The Work of Running Water
How Do Streams Erode?
The energy that makes running water move comes from gravity. As water flows downslope from a higher to a lower elevation, the gravitational potential energy stored in water transforms into kinetic energy. About 3% of this energy goes into the work of eroding the walls and beds of stream channels. Running water causes erosion in four ways:
- Scouring: Running water can remove loose fragments of sediment, a process called scouring.
- Breaking and lifting: In some cases, the push of flowing water can break chunks of solid rock off the channel floor or walls. In addition, the flow of a current over a clast can cause the clast to rise, or lift off the substrate.
- Abrasion: Clean water has little erosive effect, but sedimentladen water acts like sandpaper and grinds or rasps away at the channel floor and walls, a process called abrasion. In places where turbulence produces long-lived whirlpools, abrasion by sand or gravel carves a bowl-shaped depression, called a pothole, into the floor of the stream (figure below a, b).
- Dissolution: Running water dissolves soluble minerals as it passes, and carries the minerals away in solution.
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| Erosion and transportation in streams. |
The efficiency of erosion depends on the velocity and volume of water and on its sediment content. A large volume of fast moving, turbulent, sandy water causes more erosion than does a trickle of quiet, clear water. Thus, most erosion takes place during floods, when a stream carries a large volume of fast moving, sediment-laden water.
How Do Streams Transport Sediment?
The Mississippi River received the nickname “Big Muddy” for a reason its water can become chocolate brown because of all the clay and silt it carries. Geologists refer to the total volume of sediment carried by a stream as its sediment load. The sediment load consists of three components (figure above c):
- Dissolved load: Running water dissolves soluble minerals in the sediment or rock that it flows over, and groundwater seeping into a stream brings dissolved minerals with it. The ions of these dissolved minerals constitute a stream’s dissolved load.
- Suspended load: The suspended load of a stream usually consists of tiny solid grains (silt or clay size) that swirl along with the water without settling to the floor of the channel.
- Bed load: The bed load of a stream consists of large particles (such as sand, pebbles, or cobbles) that bounce or roll along the stream floor. Bed-load movement commonly involves saltation. During saltation, a multitude of grains bounce along in the direction of flow, within a zone that extends up from the surface of the stream bed for a distance of several centimetres to several tens of centimetres. Each saltating grain in this zone follows a curved trajectory up through the water and then back down to the bed. When it strikes the bed, it knocks other grains upward, and thus supplies grains to the saltation zone.
When describing a stream’s ability to carry sediment, geologists specify its competence and capacity. The competence of a stream refers to the maximum particle size it carries; a stream with high competence can carry large particles, whereas one with low competence can carry only small particles. Competence depends on water velocity. Thus, a fast-moving, turbulent stream has greater competence (it can carry bigger particles) than a slow-moving stream, and a stream in flood has greater competence than a stream with normal flow. In fact, the huge boulders that litter the bed of a mountain creek move only during floods. The capacity of a stream refers to the total quantity of sediment it can carry. A stream’s capacity depends on its competence and discharge. So a large river has more capacity than a small creek.
Depositional Processes
A raging torrent of water can carry coarse and fine sediment the finer clasts rush along with the water as suspended load, whereas the coarser clasts may bounce and tumble as bed load. If the flow velocity decreases, either because the slope of the stream bed becomes shallower or because the channel broadens out and friction between the bed and the water increases, then the competence of the stream decreases and sediment settles out. The size of the clasts that settle at a particular locality depends on the decrease in flow velocity at the locality. For example, if the stream slows by a small amount, only large clasts settle; if the stream slows by a greater amount, medium-sized clasts settle; and if the stream slows almost to a standstill, the fine grains settle. Because of this process of sediment sorting, stream deposits tend to be segregated by size gravel accumulates in one location and mud in another.
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| Sediments, carried and deposited by streams. The clast size depends upon the stream velocity. |
Geologists refer to sediments transported by a stream as fluvial deposits (from the Latin fluvius, meaning river) or alluvium. Fluvial deposits may accumulate along the stream bed in elongate mounds, called bars (figure above a, b). In cases where the stream channel makes a broad curve, water slows along the inner edge of a curve, so a crescent-shaped point bar bordering the shoreline of the inner curve develops. During floods, a stream may over-top the banks of its channel and spread out over its floodplain, a broad flat area bordering the stream. Friction slows the water on the floodplain, so a sheet of silt and mud settles out to comprise floodplain deposits. Where a stream empties at its mouth into a standing body of water, the water slows and a wedge of sediment, called a delta, accumulates (figure above c).
Figures credited to Stephen Marshak.
