Geography Notes for Plonk Meeting

Geography Notes for Plonk Meeting Hydrologic Cycle 1. Rainfall Water falls as rain or snow if the air is cold enough. 2. Groundwater Water that seeps into the ground may be absorbed by plants or flow into rivers, lakes or seas. 3. Runoff Water flows from highlands to lowlands into rivers, lakes or seas. 4. Evaporation The Sun s heat causes water to evaporate. Water changes into water vapour. 5. Transpiration Plants give out water vapour through their leaves. 6. Condensation Warm air cools as it rises. Water vapour changes into water droplets. Small water droplets combine to form bigger droplets. Millions of water droplets gather to form clouds. 7. Rainfall (Precipitation) Water falls as rain or snow if the air is cold enough. Interception part of the precipitation which is intercepted by vegetation Surface runoff part of the precipitation which runs off the land surface Throughflow flow of water down a slope through the soil layer Infiltration movement (transfer) of water into the soil from the surface Percolation movement (transfer) of water into underlying rocks Saturation when the soil is full of moisture Water table the level at which saturation occurs in the ground or soil Groundwater water stored in rocks following percolation Groundwater flow part of stream flow which comes from groundwater Some of the water may evaporate back into atmosphere or 2) the water may penetrate the surface and become groundwater. Groundwater either seeps its way to into the oceans, rivers, and streams, or is released back into the atmosphere through transpiration. River System River a body of water flowing in a channel Begins at a place known as its source (usually located in the mountains or highlands) Source may be a lake or melting snow Due to gravity, water flows downhill River travels along a path known as its course Upper Course

- nearest to the source - narrow and shallow river channel - small amount of water transported - steep gradient/slope Middle Course - gradient/slope not as steep - wider river channel - other smaller river/streams may join the main river - smaller rivers/streams: tributaries Lower Course - gentle gradient - river channel is wider than middle course - volume of water that flows increases - more tributaries have joined the main river River ends when it flows into a lake, sea or an ocean Part of the river that enters the sea: River Mouth A river lies within a drainage basin with its network of tributaries Drainage Basin: land area drained by the main river and its tributaries Watershed: separates one drainage basin from the next, acting as a boundary which is marked by the crests of hill ridges and mountains Factors 1. River velocity 2. River volume Speed of river (Velocity) Gradient of river the steeper the gradient, the faster is the flow Roughness of the channel the rougher the channel, the slower is the flow, because the water has to overcome the friction of the river bed and banks Shape of channel Larger wetted perimeter (banks and bed in contact with water), the river has to overcome more friction and is slowed down. Volume of river The volume of river depends on:

1. Size of drainage basin The larger the drainage basin, the more tributaries there are and the greater is the volume of water 2. Presence of Vegetation Vegetation intercepts the rain and enables the rain water to infiltrate into the ground. This reduces surface run-off and so less water flows immediately into the river. 3. Permeability of rocks If the underlying rocks in the drainage basin are permeable, rain water infiltrate into the ground quickly and there will be little surface run-off flowing into the river. 4. Climate If there is low rainfall and high precipitation, the river may have a low volume of flow or even dry up. (Vice-versa) River erosion is the gradual removal of rock material from the river banks and bed. A river erodes in the following ways: Corrasion (abrasion): - The river uses its load to grind against the river bed and river bank The action would dislodge the materials and carry them away This process operates in 2 ways (vertical--> depth; lateral--> width) Potholes are the product of corrasion Attrition: The loosened materials that are being carried away collide against the river sides and bed and against one another. Over time, they would become smaller and eventually reduced to fine particles called silt. Solution (corrosion): The solvent action of water dissolves soluble materials and carries them away in solution. For example dissolving minerals found in rocks. Hydraulic action: - The breaking down of rocks Removing and dragging rocks from the bed and banks of the river by the force of the running water (like a water jet) Usually there are lines of weakness like joints and cracks in the river The work of hydraulic action forms plunge pools (small lakes)

Waterfalls: Vertical flows of fast moving water flowing from great heights Formed by 2 ways: Due to unequal resistance of rocks or faulting - Unequal resistance-less resistant rocks downstream erodes more rapidly than resistant rocks. - Differential errotion As a result, there is a change in gradient and a sudden steepering. - By faulting- displacement of rocks results in a difference in height between 2 rocks, water plunges downwards Plunge pools: Hydraulic action by the huge impact of the water rushing down against the river bed, causing rocks to loosen and be dislodged. Abrasion or corrasion occurs when rock fragments are dragged along the river bed or sides and cause the rocks to loosen and dislodge as well, erode the base. (rock debris) Gorges: Valleys which are steep sided, deep and narrow The churning water at the base of the falls eats away the soft underlying rock layers of the gorge. The unsupported hard top rock then breaks off.

Transportation A river transports its load (mud, sand, silt, boulders, and dissolved materials) in the following ways: Suspension Movement of fine eroded materials (silt and clay) floating in water. Gives the river the murky colour. Solution Movement of minerals dissolved in water. Saltation Process by which rock fragments like gravel and pebbles are lifted briefly and then dropped on the river bed. They are thus transported in a series of hops and jumps along the river bed. (bouncing) Traction Movement of large rocks such as boulders by being rolled along the river bed. River Deposition A river deposits its load of eroded materials when it is unable to transport it. This occurs when: 1. There is not enough water to transport the load during a day season. 2. A river flows across a desert where there is a high rate of evaporation Little water leads to number 1. 3. A river flows across permeable rocks which allow water to infiltrate into the underlying rocks Little water leads to number 1. 4. A river carries a larger load than it can transport Too heavy either too little water or not enough speed to carry the load. 5. There is a sudden change in gradient (e.g. river leaves the mountain and flows onto a plain) Not enough speed to carry the load. 6. A river flows into a lake or sea. 7. One side of the river is shallower than the other (e.g. the convex bank of a meander) Tendency to get stuck on the shallower side. 8. There are aquatic plants or rocks obstructing the flow Reduce speed of flow and block or choke up. Floodplains and Levees

A floodplain is a low-lying plain on both sides of a river that has repeatedly overflowed its banks and flooded the surrounding areas. When the floods subside, alluvium is deposited on the floodplain. The larger materials, being heavier, are deposited at the river banks while the finer materials are carried and deposited further away from the river. The larger materials at the river banks build up into embankment called levees. Deltas A delta is a flat piece of land built-up from layers of sediments deposited by a river where it enters a lake or calm sea. The river may have to branch into smaller distributaries to carry the water to the sea. Types of deltas - Arcuate delta ---> triangular in shape e.g. Nile Delta - Bird's foot delta ---> e.g. Mississippi Delta - Estuarine Delta Conditions that favour the formation of deltas - A large load of sediment - Shallow sea at the river mouth - Sheltered coast with weak tides and currents - Absence of large lakes along the course of the river which will siphon off the load. Meanders Meanders are loop-like bends in a river. The water flows round the meander in a spiral manner. This causes erosion to take place on the outer bank and deposition on the inner bank. Gradually, a steep river cliff is formed on the outer bank, making it concave in shape. On the inner bank, deposition of alluvial materials produces a gentle slip-off slope and the bank takes on a convex shape. Oxbow Lakes An oxbow lake is a crescent-shaped lake formed on a river when a meander has been cut through and abandoned. When a river meanders in very big loops, the outer bank is so rapidly eroded that the river cuts through the narrow neck of the meander. The river then flows straight through the channel. When deposition seals off the cut-off from the river channel, an oxbow lake

is formed. It may silt up and eventually dry up - Lateral erosion and undercutting takes place at the outer and concave bank whereas deposition takes place at the inner and convex bank. - Pronounced meander forms after time. The neck of land in between the 2 concave banks narrow. - Subsequent flooding or after time, the neck of land in between the 2 concave banks will be cut through. A new straighter river channel is created. Part of the abandoned loop called a cut-off is hence formed. - When deposition seals off the cut-off, oxbow lake is created. Uses Dams may be built across gorges to control flooding in the lower course of the river and to store water in a reservoir. The stored water can be used for irrigation and for the generation of hydro-electric power (HEP) Waterfalls can be harnessed for generation of HEP. They are also a tourist attraction. Lakes can be used for fishing and boating. Slip-off slopes, floodplains and deltas favour cultivation and grazing because of their fertile alluvial deposits. Rivers provide people with water for domestic uses, industrial use and irrigation purposes. Rivers provide a cheap mean of transport. Problems Waterfalls and rapids make navigation along the river dangerous and difficult. In the lower course, a river sometimes overflows and floods surrounding areas, destroying crops and settlements. Silting in the lower course causes problems of drainage and navigation. Energy of a river Determined by 2 factors: the Speed of Flow of the River and the Volume of the River When the speed and volume of the river is high, the energy of the river will be high.

If the speed and volume of the river is low, the energy of the river will be low. The amount of energy a river has will determine if it is able to erode, transport or deposit materials. Factors affecting speed of river (GRP basically) Gradient of Channel - Rivers flow downhill from a high altitude due to gravity - The speed at which a river does so depends very much on the steepness (gradient) of the channel - If the gradient is steep, the river will flow quickly - If the gradient is gentle, the river will flow slowly Roughness of Channel - As water flows through a river s channel, it encounters obstacles - These obstacles cause friction between the water and the channel - Friction will slow down the river flow and reduce the speed of the river - A river with a smoother channel will have a higher speed than a river with a rougher channel - A river with a rougher channel will have a slower speed than a river with a smoother channel Wetted Perimeter - River channels can be in various shapes and sizes - The shape of the river affects the wetted perimeter - Wetted perimeter: length and breadth of the channel in contact with the river s water - Large wetted perimeter, more water comes into contact with the channel - A lot of friction is generated - Friction causes the river to flow more slowly - Small wetted perimeter, less friction generated, river will flow faster - Large wetted perimeter, more friction generated, river will flow slower Nature of Load - The greater/heavier the load, the slower the speed of flow - The smaller/lighter the load, the faster the speed of flow - The rougher the load, the greater the friction, the slower the speed of flow - The smoother the load, the lesser the friction, the faster the speed of flow

Factors affecting volume of river Size of drainage basin - Large drainage area, more surface of land for rain to fall on - Rain flows over the land as surface runoff or seeps into the ground before eventually reaching the river - The bigger the drainage area, the greater the amount of water in a river - The smaller the drainage area, the smaller the amount of water in a river Presence of Vegetation - Vegetation intercepts rain and allows more water to infiltrate/seep into the ground - Abundant vegetation allows more rainwater to infiltrate the ground - Reduces the flow of surface runoff - Less surface runoff flows into the river - Volume of river is smaller - Sparse vegetation does not allow much rainwater to infiltrate the ground - Increase the flow of surface runoff - More surface runoff flows into the river - Volume of river is larger Permeability of Rocks Climate In General - Permeable rocks allow rain falling onto the area to seep into the ground - Less surface runoff - A river flowing in an area with less permeable rocks will have a larger volume of water in its channel - A river flowing in an area with more permeable rocks will have a smaller volume of water in its channel - The higher the rainfall, the greater the volume of flow - The lower the rainfall, the smaller the volume of flow - The longer the duration of the rainfall, the greater the volume of flow - The shorter the duration of the rainfall, the smaller the volume of flow - The greater the volume, the greater the discharge - The smaller the volume, the smaller the discharge

- Discharge = Velocity x Area of Cross Section River processes Landform formed by river processes Waterfalls (The Upper Course of a River) - Waterfalls are normally formed in two ways. Erosion of Rocks of Different Resistance 1. A river flows across rocks of different resistance 2. The river erodes the less resistant rock more rapidly and this causes a change in the gradient of the river course. 3. Over time, the river plunges from a great height to hit the river bed below with tremendous force. A waterfall forms as a result. Repeated pounding of the river bed may leave a depression at the base of the waterfall. This depression which is deepened as rocks and boulders swirl around is known as a plunge pool (Hydraulic action and abrasion or corrasion) Faulting 1. A river flows over land before displacement 2. A waterfall is formed after the displacement of rocks along a fault. Repeated pounding of the river bed causes a plunge pool to form. Slides 1. It can be formed when water flows from a horizontal band of resistant rock to less resistant rock. The less resistant rock that lies downstream will be eroded faster than the more resistant rock. The differential erosion causes a change in the gradient of the river s course. 2. This will finally result over time the sudden steepening of the river s course from the resistant rock to the less resistant rock so that the water plunges down almost vertically and rapidly as a waterfall. 3. Hydraulic action by the water that plunges down from a great height and abrasion by the rock debris erode the base of the waterfall and a depression, known as a plunge pool, is formed.

Gorges When a river flows through an area with resistant rocks by its sides, it can only erode its channel vertically. Over time, a deep, narrow and steep-sided valley known as a gorge is formed. Floodplains and Levees Levees are natural embankments or walls made up of coarser alluvial materials that run along a river. These are built up by deposition during floods. Floodplains are wide and low-lying areas found on both sides of the river. They are covered with alluvium deposited by the river. Heavy rain Amount of water in the channel is more than it can contain Water overflows banks and flood surrounding areas Coarser alluvium deposited first at river banks as they are heavier Levees Finer alluvium deposited further away because they are lighter Floodplain Alluvium: Deposits of silt and soil rich in minerals Meanders: A bend/loop in the river. Differential Energy Distribution: Energy is concentrated on the concave banks and dissipated on the convex banks due to the presence of pools (areas of deep water less friction, high river energy) and riffles (areas of shallow water more friction, low river energy) on the river bed. As a current in the river flows around a bend, it erodes/undercuts the concave (outside) bank through erosion and hydraulic action, and slumping occurs to form a steep river cliff. At the same time the river s speed is very much reduced on the convex (inside) bank. This causes the river to deposit its eroded sediment (transported by a corkscrew movement of water eddy) to form a gently sloping spur called a slip-off slope. In this way, the river slowly migrates sideways across the valley floor. Factors that affect flow of river Gradient Roughness

Perimeter (Wetted) [Green Ridge Primary, Geog rapes people]