Earth Science 6.2

The Work of Streams

Streams Streams are Earth’s most important

agents of erosion

They can downcut or erode their channels

Most sediment a stream carries comes from weathering. Weathering produces huge amounts of sediment material by sheet flow and mass movements.

Eventually streams drop much of this material to create many different depositional features

Erosion Streams generally erode their

channels lifting loose particles by Abrasion Grinding Dissolving soluble materials

When the flow of water is enough, it can dislodge loose particles from the channel and carry them away in the moving water

Erosion In this way, the force of running

water rapidly erodes streambeds and stream banks.

The stronger the current, the more the powerful the erosion.

Sand and gravel carried in a stream can erode solid rock channels like sandpaper grinds down wood.

Pebbles caught in swirling stream sections can act like drill bits and bore potholes into the channel floor.

Sediment Transport

Streams transport sediments in three ways

In solution (dissolved load)

In suspension (suspended load)

Scooting or rolling along the bottom ( bed load)

Dissolved Load Most of the dissolved load enters

streams through groundwater.

Some of this load enters by dissolving rock along the stream’s course

The amount of material the stream carries in solution changes depending upon the climate and the geographical setting

Dissolved Load Usually the dissolved load is

expressed as parts per million. This means a ratio of the parts of the dissolved materials per million parts of water. This is often abbreviated as PPM.

Some rivers may have a dissolved load of 1000 PPM or more. The average figure for the world’s rivers is estimated at 115 to 120 PPM.

Suspended Load Most streams carry the largest part of

their load in suspension. (undissolved)

The visible cloud of sediment suspended in water is the most obvious portion of the stream’s load.

Streams usually carry only sand, silt, and clay this way.

During a flood, however, streams can carry larger particles because the water velocity is stronger. During floods, much more is carried in suspension.

Bed Load Bed Load is that part of a stream’s load of

solid material that is made up of material too large to be carried in suspension.

These larger, courser particles move along the streams bottom, rolling along pushed by the current.

The dissolved load and suspended load are always moving with the stream’s water but the bed load only moves when the current is strong enough to move the larger pieces.

The grinding action of the bed load’s movement is very important playing a major factor in eroding the stream channel.

Competence and Capacity

The ability of a stream to carry a load is determined by two factors

The stream’s competence

The stream’s capacity

Competence and Capacity

The competence of a stream measures the largest particles it can transport.

A stream’s competence increases with the stream’s velocity

The competence of a stream increases 4 times when the velocity doubles

Competence and Capacity

The capacity of a stream is the maximum load it can carry.

Capacity is directly related to a stream’s discharge.

The greater the volume of water in a stream, the greater it’s capacity for carrying sediment.

Deposition

When a stream slows down the process reverses and the water drops the sediments it is carrying, depositing them.

As the velocity decreases, the competence decreases as well, and sediment begins to drop out, the largest and heaviest particles first.

Deposition

Deposition occurs as streamflow drops below the critical settling velocity of a certain size particle.

The sediment in that category begins to settle out; dropping out by size, largest first.

In this way, the largest particles drop first, than proceed to drop by size with the smallest dropping last.

This process is called sorting.

Deposition

The sorted material dropped by a stream is called alluvium.

Many different depositional features are made from this alluvium.

Some occur in stream channels, others occur on the valley floor next to the channel. Still others occur at the mouth of the stream at it’s end.

Deltas

When a stream enters the calm waters of an ocean or lake, it slows down and it’s velocity drops.

As a result, all it’s sediments drop and it forms a delta

A delta is an accumulation of sediment formed where a stream enters a lake or ocean. As a delta grows outward, the stream’s gradient lessens and the water slows down.

Deltas

The channel in a delta becomes choked with sediment as the water slows and the sediment is dropped.

As a result, the river changes direction seeking a shorter route to it’s base level.

The main channel often divides into several smaller channels called distributaries

Natural Levees Some rivers occupy valleys with

broad, flat floors. Successive floods over years can build natural levees along them.

A natural levee is a ridge made up of mostly course sediments that parallels some streams.

A natural levee forms when a stream repeatedly overflows it’s banks.

It’s velocity rapidly decreases and it’s leaves coarse sediment deposits in strips that border the channel.

Stream Valleys

Narrow Valleys: a narrow deep V shaped valley shows that the stream’s primary work has been downcutting toward base level.

Rapids and waterfalls are the most prominent features of a narrow valley.

Wide Valleys Once a stream has cut it’s

channel closer to base level, downward erosion becomes less dominant. More of the streams energy becomes directed from side to side. The result is the widening of the valley.

The side-to-side cutting of the stream eventually produces a flat valley floor called a floodplain.

Wide Valleys Streams that flow on floodplains

move in meanders. Once a bend in a river forms, it grows larger.

Most of the erosion occurs on the outside of the meander, often called a cut bank, where the current is strongest.

Much of the debris removed from the cut bank moves downstream where it is deposited in point bars. Point bars form on the inside of the meander curves where the current is less strong.

Floods and Flood Control A flood occurs when the

discharge of a stream becomes so great that it exceeds the capacity of it’s channel and overflows it’s banks.

Most floods are caused by rapid spring snow melt or storms that bring heavy rains over a large region.

Floods and Flood Control Unlike far reaching regional

floods, flash floods are more limited in extent. Flash floods occur with little warning however and can be as deadly as walls of water sweep through river valleys.

Several factors influence flash floods: rainfall intensity and duration, surface conditions, and topography.

Mountain areas are susceptible to flash floods as steep slopes can send runoff water into narrow canyons.

Flood Control Strategies Measures to control

flooding include

Artificial leveesFlood control damsPlacing limits on

floodplain development

Flood Control Strategies

Artificial Levees Artificial levees are earthen

mounds built on the banks of a river. The levee increases the amount of water a channel can hold during periods of high water.

Because a river can not overflow it’s banks when a levee contains it, the sediment is deposited in the channel instead. This gradually builds the bottom up and in a few years it takes less water to overflow the banks. As a result the levees need to be raised more or the channel dredged.

Flood Control Dam

Flood control dams store floodwater and than let it out slowly

Many dams have nonrelated functions, such as providing irrigation water for crops or producing hydroelectric energy.

While dams can control floods, they can also have consequences downstream. They can trap silt so that floodplains or deltas downstream no longer build up

Flood Control Dam

While dams can control floods, they can also have consequences downstream. They can trap silt so that floodplains or deltas downstream no longer build up.

Built up sediment behind a dam can also can reduce the effectiveness of the dam for flood control as the shallower the reservoir gets, the less water can be stored without flooding.

Limiting Development

Today many scientists and engineers advocate sound floodplain management instead of building structures.

This often means preserving floodplains in their natural states. Minimizing development means that floodplains can assume their natural function allowing them to absorb floodwaters without harm to homes or businesses.

Drainage Basins

Every system has a drainage basin, also called a watershed.

A drainage basin is the land area that contributes water to a stream.

An imaginary line called a divide separates the drainage basins of one stream from another.

Concepts Covered. . . . Streams generally erode their channels by dissolving soluble material, by

lifting loose particles, or grinding

Streams transport their load of sediment in three ways: in solution (dissolved load), in suspension (suspended load), or scooting or rolling along the river bottom (bed load).

Deposition occurs as the streamflow drops below the critical settling velocity of a certain particle size

There are two general types of stream valleys: narrow V-shaped and wide valleys with flat floors.

Most floods are caused by rapid snow melt and storms that bring heavy rains over a large region

Measures to control flooding include the construction of artificial levees, building flood control dams, and placing drain limits on floodplain development.

A drainage basin is the land area that contributes water to a stream

Computer Lab Assignment Today, go to the computer lab and write a short essay, one page

double-spaced, answering the following question dealing with the following scenario.

Over the years, commercial fisherman have dredged the channels of the Mississippi River Delta so that their boats could get to the Gulf of Mexico to fish for shrimp easier.

The dredging of the channels removed much sediment from the delta that acted as a barrier against floodwaters hitting the inhabited coast.

When a hurricane came, the floodplain no longer contained enough sediment to act as a barrier that would dispense much of the hurricanes force before it hit inhabited land.

Should the Mississippi Delta be allowed to resume it’s natural course of development and not be dredged or should the commercial fisherman have priority and the river channel be kept open for boats. What type of compromise could possibly be reached so that all parties needs are met? Explain your answer fully.