Geography (9696) | The drainage basin system

🌊 Core Physical Geography: The Drainage Basin System 💧

Hello Geographers! Welcome to one of the most fundamental topics in physical geography: Hydrology and Fluvial Geomorphology. Don't worry if these names sound complicated—we are essentially studying how water moves across the land, stored in lakes and soil, and how rivers carve out the landscape.

Understanding the drainage basin system is like understanding the plumbing of the Earth. If you can master how water flows, is stored, and changes, you can explain floods, droughts, and almost every river landform! Let's dive in.

1.1 The Drainage Basin System (The Hydrological Cycle)

A drainage basin (also called a catchment area or river basin) is the area of land drained by a river and its tributaries. It is defined by a boundary known as the watershed (usually high ground like a ridge). It functions as an open system because it receives inputs (like rainfall) and has outputs (like river discharge) that cross the boundary.

Inputs, Outputs, Stores, and Flows

The system is built on these four components, which constantly interact.

A. Outputs (Water leaving the system)

• Evaporation: The process where liquid water turns into gas (water vapour) and rises into the atmosphere, often from open water surfaces (rivers, lakes).
• Transpiration: The process where water is lost from plants (mainly leaves) into the atmosphere.
• Evapotranspiration: The combined loss of water through evaporation from the land surface and transpiration from plants. This is the total water returning to the atmosphere.
• River Discharge: The volume of water flowing in the river channel out of the basin mouth (the final output of the liquid water system).

B. Stores (Water held temporarily within the system)

Analogy: Think of these stores like different containers holding water until they can be drained.

• Interception: Water that is caught and held on the leaves, branches, and stems of vegetation, preventing it from immediately hitting the ground. (This is a crucial store in heavily forested areas.)
• Soil Water: Water held in the soil layer, usually in the upper few metres of the ground.
• Surface Water: Water held in puddles, marshes, lakes, or on the ground surface (e.g., during a storm before infiltration).
• Ground Water: Water stored underground in the deeper rock layers (aquifers).
• Channel Storage: Water actually contained within the river and stream channels.

C. Flows (Water moving from one store to another)

1. Flows Above Ground:

• Throughfall: Water that drips off leaves and branches or falls through gaps in the vegetation canopy directly to the ground.
• Stemflow: Water running down the trunks and stems of plants to the ground.
• Overland Flow (or Surface Runoff): Water flowing across the ground surface, often caused when the rainfall intensity is greater than the infiltration capacity of the soil. (This flow is very fast and leads to quick discharge peaks.)
• Channel Flow: The movement of water within the river channels themselves.

2. Flows Below Ground:

Quick Review: Infiltration is the entry, percolation is the deep movement.

• Infiltration: The vertical movement of water from the surface down into the soil layer.
• Percolation: The continued, deeper vertical movement of water down from the soil into the bedrock or groundwater store.
• Throughflow: The lateral (sideways) movement of water through the soil, usually parallel to the surface slope. (Slower than overland flow.)
• Groundwater Flow: The deep lateral movement of water through the bedrock (aquifers). (Extremely slow, often taking years.)
• Baseflow: The contribution of groundwater to the river channel. This is what keeps the river flowing even during dry periods.

Underground Water Terminology

• Water Tables: The upper level of the permanently saturated zone within the ground. This level rises after heavy rain and falls during droughts.
• Ground Water: Water held beneath the water table in the saturated zone.
• Recharge: The process of water adding to the groundwater store, typically happening in winter or wet seasons when evapotranspiration rates are low.
• Springs: Points where the water table naturally intersects the ground surface, causing water to flow out.

1.2 Discharge Relationships within Drainage Basins

River discharge is key to understanding river behaviour. It measures the volume of water passing a specific point in a river channel per unit of time, usually measured in cumecs (cubic metres per second, m³/s).

Discharge (\(Q\)) is calculated using the simple formula: \[Q = A \times V\] Where A is the cross-sectional area (width x depth) and V is the velocity (speed of the water).

Components of Hydrographs

A hydrograph is a graph showing how the discharge of a river changes over a short period (usually in response to a single storm event) or over a longer annual period.

• Storm Hydrograph: Shows the change in discharge before, during, and after a storm.
• Annual Hydrograph: Shows discharge variations over a year, often revealing seasonal patterns.

Key Components of a Storm Hydrograph:

1. Baseflow: The normal, steady discharge of the river supplied by groundwater flow.
2. Storm Flow (or Runoff): The increase in discharge above the baseflow caused by the storm precipitation reaching the channel quickly (via overland flow or fast throughflow).
3. Rising Limb: The period when discharge is increasing rapidly.
4. Peak Discharge: The maximum discharge reached by the river.
5. Falling Limb: The period after the peak when discharge returns to normal baseflow.
6. Lag Time: The difference in time between the peak rainfall and the peak discharge. A short lag time means a higher flood risk!

Don't worry if this is tricky! Drawing a simple hydrograph diagram and labelling these parts is the best way to revise this concept.

Influences on Hydrographs (Flashy vs. Delayed Responses)

Hydrographs can be flashy (high, sharp peak, short lag time, common in urban basins) or delayed (lower peak, longer lag time, common in rural, forested basins). These differences are controlled by climate and drainage basin characteristics.

A. Climatic Influences

• Precipitation Type and Intensity: High intensity rainfall (a heavy downpour) causes a flashier response because the ground cannot infiltrate the water fast enough, resulting in increased overland flow. Snowmelt can cause large, delayed floods.
• Temperature and Evaporation/Transpiration: High temperatures increase Evapotranspiration, reducing the amount of water available for runoff and discharge.
• Antecedent Moisture: This is the wetness of the soil before the storm starts. If the ground is already saturated (high antecedent moisture), infiltration is zero, and all new rain becomes rapid overland flow, leading to a flashier hydrograph.

B. Drainage Basin Characteristics

• Size and Shape: Large basins generally have larger total discharge but longer lag times. Circular basins often have shorter lag times than long, elongated basins because water reaches the central channel simultaneously.
• Drainage Density: This is the total length of all river channels divided by the area of the basin. High density means more efficient water transfer to the main channel, resulting in a flashier hydrograph.
• Porosity and Permeability of Soils/Rock:
  • Porosity: How many spaces (pores) there are in the rock/soil.
  • Permeability: How easily water can move through those spaces.
  Impermeable rock (like granite) leads to less infiltration and more fast overland flow (flashy). Permeable rock (like chalk or sandstone) allows high infiltration, increasing baseflow and delaying the hydrograph.
• Slopes: Steep slopes encourage rapid overland flow, resulting in shorter lag times. Shallow slopes increase the time water has to infiltrate.
• Vegetation Type: Forests increase interception and evapotranspiration, reducing the total amount of water reaching the ground and slowing down the flow (delayed response). Sparse grass cover allows rapid overland flow.
• Land Use: See Section 1.4—urbanisation dramatically increases runoff speed and volume.

1.3 River Channel Processes and Landforms

Rivers are constantly changing their beds and banks through processes of erosion, transport, and deposition. These processes create the unique fluvial geomorphology (landforms) we see.

A. Channel Processes: Erosion

Erosion is the wearing away of the land surface and the bottom/sides of the channel.

• Abrasion/Corrasion: The load carried by the river (stones, pebbles) hits the bed and banks, scraping and wearing them away. (Think of it like sandpaper.)
• Solution: Chemical erosion where river water dissolves soluble minerals (like limestone) from the bed and banks.
• Cavitation: The erosion caused by the implosion of air bubbles (or vacuum bubbles) in the water. As water flows quickly over obstacles, pressure drops, creating bubbles. When these bubbles move to higher pressure areas (like the river bed), they collapse violently, exerting small, intense shockwaves that chip away at the rock. (This is very important near waterfalls.)
• Hydraulic Action: The sheer force of the water hitting the banks, pushing water into cracks, and compressing air. When the pressure releases, the crack enlarges and pieces of rock break off.

B. Load Transport

The river carries its load (eroded material) in four ways.

• Traction: The heaviest material (boulders, large rocks) is rolled or dragged along the river bed. (Slowest form of transport.)
• Saltation: Medium material (pebbles, gravel) bounces along the bed, lifted by the current and dropped again.
• Suspension: Fine material (silt, clay) is carried within the water column, making the water look muddy.
• Solution: Dissolved materials are carried chemically within the water.

C. Deposition and Sedimentation: The Hjulström Curve

Deposition occurs when the river loses energy and drops its load. Sedimentation is the process of the material settling.

The Hjulström Curve is a vital graph that relates water velocity (speed) to particle size, showing whether erosion, transport, or deposition is occurring.

• Erosion/Entrainment: Requires the highest velocity, especially to lift large particles off the bed.
• Transportation: Occurs at lower velocities than erosion, as the river needs less energy to carry material than to pick it up initially.
• Deposition: Occurs when velocity drops below the transportation critical velocity.

Did you know? Clay (very fine particles) needs a surprisingly high velocity to be picked up because cohesive forces (stickiness) bind the fine particles together. Once in suspension, however, it requires almost zero velocity to be dropped.

D. River Flow Characteristics

• Velocity: The speed of the water, measured in m/s. It generally increases downstream because friction relative to the water volume decreases as the channel gets bigger.
• Discharge: The volume of water flowing (as discussed in 1.2). Discharge always increases downstream.
• Laminar Flow: Water moves in smooth, parallel layers, without mixing. Rare in natural rivers, usually only occurring in very smooth, deep, slow channels.
• Turbulent Flow: Water moves erratically, swirling and mixing, especially common in rough, shallow channels or near obstacles. This flow is most effective for erosion and transport.
• Helicoidal Flow: The corkscrew-like motion of water that occurs in meanders (bends). It spirals from the inner bank (depositing) across the channel floor and up the outer bank (eroding).
• Thalweg: The line of fastest flow down a river channel. In a straight section, it is usually in the middle; in a meander, it swings to the outside bank.

E. Channel Types

• Straight Channel: Short, relatively rare. Even "straight" rivers usually have small curves.
• Braided Channel: A wide, shallow channel divided into multiple smaller channels separated by temporary islands or bars (often made of gravel/sand). These form in rivers with a high, coarse sediment load and fluctuating discharge (common in glacial meltwater areas).
• Meandering Channel: The most common type, characterised by S-shaped bends. Caused by the interplay of erosion on the outside bend and deposition on the inside bend due to helicoidal flow.

F. Fluvial Landforms

Landforms Associated with Meanders

• River Cliffs: Formed on the outside bend of a meander where the fast-flowing water (thalweg) erodes the bank via hydraulic action and abrasion.
• Point Bars (or Slip-off Slopes): Formed on the inside bend of a meander where slower water velocity leads to deposition of sediment.
• Oxbow Lakes: Formed when the river erodes the neck of a meander until the two outer banks meet, cutting off the old loop. The river now takes the shorter route, leaving the loop isolated.
• Riffle and Pool Sequences: Alternating shallow sections (Riffles, where velocity is high and energy is dissipated) and deep sections (Pools, where velocity is slower but water pressure is higher). These sequences develop due to the complex interaction between discharge, flow, and bed roughness.

Landforms of Vertical Erosion (Upper Course)

• Waterfalls: Form where a layer of hard, resistant rock overlies softer rock. The soft rock is eroded quickly (creating a plunge pool), undercutting the hard rock until it collapses, causing the waterfall to retreat upstream, leaving a gorge.
• Gorges: A steep-sided, narrow valley formed as a waterfall retreats upstream over thousands of years.

Landforms of Floodplains (Lower Course)

• Bluffs: Steep river banks marking the edges of the active floodplain. They are often the remnants of the valley side before the river started to meander and erode laterally (sideways).
• Floodplains: Wide, flat areas of land either side of the river in the middle and lower course. They are formed by lateral erosion of meanders and deposition during floods (when the river overspills its banks and deposits fine silt/alluvium).
• Levées (or Natural Embankments): Raised banks found on either side of the channel. When the river floods, the sudden reduction in velocity upon leaving the channel causes the coarsest, heaviest sediment to be dropped immediately next to the banks, building up a ridge over many floods.
• Deltas: Large areas of deposited sediment found at the river mouth where the river enters a standing body of water (sea or lake). Sediment is dropped because the water velocity decreases rapidly, and saline (salty) water causes the fine clay particles to flocculate (clump together) and settle out quickly.

1.4 The Human Impact on Drainage Basins

Human activities can dramatically modify how water flows and where it is stored in a drainage basin, often increasing the risk and severity of floods.

Modifications to Catchment Flows and Stores

• Deforestation: Removing trees leads to:
  • Decreased interception and evapotranspiration.
  • Decreased throughflow (roots no longer hold the soil structure).
  • Increased overland flow and reduced lag time (flashier hydrograph).
• Afforestation: Planting trees has the opposite effect: it increases interception, increases soil strength, and thus reduces overland flow and flood risk.
• Urbanisation: Replacing natural surfaces with impermeable surfaces (concrete, roads) causes major changes:
  • Infiltration capacity is reduced almost to zero.
  • Overland flow is vastly increased and accelerated (via drains and gutters).
  • Lag time is dramatically shortened, leading to rapid, high-peak floods.
• Abstraction: The removal of water (e.g., for irrigation or public supply) from surface stores (rivers) or underground stores (groundwater). High levels of abstraction can reduce the baseflow, causing rivers to dry up during droughts.
• Water Storage: Construction of reservoirs and dams increases surface water storage, regulating flow downstream, which reduces flood peaks but can interrupt natural sediment transfer.

Causes and Impacts of River Floods

Floods occur when river discharge exceeds the capacity of the channel.

• Physical Causes: Prolonged heavy rainfall, intense thunderstorms, rapid snowmelt, saturated ground (high antecedent moisture).
• Human Causes (Aggravating Factors): Urbanisation, deforestation, building on floodplains, and failed flood management infrastructure.

Prediction of Flood Risk

Geographers use statistics to understand flood frequency:

• Recurrence Intervals: The average time interval between flood events of a certain magnitude. A "100-year flood" has a 1 in 100 (\(1\%\)) chance of occurring in any given year.
• Prediction: Based on historical data, weather forecasting, and river monitoring (e.g., measuring river height/stage).

The Prevention and Amelioration of River Floods

Management attempts focus on preventing water from reaching populated areas, or reducing the energy and volume of flood peaks.

1. Forecasts and Warnings (Amelioration)

Non-structural solutions that do not physically alter the river. Modern technology provides fast, accurate weather forecasts and real-time river level data. This allows for timely evacuation and reduced loss of life.

2. Hard Engineering (Structural Solutions)

Physical construction that alters the flow of the river.

• Dams: Huge barriers built across the river to create a reservoir. They control the flow, storing large volumes of water during flood peaks and releasing it slowly later. (Impacts: Reduces sediment transfer downstream, high construction cost, huge environmental displacement.)
• Straightening: Cutting off meanders to create a straighter, shorter channel. This increases the gradient and velocity, moving the water away faster. (Impacts: Transfers the flood problem rapidly downstream.)
• Levées (or Embankments): Artificial banks built higher and stronger than natural levées to increase the channel capacity. (Impacts: Failure can be catastrophic if the water overtops or breaches the bank.)
• Diversion Spillways/Relief Channels: Artificial channels built parallel to the main river to take excess water during a flood, diverting it around built-up areas or into storage areas.

3. Soft Engineering (Sustainable Solutions)

Working with natural processes to reduce flood risk. Often cheaper and more environmentally friendly.

• Floodplain and Drainage Basin Management: Strict zoning laws prevent building on high-risk floodplains, reducing potential damage.
• Wetland and River Bank Conservation: Protecting or restoring natural wetlands (which act like huge sponges, soaking up excess water) and allowing fields upstream to flood naturally.
• River Restoration: Returning the river to a more natural state, sometimes by removing artificial embankments, allowing it to meander, or replanting trees along the banks (afforestation). This increases the river's capacity to store water naturally and increases lag time.

Case Study Requirement (Crucial for A-Level Success)

Remember, you must study a recent river flood event (post-1980) that details:
1. The causes (physical and human).
2. Impacts (on people and the environment).
3. An evaluation of attempts to reduce the impact (Hard vs. Soft engineering).
Example: The 2014 River Thames Floods (UK) or the 2005 New Orleans Flood (USA, related to coastal defenses but also heavy rainfall).