McKnight's Physical Geography

Lectures

Chapter 16

Fluvial Processes

© 2014 Pearson Education, Inc.

Original by Andrew MercerMississippi State University

Modified by AJ Allred for Geography 1000

Salt Lake Community College

Learning Goals of This Chapter• Differentiate between stream flow and overland flow.• Define valley, interfluve, drainage basin, watershed,

drainage divide, and tributary.• Determine the stream orders in a drainage network.• Explain erosion by stream flow and the three fractions of

stream loads.• Explain recurrence interval and 100-year flood, and

identify and explain the common misunderstanding about the latter.

• Describe the flow velocity in a cross section of a straight stream channel.

• Explain the circumstances that cause steams to develop sinuous, meandering, and braided channel patterns.

Learning Goals of This Chapter

• Differentiate between competence and capacity of a stream.

• Differentiate between perennial, intermittent, and ephemeral streams.

• Define discharge and stream gage, and explain the differences in lag time and crest height on a hydrograph for an urban versus a rural area.

Streams and Stream Systems

• Channeled flow of water – a stream

• Fluvial processes – those that involve running water

• Some water flows overland as a sheet, not in a stream channel

Streams and Stream Systems

Stream order

•1 = small streams •2 = larger streams

•3 = small rivers•4 = major rivers

[Insert Fig. 16-4 p. 376]

Streams and Stream Systems• Fluvial erosion and

deposition– Splash erosion is powerful

– Sheet erosion

• Erosion by stream flow– Once channeled, erosion is

greatly enhanced

– Chemical weathering – corrosion

These rocks in Asia are just like rocks in Utah

– heavily worn and eroded

• Transportation - bed load, dissolved, and suspended

– Stream competence – particle size a stream can transport

– Stream capacity – amount of solid material a stream can transport

Streams and Stream Systems• Deposition – streams eventually dump their loads

– Alluvium – stream deposited sediments

• Perennial and intermittent streams– Perennial streams – permanent, in humid regions

– Intermittent streams – seasonal

– Ephermal streams – flow only with occasional large storms

Streams and Stream Systems

Hard-surfacing in dry country and

urban areas greatly concentrates flood

water.

What is the “100-year” flood program?

Stream Channels

– Straight channels – steeper terrain –

water finds an easy slope

– Sinuous channels – winding meanders on shallow slopes. Water searching for a path to the ocean.

Floodplains

• Low-lying near flat alluvial valley floor that is periodically inundated with flood waters

• Floodplain landforms– Bluffs

– Cutoff meander

– Oxbow lake

– Meander scars

– Natural levees

– Backswamps

– Yazoo streams

The Shaping and Reshaping of Valleys

Valley widening

– Water moves fastest on outside of curves (cut bank)

– Slowest water on inside of curves accumulates alluvium (point bar)

Structural Relationships

• Antecedent streams – those that existed before new uplift occurs

Antecedent stream persisted in cutting its same path even as mountains grew up underneath.

Structural Relationships

• Stream drainage patterns (cont.)– Radial pattern – streams descend a concentric uplift

– Centripetal pattern – streams converge into a uniform basin

– Annular pattern – forms in areas of hard and soft domes or basins, flow follows soft bedrock and is confined by hard bedrock

The Shaping and Reshaping of Valleys

“Knick Points”

Where under-cutting softer rock underneath eventually causes

surface failure.

The “knick point” gradually moves upstream by under-cutting erosion.

Example: Niagara Falls in New York

The Shaping and Reshaping of Valleys• Delta formation

– Slowed flow when it reaches an ocean or lake results in deposited sediment

– Debris builds up and forms a delta

– Distributaries

• Alluvial fans – similar deposition at the bottom of the valley.

Floodplains• Modifying rivers to control flooding

– Humans live on floodplains – flat land, abundant water, and productive soils

– Levees & dams for flood control

– Human changes to deltas and floodplains

Living near water can be hazardous