EESA06H3 Chapter Notes - Chapter 14: Downcutting, Mass Wasting
10 May 2018
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Ch.14 - Streams and Floods
- Sea level rose and fell in Quaternary
- Fell when water froze on continents to form glaciers
- Rose when melted and returned to sea
- Rivers alternated between erosion from low sea level and deposition from high sea
level
- Resulted in complex history of cutting and filling near mouths of old rivers
- Base level can be above or below sea level if not connected to sea
- Ungraded stream uses erosional energy in down-cutting to smooth out irregularities
- Becomes graded as smooths out into a concave upward shape
Graded Upstream: exhibits delicate balance between transporting capacity and
sediment load available
- Maintained by cutting and filling irregularities
- Changes in streams gradient can change its sediment load
- Increase in gradient increases stream velocity allowing stream to erode and carry
more sediment
- Change in sediment load can change gradient
- Decrease in sediment load brings erosion of streams channel thus lowering gradient
- Streams are sediment free downstream of dams as trap sediments
- Causes sever channel erosion below dam as streams adjust to reduced load
- River’s energy is used for transporting sediment and overcoming resistance to flow
- Decreases in sediment load means more energy for other tasks
- Can erode more sediment or increase resistance to flow so excess energy is used to
overcome it
Lateral Erosion: erosion and undercutting of stream’s banks and valleys walls as
stream swings from side to side across its valley floor
- Widens valley
Headward Erosion: slow, uphill growth of valley above original source through
gullying, mass wasting and sheet erosion
Stream Terraces: steplike landforms found above a stream and its flood plain
- Benches cut in rock or steps formed in sediment by deposition and subsequent
erosion
- Terrace forms as river cuts downward into a thick sequence of its own flood-plain
deposits
- River goes from deposition to erosion is uplift
- Steepens gradient, causing river to speed up and begin erosion
- Change from dry to wet climate may increase discharge and begins eroding
- Drop in base level has same effect
- Terraces can also develop from erosion of bedrock valley floor
Document Summary
Sea level rose and fell in quaternary. Fell when water froze on continents to form glaciers. Rose when melted and returned to sea. Rivers alternated between erosion from low sea level and deposition from high sea level. Resulted in complex history of cutting and filling near mouths of old rivers. Base level can be above or below sea level if not connected to sea. Ungraded stream uses erosional energy in down-cutting to smooth out irregularities. Becomes graded as smooths out into a concave upward shape. Graded upstream: exhibits delicate balance between transporting capacity and sediment load available. Changes in streams gradient can change its sediment load. Increase in gradient increases stream velocity allowing stream to erode and carry more sediment. Change in sediment load can change gradient. Decrease in sediment load brings erosion of streams channel thus lowering gradient. Streams are sediment free downstream of dams as trap sediments.
