Welcome to the World of Rivers!
Welcome to your study notes for Hydrology and Fluvial Geomorphology. Don't let the long name scare you! Hydrology is simply the study of water on Earth, and Fluvial Geomorphology is the study of how rivers (fluvial) shape the land (geomorphology).
In this chapter, we will explore how a single drop of rain travels from the sky, through the ground, into a river, and eventually to the sea. We’ll also look at how rivers carve out canyons and build giant deltas. Whether you love the outdoors or just want to understand why floods happen, there is something here for you!
1.1 The Drainage Basin System
Think of a drainage basin as a giant bathtub. Any water that falls into this "tub" will eventually flow toward the same drain—the river's mouth. The boundary of this "tub" is called the watershed.
Inputs and Outputs
The system starts with an input: Precipitation (rain or snow). Once the water is in the basin, it can leave in three main ways (outputs):
1. Evaporation: Water turning into vapor from the ground or puddles.
2. Transpiration: Plants "breathing out" water vapor through their leaves.
(Geographers often combine these into one word: Evapotranspiration).
3. River Discharge: This is the water actually flowing out of the river into the sea or a lake.
Stores: Where water "stays" for a while
Water doesn't always move immediately. It can be stored in:
• Interception: Rain caught by leaves and branches before it hits the ground.
• Surface Water: Puddles, ponds, and lakes.
• Soil Water: Moisture trapped in the upper layers of the soil.
• Groundwater: Water deep underground in the gaps of rocks.
• Channel Storage: The water currently sitting in the river bed itself.
Flows: How water moves
Above Ground Flows:
• Throughfall: Water dripping off leaves to the ground.
• Stemflow: Water trickling down the trunks of trees.
• Overland Flow (Surface Runoff): Water flowing over the surface (common if the ground is too hard or saturated).
• Channel Flow: Water moving in the river itself.
Below Ground Flows:
• Infiltration: Water soaking into the top of the soil. (Think of a sponge soaking up a spill).
• Percolation: Water moving deeper down from the soil into the rocks below.
• Throughflow: Water moving sideways through the soil toward the river.
• Groundwater Flow: Very slow movement of water through deep rocks.
• Baseflow: The steady "background" flow of a river supplied by groundwater.
Did you know? Baseflow is why rivers keep flowing even when it hasn't rained for weeks!
Underground Water Concepts
• Water Table: The top "level" of the underground water. Above this, the ground is dry; below it, it is soaked.
• Recharge: When rain fills the groundwater back up.
• Springs: Where the water table meets the surface and water naturally bubbles out.
Key Takeaway: The drainage basin is an "open system" with inputs (rain), stores (puddles/soil), flows (run-off), and outputs (evaporation/discharge).
1.2 Discharge Relationships
How do we measure how a river reacts to a storm? We use a Storm Hydrograph. This is just a graph showing how much water is in the river over time during a rain event.
Parts of a Storm Hydrograph
• Rising Limb: The line going up as the river level rises.
• Peak Discharge: The highest point on the graph (the "flood peak").
• Lag Time: The time delay between the heaviest rain and the peak discharge. A short lag time usually means a higher flood risk!
• Falling (Recessional) Limb: The line going back down as the water drains away.
What makes a hydrograph "Flashy"?
A "flashy" hydrograph has a very steep rising limb and a short lag time. This happens when water reaches the river quickly.
Don't worry if this seems tricky at first! Just remember: anything that stops water from soaking into the ground makes the river rise faster.
Factors that increase flood risk (Flashy Hydrograph):
• Climate: Heavy, intense rain or melting snow. High antecedent moisture (the ground is already soaked from previous rain).
• Basin Shape: Circular basins lead to "flashy" hydrographs because all parts of the basin are roughly the same distance from the river.
• Rock Type: Impermeable rocks (like granite) don't let water in. Permeable rocks (like sandstone/limestone) act like a sponge.
• Soil: Clay soils have low porosity (small gaps), so they block water. Sandy soils have high permeability, letting water through.
• Vegetation: Trees act like umbrellas! More trees = more interception = slower flow.
• Land Use: Concrete and tarmac (urbanization) are impermeable. Water zooms straight into the drains and the river.
Quick Review: Short lag time = Dangerous flood risk. Long lag time = Safer, slower rise.
1.3 River Channel Processes and Landforms
Rivers are powerful "machines" that erode, transport, and deposit material.
Erosion: Tearing the land down
1. Abrasion (Corrasion): Rocks carried by the river scrape the bed and banks like sandpaper.
2. Solution (Corrosion): Weak acids in the water dissolve certain rocks (like limestone).
3. Hydraulic Action: The sheer force of the water trapping air in cracks, causing the rock to shatter.
4. Cavitation: Tiny air bubbles in the water implode, creating shockwaves that break rock.
Transport: Moving the load
• Traction: Large boulders rolling along the river bed.
• Saltation: Small pebbles bouncing along the bed.
• Suspension: Fine silt and clay floating in the water.
• Solution: Dissolved minerals carried invisibly.
The Hjulström Curve
This sounds scary, but it’s just a graph showing the relationship between velocity (speed) and particle size. It shows how fast a river must flow to pick up (erode), carry (transport), or drop (deposit) stones.
Interesting Fact: Very small particles like clay actually need more speed to erode because they stick together (cohesion), but once they are picked up, they stay in suspension even at very low speeds!
Patterns of Flow
• Laminar Flow: Water moving in smooth, straight sheets (rare in nature).
• Turbulent Flow: Water "tumbling" and swirling due to friction with the bed. This is what helps erode the river.
• Helicoidal Flow: A "corkscrew" motion of water. This is vital for making meanders (bends).
• Thalweg: The line of fastest flow in the river. It swings from side to side in a meander.
Landforms to Know
• Meanders: Bends in the river. Erosion happens on the outside (forming a river cliff). Deposition happens on the inside (forming a point bar).
• Oxbow Lakes: When a meander bend gets so curvy it cuts itself off, leaving a U-shaped lake.
• Waterfalls and Gorges: Formed when a river flows over hard rock followed by soft rock. The soft rock erodes, the hard rock collapses, and the waterfall retreats upstream, leaving a gorge.
• Levees: Natural raised banks made of silt deposited during a flood.
• Deltas: Found at the river mouth. When the river hits the sea, it slows down and drops all its sediment, building new land.
Key Takeaway: Rivers erode in the mountains (upper course) and deposit material near the sea (lower course).
1.4 The Human Impact
Humans change rivers constantly, often making floods worse by accident.
How we change the "system"
• Urbanisation: Building cities with concrete increases surface runoff and speeds up the lag time.
• Deforestation: Cutting down trees removes the "natural sponge" and interception, leading to more water reaching the river.
• Abstraction: Taking too much water out for farming or drinking lowers the water table.
Managing Floods
There are two main strategies for dealing with floods:
1. Hard Engineering (Expensive, man-made, usually concrete)
• Dams: Store water in reservoirs and release it slowly.
• Straightening: Cutting off meanders to make water flow away faster (but this can cause floods further downstream!).
• Artificial Levees: Building the banks higher to hold more water.
• Diversion Spillways: "Emergency" channels to take water away during a storm.
2. Soft Engineering (Working with nature, often cheaper)
• Afforestation: Planting trees to increase interception.
• Floodplain Management: Not building houses on land that is likely to flood.
• River Restoration: Putting meanders back into a straightened river to slow the water down naturally.
Case Study Tip: For your exam, you must study one specific recent flood. Make sure you know why it happened (e.g., heavy rain vs. human building), the impacts (e.g., houses destroyed, crops lost), and how people tried to fix it.
Quick Summary: Human activity like building cities increases flood risk. We can try to stop floods with "Hard" engineering (like dams) or "Soft" engineering (like planting trees).
Congratulations! You've reached the end of the Hydrology and Fluvial Geomorphology notes. Take a break, and then try drawing a storm hydrograph from memory!