13.2 Stream Erosion and Deposition Key Idea: As streams flow, they erode rocks and soil, transport these sediments over long distances, and deposit them in areas of quieter water.

Learning Objectives Describe how streams weather and erode Earth’s surface. Describe how streams transport and deposit sediments. Explain how deltas and alluvial fans form.

How Streams Weather and Erode Material Running water in streams and rivers wears down Earth’s surface by breaking up bedrock and by removing eroded rock and soil materials. Running water breaks up the bedrock over which it flows primarily by mechanical means. Rapidly flowing water has a lifting effect that can split off and remove rock fragments.

How Streams Weather and Erode Material Most erosion occurs when running water abrades and hammers away at its bed by using sand, pebbles, and boulders as “cutting tools”. Abrasion also wears down the cutting tools themselves, producing the rounded boulders, pebbles, and sand grains that are commonly found in te beds of streams and rivers.

River Rounded Boulders

Potholes Cutting tools (moving pebbles and boulders) are involved in the formation of potholes in the bedrock of rivers. Potholes are oval or circular basins which form when water in a river develops small whirlpools. As sand and pebbles, or even small boulders swirl around in the whirlpools they grind potholes in the bedrock.

Potholes

Along James River in Virginia potholes formed in granite

Along Mohawk River in New York the potholes formed in limestone

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

How a Plunge Pool Forms

How a Plunge Pool Looks

Chemical Weathering The rivers also exert a chemical stress on the bedrock. The water is a powerful solvent, and many minerals in rocks are easily dissolved. Such minerals are the ones which contain calcium: calcite and dolomite are the most common. Rocks containing these minerals, like limestone and marble are easily weathered away by streams and rivers.

How Streams Transport Material All the materials transported downstream by a river is called load. A river carries its load in three ways: 1. in solution 2. in suspension 3. in its bed load

Solution If the river flows over rocks that dissolve in water then the river’s load is transported in solution. Common minerals transported in solution are: calcium, magnesium, and bicarbonate. Most from rivers’ solution load comes from groundwater seeping into the river.

Materials in Suspension When the river looks muddy, it carries materials in suspension. Suspended material consists in fine particles of clay, silt , and fine sand. Although these materials are heavier than water, the turbulence of the stream flow stirs them up and keeps them from sinking.

River carrying materials in suspension

Transport of Materials on Bed Load A river may also transport materials in its bed load. The bed load materials are too heavy to be carried in suspension, and are slowly dragged or tumbled on the bed of the river, especially at high velocity. The relative amounts of river’s load depend on the -nature of the river -climate -type of bedrock -season of the year. As a general rule, most of the rivers’ load is transported in suspension. The load increases with human land use.

Bed Load Materials

Competence and Capacity The ability of a stream or a river to transport materials is described by its competence and capacity. Competence is a measure that describes the maximum size of the materials carried by the river. Capacity is the measure of the total amount of sediment a river or stream can carry. The competence and capacity increase during floods, therefore much of the erosion created by rivers occurs during floods.

Stream Deposition All rivers and streams will eventually deposit all the materials they carry. While some of the materials may be deposited along the channel, most of the load is deposited where the river empties into a sea, or lake. A delta is a common deposit, that forms when a river flows into a quiet or large body of water, such as a lake, an ocean, or an inland sea.

How Deltas Form River water comes to a standstill at a delta where most of the river’s sediment is dropped. As long as the amount of sediment is larger than the amount that can be taken away by the currents, the deposit grows. A river flowing into its delta, it splits into branches called distributaries. A delta is a delicate balance between deposition and erosion.

Mississippi Delta

Alluvial Fan A fan shaped deposit is called an alluvial fan. Alluvial fans form at the base of a mountain, where the slope changes drastically. An alluvial fan differs from a delta in several ways: -first, the deposit is formed on land, not in water. -Second, the sediments are coarse sand and gravels rather than silt and clay. -Third, its surface is sloping; it is not flat like that of a delta.

Alluvial Fans