Lesson Objectives

• Describe how surface streams produce erosion.
• Describe the types of deposits left behind by streams.
• Describe landforms that are produced as ground water flows.

Vocabulary

• alluvial fan
• base level
• bed load
• column
• competence
• delta
• dissolved load
• floodplain
• gradient
• groundwater
• headwaters
• meander
• natural levee
• saltation
• sinkhole
• stalactite
• stalagmite
• suspended load
• travertine

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 below). 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.

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 below). 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.
• A video of bedload transport is found here: http://faculty.gg.uwyo.edu/heller/SedMovs/Sed%20Movie%20files/bdld.mov

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 below). The stream moves fast and does lots of work eroding the stream bed.

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 below).

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 below).

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 below).

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 below).

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 below).

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

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 (Figure above). 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 below).

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

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 below).

You can explore a fantastic cave, Kartchner Caverns, in Arizona, by watching this video: http://video.nationalgeographic.com/video/player/science/earth-sci/exploring-kartchner-sci.html

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 below).

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.

Review Questions

1. Define the three kinds of load that make up the particles a stream carries.
2. What is a stream’s gradient? What effect does it have on the work of a stream?
3. How do streams erode their beds?
4. How does a stream produce a wide, flat floodplain?
5. What type of gradient would a river have when it is actively eroding its stream bed?
6. When would a river form an alluvial fan and when will it form a delta? Describe the characteristics of each type of deposit.
7. What are two formations that form inside caves?
8. What erosional feature formed by ground water could swallow up your house?

Points to Consider

• Would a stream at high elevations erode more than a stream at lower elevations?
• How would Earth’s surface look without streams?
• Would a flash flood along a normally dry river valley be a dangerous event?
• Do you think caves could form in your neighborhood?