Notes on Rivers
A drainage basin is the area of land that is drained by a river and its
tributaries.
A watershed is the area of high ground separating one drainage basin
from another.
A river is able to erode, transport and deposit material.
This is dependant on a number of factors:
1. River discharge ( volume of water)
2. River Velocity ( Speed, Gradient, discharge, shape of channel,
roughness of channel)
3. River flow ( turbulence)
Processes
Erosion
1.
2.
3.
4.
Hydraulic Action, Cavitation
Abrasion
Attrition
Solution
Where does a river erode?
1. Vertical Erosion
2. Lateral Erosion
3. Headward Erosion
Transportation
1.
2.
3.
4.
Solution
Suspension
Saltation
Traction
Deposition occurs when:
1. Velocity is reduced
2. Discharge is reduced
3. Load is increased
Upper course / Youthful Landforms/ Torrent Stage
Vertical erosion is dominant. Discharge (volume) is small, gradient is
steep.
V-Shaped valleys
A narrow, steep-sided valley
Processes = Vertical erosion, Hydraulic Action, Abrasion, Weathering,
Mass Movement
Example = River Moy, River Slaney
Interlocking Spurs
Are sections of land that jut (stick) out as the river follows a zigzag
course, flowing around areas of resistant rock.
Waterfalls
An interruption in the river profile, water makes a vertical drop. They
develop where the river meets a band of less resistant rock after flowing
over hard or more resistant rock. Forms as a result of differential
erosion. As waterfalls retreat steep sided gorges are formed. A gorge is
a deep narrow, steep sided valley. Example: River Shannon near Killaloe
Processes = Hydraulic Action. Abrasion, Differential erosion,
undercutting and retreat
Examples = Torc Waterfall in Killarney, Aasleagh Falls near Killary
Harbour
Middle course / Mature Landforms
Lateral erosion becomes dominant. Valley floor becomes wider and valley
sides are less steep as a result of continued weathering and mass
movement
Meanders
A series of gentle curving loops in the river
Processes = Lateral erosion, hydraulic action. Abrasion. Slumping,
Deposition, Corkscrew Flow
Pools ( deep) and Riffles (shallows) occur along the bed of a river, these
create currents, and water flows side to side (like a snake). This is called
corkscrew flow.
River Beach / point bar….inside bend
River cliff…………….outer bends
Flood Plain
Is the wide flat floor of the river valley which flood following periods of
heavy rainfall. End of flood plain is marked by a bluff point. Has very
gentle gradient. On an OS map no contours will be present on both banks
of the river.
Braided Streams
Occurs when a river breaks up into a series of interconnected channels.
Due to load of river exceeding the river’s ability to transport it
Processes: erosion, deposition and braiding.
Lower Course / Old Stage of River / Plain Stage
Process of erosion has practically stopped. Deposition is dominant. The
valley floor is very wide and flat. The gradient is very gentle
Oxbow Lakes / Cut off
Horse shoe shaped lakes
Processes = Lateral Erosion, hydraulic action, abrasion and deposition
Example = Lower course of River Shannon
Levees
Are ridges that run parallel to the river, built up over time by successive
flooding
Processes = Flooding, Loss of energy and deposition, aggradation
Example = Sections of River Moy
Delta
Is a flat area of alluvium when a river enters a big body of water like a
lake ( Lacustrine Delta, Glendalough) or the sea (Marine Delta, Mississippi
and Nile), enters an estuary ( Estuarine Delta, River Shannon)
River must be carrying a very large load, and the ability of the sea to
remove the load must be less then the amount of load deposited
Processes = Loss of energy, flocculation (sediment coagulates (sticks
together on contact with the seawater and settles rapidly), Sorting and
deposition.
Changes in Base Level
Isostatic Movement
Land level rises relative to the sea
How?
Uplift of landmass
Weight of Glacial Ice being removed
Eustatic movement
Sea level falls relative to the land
How?
Occurs at the beginning of an Ice Age ( water freezes)
Effect of River
Land emerges from the sea
Gradient steepens
Increase in river energy
Rate of erosion increases
This renewed capacity to erode is called REJUVENATION.
Forms a new river profile
The point at which new profile meets old profile is called a Knick point
Landforms
Paired terraces
Incised Meanders
Drainage Patterns
Dendritic
Trellised
Rectangular
Parallel
Radial
Absence
Deranged