1. Ch. 13 Modern Earth Science p. 242 - 255
Water & Erosion
Ch. 13
Modern Earth Science p

2. Section 13.1 – The Water Cycle
Origin of the Earth’s water supply has puzzled individuals for centuries
Initially it was believed that rivers like the Nile and Danube could be supplied by rain alone, but with new technology scientists can accurately measure the amount of water received by the Earth and the amount being carried in the rivers
Discovered that the Earth receives 5 times more water than is being carried in the rivers

3. Hydrologic (Water) Cycle – continuous movement of water from the atmosphere to Earth’s surface and then back to the atmosphere again
Evaporation – process by which liquid water changes into water vapor
500,000 km3 evaporate each year
86% evaporates from the ocean
Transpiration – process by which plants give off water vapor into the air
Evapotranspiration – combination of both evaporation and transpiration
Continents lose about 70,000 km3 of water per year
Runoff – water that flows over the land into rivers
Groundwater – water that soaks deep into the soil and rock underground
Condensation – process of water vapor changing into liquid water
Precipitation – process by which water falls from clouds to the Earth

5. Water Budget – result of continuous cycling of water through the processes of evaporation, condensation and precipitation
Overall the water budget of the planet is balanced
Local water budget or budget of a particular area is usually not balanced
Local water budgets can be seasonal

6. Water conservation
Estimated that each US citizen uses approximately 95 m3 of water each year
Approximately 90% of the water used by cities and industry is returned to rivers or oceans
Desalination – process of removing salt from ocean water; creates more drinking water
Lawn in Namibia where a hotel worker watered it for 45 minutes even though there were numerous water conservation signs

7. Section 13.2 – River Systems
Made up of the main stream and all the feeder streams, called tributaries
Watershed (drainage basin) – the landform which water runs off into these systems
Divides – ridges or areas of high ground that separates watersheds

8. Stream Erosion Path that a stream follows is called its channel
Headward erosion – process of lengthening and branching of a stream; carries away sediments from the slopes of a watershed at the upper end of a stream
Stream piracy – capture of stream in one watershed by a stream with a higher rate of erosion in another watershed

9. Stream Load
Materials carried by a stream is called the stream load, includes particles such as soil, loose rock fragments and dissolved minerals
Stream load has three forms
Suspended load – consists of fine sand and silt particles that are suspended in the water due to its velocity
Bed Load – composed of larger materials, coarse sand and gravel, moves by rolling, sliding or saltation (short jumps)
Dissolved Load – mineral matter transported in liquid solution

10. Discharge & Gradient Water & Wind Gaps
Discharge – volume of water moved at a given time; the faster a stream flows the higher the discharge
Gradient – steepness of the slope; higher gradient means a larger discharge
Gradient is steep near the headwaters of a stream
Water & Wind Gaps
Water gap – when land is uplifted and streams are eroded downwards a water gap is formed
Wind gap – when land is uplifted faster than a stream can erode it water evacuates its previous channel to create a wind gap

12. Stages of a River System
Youthful Rivers –
usually erodes its bed faster than its banks
V-shaped valley with steep sides
small amount of tributaries
carries a relatively small amount of water

13. Mature Rivers well established tributaries
carries a larger volume of water
doesn’t deepen its bed, but erosion occurs along valley walls
low gradient tends to curve back and forth across a flat valley floor
meanders form across the valley floor when the stream channel is shifted to towards the outside bank
oxbow lake – lake formed when a meander becomes so curved that it almost forms a loop

15. Old Rivers – Rejuvenated Rivers – low gradient
slower than a mature river
doesn’t erode the land
more meanders develop
few tributaries
Rejuvenated Rivers –
gradient has increased due to a lifting of the land
results in the formation of steplike terraces along both sides of the river valley

18. Section 13.3 – Stream deposition
the total load of a stream is greatest when a large volume of water is moving swiftly
as the velocity is reduced the ability of the stream to carry sediments is lessened
Deltas and Alluvial Fans
delta – fan shaped deposit at the mouth of a stream; tip of the fan is facing upstream
alluvial fan – load is deposited on land with the tip of the mound facing upstream

21. Flood Deposits
floodplain – portion of the valley that is covered by floodwater during a flood
natural levee – deposits of coarse sediments in a flood produces raised banks

22. Flood Control
indirect – includes soil and forest conservation practices to prevent excess runoff
direct – most common form is building a dam; artificial levees (only offer temporary protection)
floodway – permanent overflow channel; carries away excess water and prevents the main stream from overflowing