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Reading: Water Erosion and Deposition

INTRODUCTION

Streams—any running water from a rivulet to a raging river—complete the hydrologic cycle by returning precipitation that falls on land to the oceans (figure 1). Some of this water moves over the surface and some moves through the ground as groundwater. Flowing water does the work of both erosion and deposition.


Figure 1. As streams flow towards the ocean, they carry weathered materials.

EROSION AND DEPOSITION BY STREAMS

Erosion By Streams

Flowing streams pick up and transport weathered materials by eroding sediments from their banks. Streams also carry ions and ionic compounds that dissolve easily in the water. Sediments are carried as:

  • Dissolved load: Dissolved load is composed of ions in solution. These ions are usually carried in the water all the way to the ocean.
  • Suspended load: Sediments carried as solids as the stream flows are suspended load. The size of particles that can be carried is determined by the stream’s velocity (figure 2). Faster streams can carry larger particles. Streams that carry larger particles have greater competence. Streams with a steep gradient (slope) have a faster velocity and greater competence.
  • Bed load: Particles that are too large to be carried as suspended load are bumped and pushed along the stream bed as bed load. Bed load sediments do not move continuously. This intermittent movement is called saltation. Streams with high velocities and steep gradients do a great deal of down cutting into the stream bed, which is primarily accomplished by movement of particles that make up the bed load.
  • Here is a video of bedload transport.

Stages of Streams

As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a stream’s headwaters, often high in the mountains, gradients are steep (figure 3). The stream moves fast and does lots of work eroding the stream bed.


Figure 3. This stream begins as snow melt from the mountains.

As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (figure 4).


Figure 4. (a) At a meander, a stream actively erodes its outer banks and deposits material along the inside curves. This causes these meanders to migrate laterally over time. (b) This stream has deposited larger materials such as gravel and pebbles along the inside curve of a meander. (c) This image is a topographic map. The San Juan River eroded the land surface as the Colorado Plateau uplifted. The river’s meanders were preserved as a feature called incised meanders.

As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat level area surrounding the stream channel (figure 5).


Figure 5. The Vistula River in Poland flows onto its floodplain.

Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do.

Stream Deposition

As a stream gets closer to base level, its gradient lowers and it deposits more material than it erodes. On flatter ground, streams deposit material on the inside of meanders. Placer mineral deposits, described in the Earth’s Minerals chapter, are often deposited there. A stream’s floodplain is much broader and shallower than the stream’s channel. When a stream flows onto its floodplain, its velocity slows and it deposits much of its load. These sediments are rich in nutrients and make excellent farmland (figure 6).


Figure 6. The Mississippi floodplain is heavily farmed. Flooding can wipe out farms and towns, but the stream also deposits nutrient-rich sediments that enrich the floodplain.

A stream at flood stage carries lots of sediments. When its gradient decreases, the stream overflows its banks and broadens its channel. The decrease in gradient causes the stream to deposit its sediments, the largest first. These large sediments build a higher area around the edges of the stream channel, creating natural levees (figure 7).


Figure 7. After many floods, a stream builds natural levees along its banks.

When a river enters standing water, its velocity slows to a stop. The stream moves back and forth across the region and drops its sediments in a wide triangular-shaped deposit called a delta (figure 8).



Figure 8. (a) The Nile River delta has a classic triangular shape, like the capital Greek letter delta. (b) Sediment in the Yellow River delta. The main stream channel splits into many smaller distributaries.

If a stream falls down a steep slope onto a broad flat valley, an alluvial fan develops (figure 9). Alluvial fans generally form in arid regions.


Figure 9. An alluvial fan in Iran. The mountains are in the lower right corner of the photograph.

GROUND WATER EROSION AND DEPOSITION

Rainwater absorbs carbon dioxide (CO2) as it falls. The CO2 combines with water to form carbonic acid. The slightly acidic water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. The flow of water underground is ground water.

Ground water is a strong erosional force, as it works to dissolve away solid rock. Carbonic acid is especially good at dissolving the rock limestone.

Cave Formation

Working slowly over many years, ground water travels along small cracks. The water dissolves and carries away the solid rock gradually enlarging the cracks. Eventually a cave may form (figure 10).


Figure 10. Caverns many football fields long and as high as many meters tall form where ground water erodes away rock.

Ground water carries the dissolved minerals in solution. The minerals may then be deposited, for example, as stalagmites or stalactites (figure 11).


Figure 11. (a) Stalactites form as calcium carbonate drips from the ceiling of a cave, forming beautiful icicle-like formations. The word stalactite has a c and it forms from the ceiling. (b) Stalagmites form as calcium carbonate drips from the ceiling to the floor of a cave and then grow upwards. The g in stalagmite means it forms on the ground.

If a stalactite and stalagmite join together, they form a column. One of the wonders of visiting a cave is to witness the beauty of these amazing and strangely captivating structures. Caves also produce a beautiful rock, formed from calcium carbonate, travertine. Ground water saturated with calcium carbonate precipitates as the mineral calcite or aragonite. Mineral springs that produce travertine can be hot, warm or even cold (figure 12).


Figure 12. Travertine is a beautiful form of limestone.

You can explore a fantastic cave, Kartchner Caverns, in Arizona, by watching this video.

If the roof of a cave collapses, a sinkhole could form. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (figure 13).


Figure 13. This sinkhole formed in Florida.

LESSON SUMMARY

  • Streams erode the land as they move from higher elevations to the sea.
  • Eroded materials can be carried in a river as dissolved load, suspended load, or bed load.
  • A river erodes deeply when it is far from its base level, the place where it enters standing water.
  • Streams form bends, called meanders. Broad, flat areas are known as floodplains.
  • A delta or an alluvial fan might form where the stream drops its sediment load.
  • Caves form underground as ground water gradually dissolves away rock.

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