🌊 Comprehensive Study Notes: Hydrology and Fluvial Geomorphology (9696 Core Physical Geography)

Hello Geographer! Welcome to the exciting world of water and rivers. This chapter is fundamental to understanding how water moves across the land (Hydrology) and how rivers shape the landscape (Fluvial Geomorphology). Don't worry if some terms look intimidating; we will break down the systems, processes, and landforms step-by-step. Let's dive in!

Part 1: The Drainage Basin System (The Water "Unit")

A Drainage Basin (or catchment area) is essentially the area of land drained by a river and its tributaries. It acts as an open system, meaning water and energy can move in and out.

Analogy: Think of a drainage basin like a bathtub. The input is the tap, the stored water is the stores, and the drain is the output (river discharge).

1.1 Inputs, Stores, Flows, and Outputs

The system constantly moves water through various components:

A. Inputs (Where water enters the system):

  • Precipitation: Rain, snow, hail, etc.

B. Stores (Where water is held temporarily):

  • Interception: Water caught by vegetation (leaves and branches).
  • Soil Water: Water stored within the top layer of soil.
  • Surface Water: Water held in puddles, lakes, or marshes.
  • Ground Water (Aquifers): Water stored deep underground in porous rock.
  • Channel Storage: Water contained within the river banks.

C. Flows (How water moves through the system):

Above Ground Flows:

  • Throughfall: Water dripping directly from leaves to the ground.
  • Stemflow: Water running down the trunks or stems of vegetation.
  • Overland Flow (Surface Runoff): Water flowing over the ground surface (especially on saturated or impermeable ground).
  • Channel Flow: Water moving within the river itself.

Below Ground Flows:

  • Infiltration: Water soaking vertically down into the soil (The first step below the surface).
  • Percolation: Water moving deeper through the soil and rock layers towards the water table.
  • Throughflow: Water moving horizontally through the soil layer towards the river.
  • Groundwater Flow (Baseflow): Slow movement of water through the porous rock beneath the water table. This keeps the river flowing during dry periods.

D. Underground Water Dynamics:

  • Water Table: The upper level of the saturated zone (where all rock pores are filled with water). This level fluctuates seasonally.
  • Recharge: When water (from percolation) adds to the groundwater store, causing the water table to rise.
  • Springs: Natural outlets where groundwater emerges onto the surface, often where the water table meets the ground surface.

E. Outputs (Where water leaves the system):

  • Evaporation: Liquid water turning into gas (water vapour) directly from water surfaces or the ground.
  • Transpiration: Water vapour released from plants through their leaves.
  • Evapotranspiration (ET): The combined loss of water to the atmosphere from evaporation and transpiration.
  • River Discharge: Water leaving the basin via the river channel and flowing out to the sea or another body of water.
Quick Review: The drainage basin system is a complex flow system defined by the cycling of water between various stores and pathways (flows) before finally leaving as output (discharge or ET).

Part 2: Discharge Relationships within Drainage Basins

River Discharge is the volume of water flowing past a certain point in a river channel per unit of time. It is measured in cubic metres per second (\(m^3/s\)) or cumecs.

2.1 Understanding Hydrographs

A Hydrograph shows the relationship between rainfall (precipitation) and river discharge over time, usually in response to a single storm event (Storm Hydrograph) or over a year (Annual Hydrograph).

Key Components of a Storm Hydrograph:

  • Rising Limb: The increase in discharge following a rainfall event.
  • Peak Discharge: The point of maximum river flow.
  • Peak Rainfall: The point of maximum rainfall (shown on the accompanying bar chart).
  • Lag Time: The time delay between peak rainfall and peak discharge. A shorter lag time means a higher risk of flooding.
  • Falling Limb (Recession Limb): The period of decreasing discharge as the bank storage and surface flows drain away.
  • Baseflow: The normal, steady flow supplied by groundwater.
2.2 Influences on Hydrographs (The Flood Risk Factors)

The speed and magnitude of a river's response (i.e., the shape of the hydrograph) is governed by two sets of factors:

A. Climate Factors:

  • Precipitation Type and Intensity: Heavy, intense rainfall exceeds the infiltration capacity of the soil, leading to rapid overland flow and a sharp rise in discharge. Snowmelt can also cause floods later in the year.
  • Antecedent Moisture: How wet the ground was before the storm. If the soil is already saturated (high antecedent moisture), infiltration is low, and surface runoff is high, resulting in a quicker, higher peak.
  • Temperature and Evapotranspiration (ET): High temperatures mean high ET rates, which can reduce the amount of water available for runoff or storage, thus dampening the hydrograph peak.

B. Drainage Basin Characteristics:

Characteristic Effect on Hydrograph (High Flood Risk)
Size and Shape Smaller, circular basins respond quickly (shorter lag time) than large, elongated basins.
Drainage Density High density (many tributaries) means water gets efficiently collected and discharged quickly.
Rock Type (Permeability) Impermeable rock (e.g., granite) prevents percolation, forcing water to flow rapidly over the surface.
Porosity/Permeability of Soil Impermeable or compacted soils reduce infiltration, increasing overland flow.
Slopes (Relief) Steeper slopes encourage rapid runoff (overland flow), reducing lag time.
Vegetation Type Less vegetation (e.g., bare soil after deforestation) reduces interception, increasing surface runoff.
Land Use Urbanisation (impermeable concrete/tarmac) dramatically increases surface runoff and shortens lag time.
Key Takeaway: A flashy hydrograph (short lag time, high peak) is caused by intense rainfall, saturated ground (high antecedent moisture), and basin characteristics like steep slopes, high drainage density, and urbanisation.

Part 3: River Channel Processes and Landforms

Rivers are constantly shaping their channels and valleys through three main mechanisms: erosion, transport, and deposition.

3.1 River Erosion Processes

Erosion is the wearing away of the bed (bottom) and banks (sides) of the river channel. There are four main types:

  1. Hydraulic Action: The sheer force of the water impacting the banks and bed. Water is forced into cracks, compressing the air. When the water retreats, the compressed air expands, widening the cracks.
  2. Abrasion (Corrasion): Sediment (stones, pebbles) carried by the river scrapes and grinds against the bed and banks, wearing them away (like sandpaper).
  3. Solution: The chemical action of river water dissolving soluble minerals from the rock (especially effective in limestone areas).
  4. Cavitation: A powerful form of erosion where rapid changes in water pressure cause tiny air bubbles to implode violently, often found at the base of waterfalls.
3.2 Load Transport and Deposition

The material carried by a river is called its load. The river uses energy to move this load, and the processes change depending on the size of the material and the river's velocity.

Load Transport Mechanisms:

  • Traction: Large, heavy pebbles and boulders are rolled or dragged along the river bed. (Think of a tractor dragging a log.)
  • Saltation: Smaller stones and pebbles are bounced along the bed. (Think of small, rapid leaps.)
  • Suspension: Fine, light material (silts and clays) is carried within the water flow. This gives the river a muddy appearance.
  • Solution: Dissolved chemicals and minerals are carried completely invisible within the water.

Deposition and Sedimentation: The Hjulström Curve

The Hjulström Curve is a graph that shows the relationship between river velocity (speed) and the size of sediment particles (load). It tells us whether erosion, transport, or deposition will occur.

  • If velocity is high, the river has enough energy for erosion and transport.
  • If velocity drops below the critical settling velocity for a particle size, deposition (sedimentation) occurs.
  • Fun Fact: Clay particles are surprisingly hard to erode, even though they are small, because they stick together cohesively.
3.3 River Flow and Channel Types

River Flow Patterns:

  • Laminar Flow: Very smooth, straight flow, typically occurring in small, slow-moving streams or deep, fast-moving sections with little friction.
  • Turbulent Flow: Rough, swirling flow characterised by eddies and spirals. This is the most common type and increases energy for erosion.
  • Helicoidal Flow: A corkscrew or spiral motion of water found in meanders, essential for maintaining the alternating pools and riffles and contributing to erosion/deposition on opposing banks.
  • Thalweg: The line of fastest flow down the river channel. It tends to swing from side to side in a meandering river.

Channel Types:

  • Straight: Rare in nature, usually engineered by humans (channelisation).
  • Braided: Characterised by multiple channels separated by islands or bars of deposited sediment. Occurs when discharge fluctuates greatly and the river has a large, coarse load (often in glacial outwash areas).
  • Meandering: Sinuous (winding) channel pattern. The most common type in temperate lowlands, driven by helicoidal flow.
3.4 Fluvial Landforms

Landforms are typically divided by the river stage (Upper, Middle, or Lower course), which reflects the dominant process (erosion, transport, or deposition).

Erosional Landforms (Upper Course, High Energy):

  • Waterfalls and Gorges: Formed when a river flows over a resistant rock layer (cap rock) above a softer rock layer. The softer rock is undercut by hydraulic action and abrasion, creating a plunge pool. Over time, the cap rock collapses, and the waterfall retreats upstream, leaving behind a steep-sided valley called a gorge.

Meander-Related Landforms (Middle/Lower Course, Erosion and Deposition):

  • Meanders: Bends in the river caused by the Thalweg swinging.
  • River Cliffs (Cut Banks): Formed on the outside bend where the water is fastest, leading to maximum erosion.
  • Point Bars (Slip-off Slopes): Depositional features found on the inside bend where the water is slowest, leading to sediment build-up.
  • Oxbow Lakes: Formed when a meander neck is breached during a flood, the main river flow takes the shortest, straight path, leaving the old meander loop cut off and eventually drying up.
  • Riffle and Pool Sequences: Found within the meandering channel. Pools are deep sections (outside bends) where the water flows faster during low discharge, while Riffles are shallow sections (straight parts) built of coarse sediment.

Depositional Landforms (Lower Course, Low Gradient/Energy):

  • Floodplains: Wide, flat areas of land either side of the river, formed by the deposition of fine silt (alluvium) when the river overflows its banks during a flood.
  • Levées (Natural Embankments): Raised banks along the river channel. When a river floods, the heaviest, coarsest material is immediately deposited close to the channel, building up these natural walls over many floods.
  • Bluffs: Steep, sometimes tiered banks marking the edge of the active floodplain.
  • Deltas: Large, triangular depositional areas formed where a river enters a still body of water (like a sea or lake), causing the velocity to drop rapidly and the load to be deposited.
Memory Aid: Erosion happens where velocity is Fast (outside bends, waterfalls). Deposition happens where velocity is Slow (inside bends, floodplains, deltas).

Part 4: The Human Impact on the Drainage Basin

Human activities profoundly modify the natural processes in the drainage basin, often increasing the risk of flooding and altering the natural landscape.

4.1 Modifications to Flows and Stores

Land-Use Changes:

  • Urbanisation: Replaces permeable land with impermeable concrete and tarmac. This drastically reduces infiltration and increases overland flow, resulting in higher peak discharge and shorter lag times (flash floods).
  • Deforestation: Removing trees reduces interception and evapotranspiration. Rainfall hits the ground directly, increasing soil erosion and overland flow, leading to rapid runoff.
  • Afforestation: Planting trees increases interception and ET. This is a beneficial modification that slows the rate at which water reaches the river, reducing flood risk.

Direct Water Management:

  • Abstraction: Removing water from the river or groundwater stores (often for agriculture or public supply). This reduces baseflow and discharge, potentially lowering water tables and harming ecosystems.
  • Water Storage (Reservoirs/Dams): Artificial stores that hold large volumes of water, modifying the natural seasonal flow of a river. This can control downstream flooding but starves the lower river course of sediment and water.
4.2 The Causes, Impacts, and Prediction of River Floods

Flooding occurs when river discharge exceeds the capacity of the channel (bankfull discharge).

  • Causes: High intensity rainfall, saturated ground (antecedent moisture), rapid snowmelt, and human modifications (urbanisation, deforestation).
  • Prediction: Hydrologists use models to predict flood risk. A key concept is the Recurrence Interval (or return period), which estimates the probability of a flood of a given magnitude occurring in any one year.
    • Example: A 100-year flood has a 1 in 100 (or 1%) chance of happening in any single year.
  • Impacts: Include loss of life, damage to property/infrastructure, economic disruption, destruction of habitats, but also potentially beneficial deposition of fertile silt on floodplains.
4.3 Prevention and Amelioration of River Floods (Flood Management)

Flood management strategies are categorised into 'Hard' (structural, expensive) and 'Soft' (natural, sustainable) approaches.

A. Hard Engineering (Physical Structures):

  • Dams and Reservoirs: Large structures built across rivers to store water, especially during high-flow periods, and release it slowly. (Benefit: Controls floods, generates HEP. Drawback: High cost, floods upstream valleys, blocks sediment transport.)
  • River Straightening (Channelisation): Cutting off meanders to create a shorter, straighter channel. This increases velocity and moves water out of the area faster. (Drawback: Increases flood risk downstream.)
  • Levées / Embankments: Artificially built raised banks (often reinforced with concrete) to increase channel capacity. (Drawback: Failure can cause catastrophic flooding; limits use of the floodplain.)
  • Diversion Spillways/Channels: Man-made channels that divert excess water away from key areas (like cities) into designated flood storage areas.

B. Soft Engineering (Sustainable, Natural Methods):

  • Forecasts and Warnings: Using monitoring stations and modelling to predict floods, allowing for timely evacuation and preparation (amelioration).
  • Floodplain Zoning/Drainage Basin Management: Restricting development on high-risk areas of the floodplain, often reserving them for lower-value uses (e.g., parks or agriculture).
  • Afforestation: Planting trees in the upper basin to increase interception and reduce overland flow.
  • Wetland and River Bank Conservation: Protecting natural wetlands which act as huge sponges, storing excess water. Restoring natural meanders or removing artificial embankments to reconnect the river to its floodplain.
Key Takeaway: Hard engineering is often effective immediately but has high environmental costs and transfers the problem downstream. Soft engineering is sustainable and works with nature but requires broader planning and time.