Rivers
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RIVERS Water cycle, drainage basins, energy and processes D o n e b y : A . S m i t h 1
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Transcript of Rivers
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RIVERS Water cycle, drainage basins, energy and processes
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THE WATER CYCLE
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THE WATER CYCLE
http://www.epa.gov/ogwdw/kids/flash/flash_watercycle.html
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THE WATER CYCLE
The Water Cycle (also known as the hydrologic cycle) is the journey water takes as it circulates from the land to the sky and back again
The Sun's heat provides energy to evaporate water from the Earth's surface (oceans, lakes, etc.).
Plants also lose water to the air (this is called transpiration).
The water vapor eventually condenses, forming tiny droplets in clouds.
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THE WATER CYCLE
When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and water returns to the land (or sea). Some of the precipitation soaks into the ground.
Some of the underground water is trapped between rock or clay layers; this is called groundwater. But most of the water flows downhill as runoff (above ground or underground), eventually returning to the seas as slightly salty water.
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THE WATER CYCLE
More than 97% of the world’s water is stored in oceans and seas.
These water bodies make up about 70% of the surface of the earth.
The rest of the water is stored as: 2.1% as ice and snow 0.6% as groundwater 0.1% in rivers and lake 0.001% in the atmosphere as water vapor and
clouds
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THE WATER CYCLE
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PROCESSES IN THE WATER CYCLE
Evaporation Condensation Precipitation
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EVAPORATION
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EVAPORATION
This is the process by which water changes from a liquid into water vapor which is a gas
Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.
Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world
The energy to carryout this process comes from the sun’s heat and the wind.
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CONDENSATION
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CONDENSATION
Condensation is the change of water from its gaseous form (water vapor) into liquid water.
Condensation generally occurs in the atmosphere when warm air rises, cools and looses its capacity to hold water vapor.
Excess water vapor condenses to form cloud droplets.
The upward motions that generate clouds can be produced by convection in unstable air.
The level at which water vapor cools and changes to water is called the dew point.
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PRECIPITATION
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HOW WATER FLOWS ON REACHING THE SURFACE
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WATER ON THE SURFACE
When water reaches the surface it is not stagnant. It follows a number of paths that eventually ends up in the sea.
A small amount goes directly into rivers. The rest falls on the vegetation or the ground.
When it rains and the ground becomes over saturated the water falls on the surface and run off; this is called surface runoff or overland flow.
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WATER ON THE SURFACE
If the soil is not soaked, the rain water will soak through.
If the rock below the ground is permeable it will allow water to soak through until it reaches impermeable rock.
After that it begins to build up towards the surface after which gravity causes it to flow down slope.
The upper level of the groundwater is called the water table. The water in the rocks is know as groundwater while the water moving through the rocks is called groundwater flow.
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WATER ON THE SURFACE
Water flowing through the soil is called infiltration.
Percolation is the downward movement of water within a soil or rock.
Rainwater can be intercepted by vegetation Vegetation takes in water through the roots. It looses some of it through transpiration.
Surface water is also lost by evaporation. The two processes together is known as evapotranspiration.
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WATER ON THE SURFACE
Where water enters the system these are known as Inputs.
Where water is held are called stores. Where water flows through the system is
known as transfers. Where water is lost to the system are called
outputs.
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WHAT IS A DRAINAGE BASIN
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DRAINAGE BASIN
This is the area of land drained by a river system (a river and its tributaries). It includes the surface run-off in the water cycle, as well as the water table.
Drainage basins are separated by watersheds.
The drainage basin system is demarked by a watershed, this is the upper limit around the drainage basin that separates one drainage basin from another.
This is a ridge of island that separates drainage basins.
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DRAINAGE BASIN
A tributary joins the main river a t a confluence
A main river and all its tributaries form a river system
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DRAINAGE BASIN
The Mississippi drainage basin is one of the largest in the world. It drains over 1/3 of the USA. Watersheds in the Caribbean are pear-shaped; broad along the upstream divide and relatively narrow near the sea.
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SOURCE OF A RIVER
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SOURCE OF A RIVER
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SOURCE OF A RIVER
The source of a river or stream is the original point from which the river flows.
The source of a river or stream may be a lake, a marsh, a spring or a glacier. This is where the stream starts.
The farthest stream is called the headstream or headwater. The source is the farthest point of the river stream from its estuary or its confluence with another river or stream.
Where a river is fed by more than one source, it is usual to regard the highest as its source. The source is where a river begins, and the mouth is where it joins the sea.
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DRAINAGE DENSITY
Drainage density is defined as the total length of the streams in a drainage basin divided by the area of the basin.
Climate affects drainage density both directly and indirectly. The amount and the type of precipitation influence directly the quantity and character of runoff
When a river has few tributaries the density is low
When a river has a lot of tributaries the density is high.
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HIGH DRAINAGE DENSITY
High drainage densities occur where: The bedrock is impermeable The soils are easily saturated Precipitation is high Slopes are steep Interception by vegetation is limited
In high drainage density water moves through the system rapidly and reaches the stream quickly
The risk of flood is very high
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DRAINAGE PATTERNS
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DRAINAGE PATTERNS
Distinctive patterns are acquired by stream networks in consequence of adjustment to geologic structure.
River streams forms a distinctive pattern depending on the type of rock on which it flows.
Drainage basins are commonly viewed by scientists as being open systems
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DENDRITIC DRAINAGE PATTERN
Dendritic resembles the branches of a tree. This pattern develops on gentle slopes that
has a uniformed rock type. Streams flow into each other almost at
random. This is the most common in the Caribbean, e.g. Caroni river in northern Trinidad
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TRELLISED DRAINAGE PATTERN
Trellised drainage pattern resembles a rectangular grid.
This occurs when there arealternate bands of hard and softrock at right angles to the maindirection of slope. The main river can cut throughboth the hard rock and soft rockhowever the tributaries mainlyform in the soft rock and joins the river at right angles. E.g. Western Barbados
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RADIAL DRAINAGE PATTERN
Radial drainage pattern resembles the spokes of a wheel
The river radiates outwards in all directions from a high central point.
E.g. the volcanic islands in the eastern Caribbean
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RIVER PROCESSES
Energy is needed for transfers to occur. 95% of the rivers energy is used to overcome friction. The remaining 5 is used to erode the river channel, transport and deposit the load.
Near the source the river channels are shallow and narrow.
The bed is usually strewn and uneven with boulders
The water flows more slowly upstream than downstream.
Processes of the river: Erosion Transportation Deposition
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RIVER PROCESSES
Every river is unique but most show similar changes from source to mouth.
The river has three distinct sections: The upper course The middle course Lower course
From source to mouth the rivers; Gradient decreases Depth increases Width increases Volume increases Discharge increase
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RIVER PROCESSES
The volume is the amount of water in the river
The velocity is the speed of the water The discharge is the volume times the
velocity The discharge is the amount of water passing
a specific point at a given time The Caribbean rivers are small and does not
carry a heavy discharge
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EROSION
Rivers are usually the main causes of erosion and deposition
A river may erode in any of the following ways Attrition Corrasion Corrosion (solution) Hydraulic action
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ATTRITION
This occurs when boulders and rocks are knocked against each other and gradually reduce this stage.
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CORRASION
Corrasion occurs when smaller material gets thrown against the riverbanks and erode them.
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CORROSION
Corrosion occurs when acids in the river dissolve rocks, such as limestone.
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HYDRAULIC ACTION
Hydraulic action is when the sheer force of the river dislodges rocks from the riverbanks and bed
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EROSION
Most erosions occurs when discharge is high and rivers are in flood.
Erosion acts on the landscape in three ways: Vertical erosion Lateral erosion Headward erosion
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TRANSPORTATION
The river transport what is called its load There are four processes by which a river can
transport its load: Traction Saltation Suspension Solution
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DEPOSITION
A river deposits its load when the speed is checked due to its gentle gradient such as in a flat or gently sloping plain :
A river has three courses/sections/stages Upper (youthful stage) Middle (mature stage) Lower (old age stage)
The gradient decreases significantly Discharge falls during a dry period The current slows down on the inside of a
meander The river enters a lake or the sea.
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DEPOSITION
When a river loses energy the first part of the load that is deposited is the large, heavy material called the bedload.
The lighter material is carried farther The gravel, sand and silt deposited is called
alluvium. The lightest material is carried in suspension
which is clay
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RIVER LANDFORMS The upper course
V-shaped valley Steep gradient Interlocking spurs Potholes Waterfalls, rapids and gorges
The middle course River cliffs Slip-off slopes Confluence Flood plains and levees
Lower course Meanders Braiding Ox-bow lakes Deltas
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V-SHAPED VALLEY AND STEEP GRADIENT
Most of the energy in the upper course is used to overcome friction
There are large boulders in this section which erode the bed rapidly during flood. The river eventually cuts downward in the bed in vertical erosion This creates the V-shaped valley
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INTERLOCKING SPURS
This occurs when the river winds its way around obstacles of hard rock.
Erosion is concentrated on the outside banks. This creates spurs which alternate on each side of the river, so they interlock. A spur is a ridge or high land.
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POTHOLES
Potholes are created in areas where the river bed is uneven. Pebbles may get stuck temporarily; the swirling currents cause the stone to move in a circular motion which creates the potholes.
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WATERFALLS
A waterfall is found in the upper course but can also be found in the middle course.
It occurs when there is a sudden change in the course of the river usually caused by differences in rock hardness. Other causes may be: A fault line has created an escarpment over which
the river flows Glaciation has left a tributary valley hanging high
above the main valley A steep drop at the edge of a plateau has been
formed by uplift of the land A lava flow crosses the path of the river which
pours over its edge as a waterfall.
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RAPIDS
Rapids are created alternate bands of hard rock and soft rock cross the course of a river.
This makes the bed uneven and cause zones of turbulence.
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GORGES
A gorge is formed after a plunge pool is created and the waterfall retreats upstream. The steep sided section that remains behind is called a gorge.
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RIVER CLIFFS
A river cliff is created when a bank undercut collapse and retreats. Then erosion on the outside bend creates a very steep slope.
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SLIP OFF SLOPES
When a river cliff is created the current on the other side of the meander is slower.
When it slows it drops its load which eventually builds up to form a gently sloping slip-off.
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CONFLUENCE
A confluence of a river is a meeting of two or more bodies of water; A flowing together of two or more streams.
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FLOOD PLAINS AND LEVEES
o A flood plain is the area of almost flat land on both sides of the river.
o When the river deposits the coarse material it forms natural embankments, these are called levees.
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MEANDERS
A meander is a curve or bend in a river’s course swinging from side to side in wide loops.
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OX BOW LAKE
This is formed when the neck of a meander is cut across by the river which then takes a straight path leaving a cut-off.
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DELTAS
A delta is formed where the mouth of a river drowns and forms an estuary salt water mixes with fresh water. The river channel widens and forms a V-shaped pattern and silt accumulates at the mouth.
