Chapter 9 – Section – 9.2Stream Development Surface Water
Stream Development: Objectives • Describe some of the physical features of stream development. • Describe the relationship between meanders and stream flow. • Explain the process of rejuvenation in the stream development.
Streams erode paths through sediment and rock, forming V-shaped stream valleys. Review Vocabulary abrasion: process of erosion in which windblown or waterborne particles, such as sand, scrape against rock surfaces or other materials and wear them away
New Vocabulary • stream channel • stream bank • stream capture • Stream gradient • base level • meander • delta • alluvial fans • rejuvenation
Supply of Water: Stream Channels The region where water first accumulates to supply a stream is called the headwaters. Falling precipitation accumulates in small gullies at higher elevations and forms briskly moving streams. Moving water carves a narrow pathway into the sediment or rock called the stream channel. Stream banks hold the moving water within them.
Supply of Water: Stream Channels • When small streams erode away the rock or soil at the head of a stream, it is known as headward erosion. • If the headward erosion of Stream A cuts into Stream B and draws water away from Stream A into Stream B, then the process is called stream capture.
Formation of Stream Valleys The energy of a stream comes from the movement of water down a slope. The slope of a stream channel is called the stream gradient. The gradient of the stream depends on its base level, which is the elevation at which the stream enters another stream or body of water.
Formation of Stream Valleys • The height of a stream above its base level determines how much downcutting energy the stream will have.
Formation of Stream Valleys: Meanders • A bend or curve in a stream channel caused by moving water is called a meander. • Water moving along the outside of a meander curve experiences the greatest velocity within the meander and erodes the side of the streambed. • Along the inside of a meander, the water moves more slowly and deposition is dominant. • The meanders of a stream are accentuated by differences in the velocity of water in the channel.
Deposition of Sediment • When streams lose velocity, they lose some of the energy needed to transport sediment, and deposition of sediment occurs. • Two Common landforms are • Alluvial Fans • Deltas
Deposition of Sediment: Alluvial Fans • Alluvial fans are fan-shaped, sloping depositional features that form when water flows down steep slopes onto flat, dry lake beds. • Alluvial fans are composed mostly of sand and gravel.
Deposition of Sediment: Deltas • The triangular deposit that forms where a stream enters a large body of water is called a delta. • Delta deposits usually consist of layers of silt and clay particles.
Rejuvenation • Rejuvenation is the process of down cutting of a stream actively towards its base level. • This causes an increase in the stream’s velocity, and the stream’s channel once again cuts downward into the existing meanders.
Summary • Streams erode paths through sediment and rock, forming V-shaped stream valleys. • Water from precipitation gathers in gullies at a stream’s headwaters. • Stream water flows in channels confined by the stream’s banks. • Alluvial fans and deltas form when stream velocity decreases and sediment is deposited. • During the process of rejuvenation, deep – sided canyons can be caused.