.22). A water gap is a place where a stream has cut through a resistant ridge of rock.

Superimposed streams are like antecedent streams in that they cut through resistant layers of rock and form water gaps. In the case of superimposed streams, however, the land surface is lowered through fluvial erosion rather than being lifted through tectonic forces.

Distinguishing a water gap caused by antecedence from one caused by superimposition is not possible with only a quick glance at the surface. The tectonic and erosional history of a region must be known before determining how the water gap formed. In some cases, streams with water gaps no longer flow in their former channels. When that occurs, a wind gap results.

Stream Piracy

Stream piracy is the diversion of one stream into another as headward erosion merges the two streams. When a stream’s tributaries migrate upslope and cut through the drainage divide of another drainage basin, streams within that drainage basin can be diverted into the new stream channel, as illustrated in Figure 16.23.

526