Fluvial Geomorphology: Drainage Basins & Stream Orders

Fluvial Geomorphology

• Studies how running water shapes the Earth's surface

Drainage Basin

• Also known as a river basin
• An area of land channeling water and sediment to a common outlet

Elements of a Drainage Basin

• Catchment Area: The area within the drainage basin
• Watershed: Highland edge surrounding a drainage basin, marking its boundary
• Source: River's beginning
• Confluence: Point where two rivers or streams meet
• Tributary: A smaller river joining a larger one
• Mouth: River's end point, often entering a sea

Stream Ordering

• Developed by Robert E. Horton and Arthur Strahler in 1952
• A method for classifying streams based on the number of tributaries
• Determines river size based on tributary hierarchy

How to do Stream Ordering

• A stream with no tributaries is considered a first-order stream
• Two first-order streams merging form a second-order stream
• Two second-order streams merging form a third-order stream
• Lower order streams joining a higher order stream do not change the higher stream's order

Countries Using Stream Ordering

• United States: For river network analysis and basin management, also for water resource management, flood control, and environmental protection, such as in the Mississippi River Basin Management
• United Kingdom: For classifying and analyzing river networks, assisting in river basin management and flood risk assessment, with the River Thames serving as an example
• Canada
• Australia
• Germany
• India

River Morphology

• Fluvial geomorphology
• The scientific study of river system shapes, structures, and surface modification processes

River Morphology Characteristics

• Rivers flow from higher to lower elevations, such as mountains to the sea
• Rivers start fast and narrow
• Rivers widen in the middle
• Rivers slow down towards the end
• A river course, the path from source to mouth
• Rivers divided into upper, middle, and lower courses
• Each river section has distinctive features

Cross-Sectional Shape of a River

• The view when looking at a river from the side

Longitudinal Profile

• Graphical representation of elevation changes along a river's length from source to endpoint
• Helps in understanding how a river adjusts to its environment

Understanding the Long Profile

• Illustrates a river's gradient (slope) from its source to its mouth
• Displays the channel slope along its entire length
• Changes downstream due to erosion, transportation, and deposition

Fluvial Processes

• Geological processes involving running water

Key Fluvial Processes

• Erosion
• Transportation
• Deposition

River Erosion

• Process by which a river wears away land, dependent on velocity
• Vertical (downwards) erosion: occurs in the upper stages of the stream
• Lateral (sidewards) erosion: occurs in the middle and lower stages

Lateral and Vertical Erosion

• Vertical erosion deepens the river bed, mainly through hydraulic action, in the upper course.
• Lateral erosion erodes the river banks, more common in middle and lower courses.

Processes of Erosion

• Hydraulic Action: Force of fast-flowing water hitting and entering cracks in the bedrock
• Abrasion/Corrosion: Boulders and stones wearing away river banks and bed
• Attrition: Sediment particles knocking against each other, becoming smaller and more rounded
• Solution/Corrosion: Acidic water dissolving rocks like chalk or limestone

Fluvial Transportation

• Eroded materials are carried along with the water after erosion
• Transportation is when the stream carrying eroded materials
• Eroded materials transported in four ways

Transportation Methods

• Suspension: Fine, light alluvium carried in water, called suspended load; varies with river's velocity
• Solution: Dissolved minerals are carried in the water and undergoes chemical change
• Saltation: Small pebbles and stones bounce along the river bed
• Traction: Heavier, larger rock fragments roll along the river bed

River Deposition

• Occurs when a river drops transported material due to energy loss
• Deposited material is sediment
• Lower velocity causes larger materials to deposit first
• Finer materials, like clay, deposit last
• Deposition happens when a river’s energy and velocity decrease

Stages of a River

• Rivers divided into three parts/stages: upper, middle, and lower course

Three Main Stages

• Upper Course (Steep Gradient): Vertical erosion with source and tributaries
• Middle Course (Gentle Gradient): Lateral erosion and transportation with river beaches and meanders
• Lower Course (Very Low Gradient): Deposition with floodplains, oxbow lakes and deltas

Upper Course of a River

• Found in high relief areas, where the long profile begins at the river source
• Narrow, with less water, and a fast flow rate
• Vertical erosion is dominant
• Source is where the river starts
• Tributaries are streams originating from different locations

Upper Course Characteristics

• Channel flows downwards and follows steep gradient
• The steep channel provides high potential energy
• Rapids and waterfalls through steep V-shaped valleys

Interlocking Spurs

• Fingers of land jutting into the river valley

Rapids

• Fast-flowing, turbulent sections in the upper course of the river

Waterfalls

• Form where water flows down steep hillsides in upland areas
• Often found in the upper valley
• Height and number affected by rock types eroded

Gorge

• A gorge forms as a waterfall retreats up a valley
• Occurs when river flows over hard rock overlying softer rock
• Undercutting of the softer rock leads to overhang of the hard rock, which eventually collapses
• Process repeats, causing retreat and leaving a steep, narrow gorge

Upper Course Processes

• Erosion is mainly vertical, driven by hydraulic action
• Transportation primarily involves large boulders moved by traction
• Deposition consists of large boulders

Middle Course

• The matured stage of a river, where the valley widens
• Wider river with more water, flowing less quickly
• Lateral erosion widens the valley
• The river's water volume increases and the slope is moderate
• The depth of the river is deep

Lower Course

• The final river stage, broad valleys, and gentle slopes
• Wider river with more water and flow rate is diminished
• The valley is nearly flat, forming flood plains
• Deposition is the main river activity
• River depth is shallow
• Main river splits into distributaries

Landforms by the Erosional Work of Rivers

• V-Shaped Valleys, Gorge and Canyon
• Waterfalls
• Meanders, Oxbow Lakes
• Alluvial Fans and Deltas

V-Shaped Valleys

• Narrow valleys carved by fluvial erosion in upland areas
• The water erodes the valley sides

Gorges and Canyons

• Form as fluvial erosion deepens and widens valleys
• Creates steep-sided landforms

Canyons

• The Grand Canyon on the Colorado River, Arizona, is the largest canyon in the world
• It is 482.8 kilometers long and 2088.3 meter in depth

Waterfalls

• Form where a river meets resistant rock or gradient change
• Water erodes softer rock, creating a waterfall
• Niagara Falls and Victoria Falls are cited as examples

Meander

• Bends or loops in a river's course, formed by erosion and deposition
• A winding curve or bend

Oxbow Lake

• Formed when a meandering river cuts off a meander bend
• Erosive process cuts through the meander neck and creates a separate water body

Landforms by the Deposition in Rivers

• River Deltas
• Floodplains
• Levees
• Alluvial Fan

River Delta

• Large, fan-shaped landforms at river mouths
• Sediment is deposited as river slows down, forming distributaries
• Highly fertile, with rich agricultural activity

Levees

• Raised river banks due to flooding and sediment deposition
• A levee is a natural or artificial wall blocking water

Human Impact

• Activities like building, farming, and industry significantly impact river landscapes, and rivers

Effects of Flooding on People

• Injuries or fatalities
• Contaminated floodwater causing illness
• Disrupted power supplies
• Businesses forced to close
• Services, like hospitals and schools, shut down
• Affected transport networks and damaged infrastructure
• Flooded homes and properties
• Displacement of people
• Damaged or washed-away belongings

Environmental Impact of Flooding

• Destruction of wildlife habitats
• Contamination of rivers and habitats
• Silt and sediment destruction of crops
• Widened rivers, increased downstream deposition
• Trees being uprooted by high-velocity water flow