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

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

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

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

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

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

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

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

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

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

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

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

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

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

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