High School Earth Science/Water Erosion and Deposition

Rivers and streams complete the hydrologic cycle by returning precipitation that falls on land to the oceans (Figure 10.1). Ultimately, gravity is the driving force, as water moves from mountainous regions to sea level. Some of this water moves over the surface and some moves through the ground as groundwater. As this water flows it does the work of both erosion and deposition. You will learn about the erosional effects and the deposits that form as a result of this moving water.

Lesson Objectives

Describe how surface rivers and streams produce erosion.
Describe the types of deposits left behind by rivers and streams.
Describe landforms that are produced as groundwater flows.

Erosion and Deposition by Rivers and Streams

Erosion from Runoff

As streams move over the ground, they transport weathered materials. Streams continually erode material away from their banks, especially along the outside curves of meanders. Some of these materials are carried in solution. Many minerals are ionic compounds that dissolve easily in water, so water moves these elements to the sea as part of the dissolved load that the stream carries. As groundwater leaches through layers of soil and rock, minerals dissolve and are carried away. Groundwater contributes most of the dissolved components that streams carry. Once an element has completely dissolved, it will likely be carried to the ocean, regardless of the velocity of the stream. In some circumstances, the stream water could become saturated with dissolved materials, in which case elements of those minerals might precipitate out of the water before they reach the ocean.

Another way that rivers and streams move weathered materials is as the suspended load. These are pieces of rock that are carried as solids as the river flows. Unlike dissolved load, the size of the particle that can be carried as suspended load is determined by the velocity of the stream. As a stream flows faster, it can carry larger and larger particles. The larger the size particle that can be carried by a stream, the greater the stream's competence. If a stream has a steep slope or gradient, it will have a faster velocity, which means it will be able to carry larger materials in suspension. At flood stage, rivers flow much faster and do more erosion because the added water increases the stream's velocity. Sand, silt and clay size particles generally make up the suspended load for a stream (Figure 10.2). As a stream slows down, either because the stream's slope decreases or because the stream overflows its banks and broadens its channel, the stream will deposit the largest particles it has been carrying first.

Figure 10.2: Rivers carry sand, silt and clay as suspended load. During flood stage, the suspended load greatly increases as stream velocity increases.

The last way that rivers and streams move weathered materials is as bed load. This means that although the water in the stream is capable of bumping and pushing these particles along, it is not able to pick them up and carry them continuously. Bed load is named for the fact that these particles get nudged and rolled along the stream bed as the water flows. Occasionally a larger size particle will get knocked into the main part of the stream flow, but then it settles back down to the stream bed because it is too heavy to remain suspended in the water. This is called saltation, which we will learn about later in this chapter with transport of particles by wind. 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. Particles that move along as the bed load of a stream do not move continuously along, but rather in small steps or jumps with periods of remaining stationary in between.

Stream and River Erosion

As a stream moves water from high elevations, like mountains, towards low elevations, like the ocean, which is at sea level, the work of the stream changes. At high elevations, streams are just beginning streams that have small channels and steep gradients. This means that the stream will have a high velocity and will do lots of work eroding its stream bed. The higher the elevation, the farther the stream is from where it eventually meets the sea. Base level is the term for where a stream meets sea level or standing water, like a lake or the ocean. Streams will work to downcut their stream beds until they reach base level.

As a stream moves out of high mountainous areas into lower areas closer to sea level, the stream is closer to its base level and does more work eroding the edges of its banks than downcutting into its stream bed. At some point in most streams, there are curves or bends in the stream channel called meanders (Figure 10.3). The stream erodes material along its outer banks and deposits material along the inside curves of a meander as it flows to the ocean (Figure 10.4). This causes these meanders to migrate laterally over time. The erosion of the outside edge of the stream's banks begins the work of carving a floodplain, which is a flat level area surrounding the stream channel.

Figure 10.3: Here a stream can be seen actively eroding its outer banks along a meander.

Figure 10.4: This stream has deposited larger materials like gravel and pebbles along the inside curve of a meander.

Stream and River Deposition

Once a stream nears the ocean, it is very close to its base level and now deposits more materials than it erodes. As you just learned, one place where a river deposits material is along the inside edges of meanders. If you ever decide to pan for gold or look for artifacts from an older town or civilization, you will sift through these deposits. Gold is one of the densest elements on Earth. Streams are lazy and never want to carry more materials than absolutely necessary. It will drop off the heaviest and largest particles first, that is why you might find gold in a stream deposit. Imagine that you had to carry all that you would need for a week as you walked many kilometers. At first you might not mind the weight of what you are carrying at all, but as you get tired, you will look to drop off the heaviest things you are carrying first!

When a river floods or overflows its channel, the area where the stream flows is suddenly much broader and shallower than it was when it was in its channel. This slows down the velocity of the stream’s flow and causes the stream to drop off much of its load. The farmers who use the floodplain areas around the Nile River rely on these deposits to supply nutrients to their fields each year as the river floods its banks. At flood stage, a river will also build natural levees as the largest size particles build a higher area around the edges of the stream channel (Figure 10.5).

When a river meets either standing water or nearly flat lying ground, it will deposit its load. If this happens in water, a river may form a delta. From its headwaters in the mountains, along a journey of many kilometers, rivers carry the eroded materials that form their stream load. Suddenly the river slows down tremendously in velocity, and drops the tremendous load of sediments it has been carrying. Deltas are relatively flat topped, often triangular shaped deposits of sediments that form where a large river meets the ocean. The name delta comes from the capital Greek letter delta, which is a triangle, even though not all deltas have this shape. A triangular shaped delta forms as the main stream channel splits into many smaller distributaries. As the channel shifts back and forth dropping off sediments and moving to a new channel location a wide triangular deposit forms.

There are three types of beds that make up a delta (Figure 10.6). The first particles to be dropped off are the coarsest sediments and these form sloped layers called foreset beds that make up the front edge of the delta. Further out into calmer water, lighter, more fine grained sediments form thin, horizontal layers. These are called bottomset beds. During floodstage, the whole delta can be covered by finer sediments which will overlie the existing delta. These are called topset beds. These form last and lie on top of the rest of the delta.

Figure 10.6: The three types of beds that form the layers of a delta.

Not all large rivers form deltas as they meet the ocean. Whether a delta forms depends on the action of waves and tides. If the water is quiet water such as a gulf or shallow sea, a delta may form. If the sediments are carried away, then no delta will form. Sediments brought to the shore and distributed along coastlines by longshore transport form our beaches and barrier islands.

If a river or stream suddenly reaches nearly flat ground, like a broad flat valley or plain, an alluvial fan develops at the base of the slope (Figure 10.7). An alluvial fan is a curved top, fan shaped deposit of coarse sediments that drop off as the stream suddenly loses velocity. The fan spreads out in a curve in the direction of the flat land as many stream channels move across the curved surface of the alluvial fan, forming and unforming many channels as sediments are deposited. Alluvial fans generally form in more arid regions.

Figure 10.7: This satellite photo of an alluvial fan in Iran shows the typical fan shape of these deposits. The stream forming the alluvial fan runs from the mountains in the southwest (lower left) corner of the photograph toward the flatter land to the northeast (upper right). The green rectangles are farm fields which utilize the distributed water.

Groundwater Erosion and Deposition

Not all water that falls on the land flows through rivers and streams. When it rains, much of the water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. This water necessarily moves slowly, mostly under the influence of gravity. Yet groundwater is still a strong erosional force, as this water works to dissolve away solid rock. If you have ever explored a cave or seen a sinkhole, you have some experience with the work of groundwater (Figure 10.8).

Figure 10.8: Groundwater forms when water sinks into the ground rather than forming rivers or streams.

Figure 10.9: Caverns form where groundwater dissolves rock.

As groundwater moves through spaces between mineral grains, it works to dissolve and carry away different elements. Some types of minerals are easily dissolved by groundwater. Rainwater absorbs carbon dioxide (CO₂) as it falls through the air. The carbon dioxide combines with water to form carbonic acid. This naturally occurring weak acid readily dissolves many types of rock, including limestone. If you have ever watched an antacid tablet dissolve in water, you have seen an example of just how quickly this type of rock is eroded away. Caves are one of nature's most spectacular demonstrations of erosion (Figure 10.9). Working slowly over many years, groundwater dissolves and carries away elements of once solid rock in solution. First it travels along small cracks and fractures, gradually enlarging them. In time, caverns many football fields long and as high as many meters tall can form.

A sinkhole could form if the roof of an underground cave collapses. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (Figure 10.10). As groundwater dissolves away solid rock, it carries those minerals in solution as it travels. As groundwater drips through openings, several interesting types of formations occur. Stalactites are icicle like deposits of calcium carbonate which form as layer on layer of calcite drips from the ceiling, coating the 'icicle' (Figure 10.11). As mineral rich material drips to the floor of a cave, stalagmites form rounded deposits of calcium carbonate on the floor of the cave. The word stalactite has a 'C', so you can remember it forms from the ceiling, while the 'G' in stalagmite reminds you 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 type of rock, formed from calcium carbonate called travertine. This happens when groundwater saturated with calcium carbonate suddenly precipitates out as the mineral calcite or aragonite. Mineral springs that produce travertine can be hot springs or the water may just be warm or could even be cold (Figure 10.12).

When lots of calcium carbonate is carried by groundwater, we call the water 'hard'. If the water in your area is hard, it might be difficult to get soap to lather or make soapsuds. Hard water might also have a taste to it, perhaps one that some people don't like as much as pure water. If your water is 'hard', you may treat your water with a filter before you drink it. Zeolites are minerals that help to absorb ions from the water as it passes through the filter. When the water passes through the filter, it comes out tasting good!

Figure 10.10: This sinkhole formed in Florida.

Figure 10.11: Stalactites form as calcium carbonate drips from the ceiling of a cave, forming beautiful icicle-like formations.

Figure 10.13: Travertine is a beautiful form of limestone that forms as calcium carbonate precipitates.

Lesson Summary

Rivers and 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 will deeply erode the land when it is far from its base level, the elevation where it enters standing water like the ocean.
As a river develops bends, called meanders, it forms a broad, flat area known as a floodplain.
At the end of a stream, a delta or an alluvial fan might form where the river drops off much of the load of sediments it carries.
Caves form underground as groundwater gradually dissolves away rock.

Review Questions

What are the three kinds of load that make up the particles a stream carries. Name and define each type.
What is a stream's gradient? What effect does it have on the work of a stream?
Describe several erosional areas produced by streams. Explain why erosion occurs here.
What type of gradient or slope would a river have when it is actively eroding its stream bed? Explain.
When would a river form an alluvial fan and when will it form a delta? Describe the characteristics of each type of deposit.
What are two formations that form inside caves?
What erosional feature formed by groundwater could swallow up your house?

Vocabulary

alluvial fan: Curved top, fan shaped deposit of coarse sediments that forms when a stream suddenly meets flat ground.
base level: The elevation at which a river meets standing water; a stream cannot erode below this level.
bed load: The largest particles moved by streams; move by rolling or bumping along the stream bed.
competence: A measure of the largest particle a stream can carry.
delta: A flat topped, triangular shaped deposit of sediments that forms where a river meets standing water.
dissolved load: The elements carried in solution by a stream.
distributaries: Smaller branching channels that spread out over the surface of a delta.
floodplain: Broad, flat lying plain surrounding a stream channel; created by the stream.
gradient: The slope of a stream.
groundwater: Water that moves through pore spaces and fractures in soil and rock.
meander: A bend or curve in a stream channel.
natural levees: Coarse grained deposits of sediments that build up along a stream's banks as it floods.
saltation: The intermittent movement of bed load particles, as they are carried by the flow and then settle back down.
sinkhole: Circular hole in the ground that forms as the roof of a cave collapses.
stalactite: An icicle like formation of calcium carbonate that forms as saturated water drips from the ceiling of a cave.
stalagmite: Rounded, cone shaped formation of calcium carbonate that forms in caves as water drips onto the floor.
suspended load: Solid particles that are carried in the main stream flow.
travertine: Beautiful deposit of calcium carbonate that forms around hot springs.

Points to Consider

Would a stream that begins at high elevation be likely to do more erosion than a stream that begins at lower elevations?
What differences would there be on Earth’s surface without rivers and streams?
Do you think a flash flood along a normally dry river valley would be a dangerous event?
Do you think caves could form in your neighborhood?

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