13.2 Stream Erosion and 13.2 Stream Erosion and DepositionDeposition

Key Idea: Key Idea: As streams flow, they erode rocks and soil, As streams flow, they erode rocks and soil,

transport these sediments over long transport these sediments over long distances, and deposit them in areas of distances, and deposit them in areas of

quieter water.quieter water.

Learning ObjectivesLearning Objectives

1.1. Describe how streams weather and erode Describe how streams weather and erode Earth’s surface.Earth’s surface.

2.2. Describe how streams transport and Describe how streams transport and deposit sediments.deposit sediments.

3.3. Explain how deltas and alluvial fans form.Explain how deltas and alluvial fans form.

How Streams Weather and Erode How Streams Weather and Erode MaterialMaterial

Running water in streams and rivers wears Running water in streams and rivers wears down Earth’s surface by breaking up down Earth’s surface by breaking up bedrock and by removing eroded rock and bedrock and by removing eroded rock and soil materials.soil materials.

Running water breaks up the bedrock over Running water breaks up the bedrock over which it flows primarily by mechanical which it flows primarily by mechanical means. Rapidly flowing water has a lifting means. Rapidly flowing water has a lifting effect that can split off and remove rock effect that can split off and remove rock fragments.fragments.

How Streams Weather and Erode How Streams Weather and Erode MaterialMaterial

Most erosion occurs when running water Most erosion occurs when running water abrades and hammers away at its bed by abrades and hammers away at its bed by using sand, pebbles, and boulders as using sand, pebbles, and boulders as ““cutting tools”. cutting tools”.

Abrasion also wears down the cutting tools Abrasion also wears down the cutting tools themselves, producing the rounded themselves, producing the rounded boulders, pebbles, and sand grains that are boulders, pebbles, and sand grains that are commonly found in te beds of streams and commonly found in te beds of streams and rivers. rivers.

River Rounded BouldersRiver Rounded Boulders

PotholesPotholes

Cutting tools (moving pebbles and boulders) Cutting tools (moving pebbles and boulders) are involved in the formation of potholes in are involved in the formation of potholes in the bedrock of rivers. the bedrock of rivers.

Potholes Potholes are oval or circular basins which are oval or circular basins which form when water in a river develops small form when water in a river develops small whirlpools. As sand and pebbles, or even whirlpools. As sand and pebbles, or even small boulders swirl around in the whirlpools small boulders swirl around in the whirlpools they grind potholes in the bedrock.they grind potholes in the bedrock.

PotholesPotholes

Along James River in Virginia Along James River in Virginia potholes formed in granitepotholes formed in granite

Along Mohawk River in New York Along Mohawk River in New York the potholes formed in limestonethe potholes formed in limestone

What Is a Plunge Pool?What Is a Plunge Pool?

A plunge pool is a basin that has been worn A plunge pool is a basin that has been worn away at the base of a waterfall by the action away at the base of a waterfall by the action of falling water. Abrasion by churning of falling water. Abrasion by churning particles also plays a role in forming a particles also plays a role in forming a plunge pool. Many waterfalls have plunge plunge pool. Many waterfalls have plunge pools. pools.

How a Plunge Pool FormsHow a Plunge Pool Forms

How a Plunge Pool LooksHow a Plunge Pool Looks

Chemical WeatheringChemical Weathering

The rivers also exert a chemical stress on the The rivers also exert a chemical stress on the bedrock. The water is a powerful solvent, bedrock. The water is a powerful solvent, and many minerals in rocks are easily and many minerals in rocks are easily dissolved. Such minerals are the ones which dissolved. Such minerals are the ones which contain calcium: calcite and dolomite are the contain calcium: calcite and dolomite are the most common. most common.

Rocks containing these minerals, like Rocks containing these minerals, like limestone and marble are easily weathered limestone and marble are easily weathered away by streams and rivers.away by streams and rivers.

How Streams Transport MaterialHow Streams Transport Material

All the materials transported downstream by All the materials transported downstream by a river is called a river is called load. load. A river carries its load A river carries its load in three ways:in three ways:

1. in solution 1. in solution 2. in suspension2. in suspension 3. in its bed load 3. in its bed load

SolutionSolution

If the river flows over rocks that dissolve in If the river flows over rocks that dissolve in water then the river’s load is transported in water then the river’s load is transported in solution. solution.

Common minerals transported in solution Common minerals transported in solution are: calcium, magnesium, and bicarbonate. are: calcium, magnesium, and bicarbonate. Most from rivers’ solution load comes from Most from rivers’ solution load comes from groundwater seeping into the river. groundwater seeping into the river.

Materials in SuspensionMaterials in Suspension

When the river looks muddy, it carries When the river looks muddy, it carries materials in suspension. materials in suspension.

Suspended material consists in fine particles Suspended material consists in fine particles of of clay, siltclay, silt , and , and fine sand.fine sand.

Although these materials are heavier than Although these materials are heavier than water, the turbulence of the stream flow stirs water, the turbulence of the stream flow stirs them up and keeps them from sinking.them up and keeps them from sinking.

River carrying materials in River carrying materials in suspensionsuspension

Transport of Materials on Bed LoadTransport of Materials on Bed Load

A river may also transport materials in its bed load. The A river may also transport materials in its bed load. The bed load materials are too heavy to be carried in bed load materials are too heavy to be carried in suspension, and are slowly dragged or tumbled on the bed suspension, and are slowly dragged or tumbled on the bed of the river, especially at high velocity.of the river, especially at high velocity.

The relative amounts of river’s load depend on the The relative amounts of river’s load depend on the -nature of the river-nature of the river -climate-climate -type of bedrock-type of bedrock -season of the year. -season of the year. As a general rule, most of the rivers’ load is transported in As a general rule, most of the rivers’ load is transported in

suspension. The load increases with human land use.suspension. The load increases with human land use.

Bed Load MaterialsBed Load Materials

Competence and CapacityCompetence and Capacity

The ability of a stream or a river to transport materials The ability of a stream or a river to transport materials is described by its competence and capacity.is described by its competence and capacity.

Competence Competence is a measure that describes the is a measure that describes the maximum size of the materials carried by the river. maximum size of the materials carried by the river.

Capacity Capacity is the measure of the total amount of is the measure of the total amount of sediment a river or stream can carry.sediment a river or stream can carry.

The competence and capacity increase during The competence and capacity increase during floods, therefore much of the erosion created by floods, therefore much of the erosion created by rivers occurs during floods. rivers occurs during floods.

Stream DepositionStream Deposition

All rivers and streams will eventually deposit all All rivers and streams will eventually deposit all the materials they carry. the materials they carry.

While some of the materials may be deposited While some of the materials may be deposited along the channel, most of the load is deposited along the channel, most of the load is deposited where the river empties into a sea, or lake. where the river empties into a sea, or lake.

A A deltadelta is a common deposit, that forms when a is a common deposit, that forms when a river flows into a quiet or large body of water, such river flows into a quiet or large body of water, such as a lake, an ocean, or an inland sea. as a lake, an ocean, or an inland sea.

How Deltas FormHow Deltas Form

River water comes to a standstill at a delta where River water comes to a standstill at a delta where most of the river’s sediment is dropped. most of the river’s sediment is dropped.

As long as the amount of sediment is larger than As long as the amount of sediment is larger than the amount that can be taken away by the the amount that can be taken away by the currents, the deposit grows. currents, the deposit grows.

A river flowing into its delta, it splits into branches A river flowing into its delta, it splits into branches called called distributaries. distributaries.

A delta is a delicate balance between deposition A delta is a delicate balance between deposition and erosion. and erosion.

Mississippi Delta Mississippi Delta

Alluvial FanAlluvial Fan

A fan shaped deposit is called an A fan shaped deposit is called an alluvial fan. alluvial fan. Alluvial fans form at the base of a mountain, where Alluvial fans form at the base of a mountain, where the slope changes drastically. the slope changes drastically.

An alluvial fan differs from a delta in several ways: An alluvial fan differs from a delta in several ways: -first, the deposit is formed on land, not in water. -first, the deposit is formed on land, not in water. -Second, the sediments are coarse sand and -Second, the sediments are coarse sand and

gravels rather than silt and clay. gravels rather than silt and clay. -Third, its surface is sloping; it is not flat like that of -Third, its surface is sloping; it is not flat like that of

a delta. a delta.

Alluvial Fans Alluvial Fans