Surface Water: Rivers. Where does the water come from? (the hydrologic cycle) Sediment in streams and rivers. How does a stream change along its course?

Surface Water:Surface Water:RiversRivers

Surface Water: Rivers

• Where does the water come from? (the hydrologic cycle)

• Sediment in streams and rivers.• How does a stream change along its course?• How do floodplains form?• How do our activities affect streams?• How do stream-carved landscapes change

through geologic time?

• They provide us with water. • They deposit fertile soils for agriculture.• They are major transportation pathways for

commerce and trade.• They flood, erode, and ultimately carve out the

landscapes we see on Earth’s surface.• Floods may affect major cities.• They provide us with recreational opportunities.

Why study rivers?

Where does the water come from?The hydrologic cycle describes the movement of water on and in Earth. Evaporation and transpiration moves water into the atmosphere. Precipitation brings it back to the ground. It may then flow across the surface as runoff into river channels, or mayinfiltrate into the subsurface and become ground water.

Earth’s Water Budget

Where does the water come from?Where does the water come from?

• The total land surface area that drains into a stream is called a drainage basin.

• Basins are separated by ridges called divides.

• Larger rivers (e.g., major rivers such as the Mississippi) contain water from the drainage basins of all the smaller streams and rivers that feed into it.

Where does the water come from?Where does the water come from?

Where does the water come from?Map showing major drainage divides of the North American continent. Most of Canada drains into the Hudson Bay. Most of the U.S. drains into the Gulf of Mexico. Western north America

drains into either the Sea of Cortez or the Pacific ocean.

The Great Basin does not drain into the ocean, but instead is an area of internal drainage, forming lakes and playas (dry lakes).

Example: The Columbia River Drainage Basin

Where does the water come from?Discharge is a measure of the amount of water that flows through a river channel. It is calculated by determining the cross-sectional area of a stream and multiplying this times the velocity. Discharge is expressed as volume per unit time, usually m3/s.

Sediment in Streams and Rivers

Fig 11.12

Particles of decomposing rocks that are created by mechanical weathering are picked up in surface runoff and transported to the stream channel.

The stream itself may erode the banks of its channel and thus add sediment that is carried away. This often happens during times of dramatically increased discharge, e.g. storms.


• How much total sediment does a stream carry?• The particles a stream carries are called its

sediment load, this material is a product of mechanical weathering (clastic material).

• Delivery of sediment is a function of drainage basin slope and climate.

Greater slope = higher delivery of sediment via erosion.Greater rainfall = higher sediment delivery.More vegetation = less sediment delivery.

• Dissolved load – the “hidden” load• Ions in solution from chemical weathering are also

carried by water in streams. Dissolved load is a smaller mass than the normal sediment load, but still significant.


Fig 16.7

Sediment loads of major rivers on Earth.


Streams on solid rock (bedrock) erode their channels by abrasion. The impact of tumbling rock particles fractures small pieces of rock, which then tumble and break up and/or break off other small pieces. This adds to the clastic sediment load.


Joints, fractures, and bedding planes also provide zones of weakness that allow sediment to be produced and carried away.


• Clastic sediment moves as both bedload and suspended load.• Bedload: larger particles that cannot be picked up

by the current, but instead simply roll, bounce, and slide along the bottom, moving downstream.

Bedload may form structures on the stream bed such as ripples and cross-bedding.

• Suspended Load: smaller particles that are physically picked up and carried by the current downstream.


Dissolved load, bed load and suspended load are all carried downstream by the force of moving water.


• All streams transition from regimes of erosion and transportation to those of deposition as they journey towards the ocean.• Decrease of water velocity will cause deposition.• This usually occurs downstream due to changes in

the gradient of the stream, and ultimately entry into lakes or oceans.

• Gradient is defined as rise/run (vertical/horizontal).


Typical Stream Profile



Example: Streams in arid landscapes such as where we live - the depth goes to zero very rapidly. The result is that a fan of alluvium forms where the stream goes from confined to unconfined conditions exiting a mountainous area. The streams transporting power drops to zero and sediment is deposited.


An alluvial fan forming in Death Valley, California.


• Deposition occurs where velocity decreases - transporting power is a function of water velocity.

• Example: when a river enters a body of water such as an ocean or lake, the drop in velocity causes deposition of sediments. This may form a delta.


The Mississippi River delta. Delta’s are progressively built out over time.

Suspended load and dissolved load is basically all that is left in the river at this point. Bed load is lower due to low transporting power of the slow moving water.


Typical Stream Profile

How Does A Stream Change Along It’s Course?

• Sediment size decreases downstream.• Abrasion explains part of this, particles are reduced in

size by mechanical action.• Transporting power of the stream decreases downstream

also as the slope decreases.

How Does A Stream Change Along It’s Course?

• Gradient decreases downstream.• Streams achieve a concave upwards profile due to

erosion upstream and deposition downstream.• The gradient is at it’s shallowest as the stream

approaches base level.• Base level is the lowest a stream can go – the oceans

are the ultimate base level for Earth.

How Do Floodplains Form?

• A floodplain is the flat land surface adjacent to a river channel. It is constructed by the river during times of flooding.

• In most areas the flow in streams exceeds the capacity of the channel every few years - floods are not uncommon.

• Floodplains are formed by the process of meandering and by bedload and suspended load. Both may be deposited when rivers overflow their banks during floods.

• Most floodplains consist of both materials.

Floodplain of the Owens River



Stream valleys naturally widen through time due to meandering.

Rivers meander, eroding along the outer parts of bends (forming cutbanks), and deposition along the inner parts of bends (forming point bars).

Areas where the river is currently not flowing will contain sediment deposited by the flowing river when it was there, along with sediment deposited during flood events.


Cutbanks in a meandering stream may erode towards one another. When they meet, the meander gets “cut off”, forming an oxbow lake.


How Do Our Activities Affect Streams?

• Dams and reservoirs now interrupt nearly all major rivers and many small streams. They are built to control floods, provide electrical power, assist navigation, and to store water for us.

• Dams trap 90–100% of the incoming sediment load. The water level in a lake behind a dam creates a new base level reference up- and downstream of the dam.

Dams increase deposition upstream due to decreased velocity. They increase erosion downstream due to increased slope below the dam. They also produce

sediment-free water downstream, which has increased capacity for erosion and carrying more sediment.


Changes in river discharge occur due to urbanization. Natural landscapes with vegetation slow the delivery of water to streams. However, urban settings divert water from themselves because of e.g., pavement and storm sewers. This changes the water delivery to streams and may trigger floods.


How do stream-carved landscapes change through geologic time?

Terraces are evidence of downcutting and filling of valleys.

Bedload deposits well above a current floodplain indicates the stream was at a higher elevation in the past.

Sea level changes may drive terrace formation. When sea level drops downcutting occurs. When sea level rises deposition occurs.

16.12 How do stream-formed landscapes 16.12 How do stream-formed landscapes change through geologic time?change through geologic time?

Stream Terraces

A stream with well developed meanders is formed.

Sea level drops, e.g. during an ice age when more water is stored in glaciers.

Sea level drop produced a new lower base level.

Streams respond by downcutting and incised meanders may develop.



Surface Water: Rivers. Where does the water come from? (the hydrologic cycle) Sediment in streams and rivers. How does a stream change along its course?

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