Long Profile/Channel Characteristics

Question 1

what does the long profile show

Answer 1

how the gradient of the river channel changes form the rivers source to its mouth by showing the height of the river bed above the base level for the whole length of the river

Question 2

what is the base level

Answer 2

the lowest point that the river can erode to - usually sea level

Question 3

when, generally, to landforms such as waterfalls occur - and how does this affect the profile of the river

Answer 3

when the rates of erosion and deposition change along the course of a river
It makes the profile uneven

Question 4

what is balanced along the course of a river

Answer 4

total amount of erosion and deposition

Question 5

what is a graded profile, and when does it occur

Answer 5

when over time the long profile goes from being uneven to being a curve.
occurs as the total amount of deposition and erosion is balanced, and the rate of erosion of landforms like waterfalls is equal to the rate of deposition elsewhere along the river

Question 6

describe the upper stage of the river

Answer 6

• steep gradient
• river is high above sea level
• this gives it a lot of potential energy

Question 7

describe the middle stage of the river

Answer 7

• as gradient decreases, potential energy is converted to kinetic energy - the river gains velocity

Question 8

describe the lower stage of the river

Answer 8

little potential energy
lots of kinetic energy
highest velocity

Question 9

what increases downstream as you move from source to mouth

Answer 9

velocity

discharge

Question 10

why does discharge increase downstream

Answer 10

tributaries (smaller streams and rivers) and more surface runoff join the main channel

Question 11

what is river velocity influenced by

Answer 11

gradient, discharge and channel characteristics (shape and roughness)

Question 12

what will an efficient river have

Answer 12

high velocity, high discharge and little friction

Question 13

how is efficiency measured

Answer 13

hydraulic radius

- larger HR means more efficient

Question 14

what is the hydraulic radius

Answer 14

channels cross section area divided by length of its wetted perimeter

• contact between water and wetted perimeter creates frictions,which increases energy loss and slows the river down
• larger hydraulic radius means smaller proportion of water is in contact with the wetted perimeter. So friction is lower, which reduces energy loss,increasing velocity and discharge
• smooth, narrow, deep channels have larger HR

Question 15

how does channel roughness affect efficiency

Answer 15

large angular boulders/protruding banks increase wetted perimeter and cause more frictions. reduces E,V,D

Question 16

what happens when channel roughness increases

Answer 16

so does turbulence

causes greater erosion

Question 17

what happens when channel roughness increases

Answer 17

so does turbulence

causes greater erosion as turbulent flow is more effective at picking up particles from the river bed

Question 18

explain channel roughness in the upper/lower stages

Answer 18

• greatest in the upper stages. altho gradient is steep, the river loses a lot of energy to friction, so discharge and velocity are lowest here during normal conditions
• in the lower stages, the banks and bed of the river are smooth, so theres much less friction. this means less energy is lost, so discharge and velocity are the highest in this stage

Question 19

what is erosion like in the upper stage

Answer 19

• mainly vertical and by abrasion (some hydraulic action)
• erosion occurs when there are high energy conditions (i.e when velocity and discharge are high after heavy rain/ice melt
• rough channel causes turbulence and the large angular bedload is dragged along the river bed causing intense downward erosion

Question 20

what is transportation like in the upper stage

Answer 20

• mainly large particles such as boulders carried by traction or saltation during high energy conditions

Question 21

what is deposition like in the upper stage

Answer 21

• little deposition; mainly largest particles deposited in the river bed as energy levels drop

Question 22

what is erosion like in the middle stage

Answer 22

• mainly lateral and by abrasion

- attrition of larger particles in this stage means that sediment particle size decreases from source to mouth

Question 23

what is transportation like in the middle stage

Answer 23

• more material carried in suspension as particle size decreases
• some larger particles moved by saltation

Question 24

what is deposition like in the middle stage

Answer 24

• sand and gravel are deposited across the flood plain as the river floods and friction reduces the rivers energy

Question 25

what is erosion like in the lower stage

Answer 25

although velocity and discharge are highest, theres less erosion as turbulence is lower and sediment particle size is reduced
- some lateral erosion occurs during the formation of meanders

Question 26

what is transportation like in the lower stage

Answer 26

• mainly smaller particles such as silt and clay carried by suspension or substances carried in solution

Question 27

what is deposition like in the lower stage

Answer 27

• smaller particles such as sand silt and clay are deposited on the flood plain when the river floods and in the river mouth as the sea absorbs river energy

Question 28

whats the cross profile like at upper/middle/lower stage

Answer 28

upper: steep v shapes, vertical erosion crates narrow valley floors and steeply sloped sides
middle: wider, caused by later erosion, deposition creates floodplain on the valley floor
lower: wide with gently sloping sides, much wider flood plain caused by deposition