Surface Water:
Rivers

Surface Water: Rivers
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• 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?

Why study rivers?
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They provide us with water.
They deposit fertile soils for agriculture.
They are major transportation pathways for commerce and trade.
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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.

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 may infiltrate into the subsurface and become ground water.

Earth’s Water Budget

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?
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 m 3 /s.

Sediment in Streams and Rivers
Particles of decomposing rocks that are created by mechanical weathering are picked up in surface runoff and transported to the stream channel.
Fig 11.12

Sediment in Streams and Rivers
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.

Sediment in Streams and Rivers
• 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.

Sediment in Streams and Rivers
Sediment loads of major rivers on Earth.
Fig 16.7

Sediment in Streams and Rivers
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.

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

Sediment in Streams and Rivers
• 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.

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

Sediment in Streams and Rivers
• 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).

Sediment in Streams and Rivers

Sediment in Streams and Rivers

Sediment in Streams and Rivers
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.

Sediment in Streams and Rivers
An alluvial fan forming in Death Valley, California.

Sediment in Streams and Rivers
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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.

Sediment in Streams and Rivers
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.

How Does A Stream Change Along It’s Course?

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.
• 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?
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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.

How Do Floodplains Form?
Floodplain of the Owens River

How Do Floodplains Form?

How Do Floodplains Form?
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.

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

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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.

How Do Our Activities Affect Streams?
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.

How Do Our Activities Affect Streams?
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.

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.

Stream Terraces

How do stream-carved landscapes change through geologic time?
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.

How do stream-carved landscapes change through geologic time?