Comprehensive Study Notes on Drainage Basins and Fluvial Processes
Core Principles of the Drainage Basin and Weathering Processes
A drainage basin refers to the entire area of land that is drained of water into a single exit point or river mouth. Within this area, weathering processes work to wear down the surface on either side of the river channel over time. These processes break exposed rocks into smaller fragments. In the United Kingdom, there are three primary types of weathering. Freeze-thaw weathering occurs when water flows into the pores, cracks, or joints of a rock. During the night, when temperatures drop, the water freezes and expands, which causes the crack to widen. Over time, the repeated cycle of freezing and thawing causes the rock to break up entirely. Biological weathering involves the contribution of burrowing animals to the breakdown of rocks or the action of plant roots reaching into cracks and joints, forcing them to widen and eventually break apart. Chemical weathering occurs when dissolved carbon dioxide in rainwater creates a weak carbonic acid. This acid then dissolves specific rock types, such as limestone and chalk.
Fluvial Erosion, Transportation, and Deposition Processes
Erosion processes wear away the river channel over time through several distinct mechanisms. Hydraulic action is the force of the water itself forcing air into gaps in the rock or soil; this air is compressed, weakening the surrounding material. Abrasion, also known as corrasion, happens when the river carries a load of sand, gravel, or pebbles that wear away the river bank and bed in a manner similar to sandpaper. Corrosion, or solution, is a chemical process where weak acids dissolve the calcium carbonate in rocks such as limestone and chalk, effectively dissolving the channel boundaries. Attrition is the process where large, angular stones are converted into round, smooth ones because they constantly bump against each other while being carried by the flow. Transportation processes move the river's load downstream. Solution involves soluble minerals, such as calcium carbonate, being carried in an invisible, dissolved state. Suspension occurs when very fine particles like sand and silt are carried by the flow, requiring very little energy. Saltation involves small stones and pebbles jumping and bouncing along the river bed, which requires a fairly fast flow. Traction requires the highest amount of energy, as it involves large boulders and rocks being rolled along the river bed. Deposition occurs when a river loses its energy. The river deposits its load in order of weight; the heaviest load is deposited first, while the sand and silt are deposited last.
Drainage Basin Components and Terminology
A drainage basin is defined by several key geographical features and terminology. The watershed acts as a ridge of high land that serves as a boundary separating two drainage basins. The source is the starting point of the stream or river, while the mouth is the final place where the river flows into a larger body of water, such as a sea or lake. A confluence is the specific point where two rivers join. A tributary is a smaller stream or river that flows into a larger main river. The channel is the specific area where the water flow is contained from the source to the mouth. A floodplain is the flat area of land adjacent to a river channel, typically in the lower course, which becomes covered in water during flood events.
The Drainage Basin as an Open System
The drainage basin is considered an open system because it functions as part of the global hydrological cycle with inputs and outputs occurring beyond the basin's physical boundaries. Inputs consist of precipitation entering the basin, either as a liquid (rain) or a solid (snow, sleet, and hail). The amount and form of this precipitation change seasonally. Transfers are the movements of water from one store to another in liquid form, such as water moving from the soil into the rock. Stores are areas where water accumulates in liquid form, such as lakes or within the soil. Outputs refer to water leaving the system, either into the ocean or into the atmosphere via evaporation (a gas transfer). Transfers, or flows, within the system include overland flow (surface runoff when soil stores are full), infiltration (downward movement from the surface into the soil), throughflow (lateral movement through the soil into the channel), percolation (slow downward movement through rock pores or cracks), and groundwater flow (slow transfer through permeable rock). Evapotranspiration is the transfer of water vapour from plants into the atmosphere. Specific stores include surface stores (lakes, ponds, or snow in winter), soil moisture stores (water held in pore spaces until the soil is saturated), groundwater stores (water held deep underground in permeable rock with its surface referred to as the water table), and vegetation stores (where water is intercepted by leaves or absorbed by roots).
The Global Hydrological Cycle as a Closed System
The global hydrological cycle, or water cycle, is the constant circulation of water around the world. Unlike the drainage basin, it is a closed system because there is no input or output beyond the Earth's atmosphere. This cycle is driven by the sun's energy, which evaporates liquid water into water vapour. As this vapour rises, it cools and condenses back into liquid, returning to the surface as precipitation. Key stores in the global cycle include surface stores (oceans, glaciers, ice sheets), vegetation stores, terrestrial stores (soil and rock pore spaces), and the atmospheric store (water vapour). The cryosphere is a specific store of water in solid form, such as ice, which must change state to liquid before it can flow. Flows move water between these reservoirs. For example, water moves from surface stores to the atmosphere via evaporation, or from plants via evapotranspiration (which is fastest in summer). Groundwater flows can take centuries to reach rivers or oceans, while atmospheric precipitation can be a very rapid flow.
River Profiles and Course Characteristics
The changing characteristics of a river are analyzed through its long profile and cross profile. The long profile is the change in the river's gradient from the source to the mouth, also known as the river hydroprofile. The cross profile is the cross-section representing the change in width and depth across the channel. In the Upper Course (near the source), the gradient is steep, the channel is narrow and rough, and the load consists of large boulders; erosion is primarily vertical, creating V-shaped valleys, waterfalls, and gorges. Friction is high and discharge is low. In the Middle Course, the gradient is gradual and the channel becomes wider. The load consists of stones and pebbles, and erosion is both vertical and horizontal, forming meanders and oxbow lakes. In the Lower Course (near the mouth), the gradient is flat and the cross profile is very wide and uniform. The load consists of sand and silt, and the process is mainly deposition, creating meanders, oxbow lakes, levees, and floodplains. Discharge increases downstream as tributaries join, and velocity increases as friction decreases. Geology, or rock resistance, affects the rate of erosion throughout these profiles.
Evolution of Landforms: From Waterfalls to Meanders
Waterfalls are formed due to differential erosion caused by changes in geology. As water flows from hard rock to soft rock, a step is formed. The water plunges over the step, creating a plunge pool in the soft rock. Hydraulic activity and abrasion deepen the plunge pool, which undercuts the hard rock above. Eventually, the overhang collapses, and the waterfall retreats upstream, leaving a gorge. In the middle and lower courses, increased energy leads to horizontal erosion and the formation of meanders (bends in the river). Water erodes the outer bank to create a river cliff and flows slower on the inner bank, depositing alluvium to form a slip-off slope. As the meander neck narrows and eventually cuts through, an oxbow lake is formed. Floodplains are created as meanders move horizontally across the valley floor. During floods, alluvium is deposited; the heaviest load is deposited first right next to the channel, creating natural embankments called levees. These levees grow with each flood, providing a natural defense and creating fertile agricultural land.
Human Interaction and River Management Strategies
People utilize rivers and their landforms in various ways: floodplains provide flat, fertile land for farming and cheap land for construction; river flows are used for hydro-electric power by diverting water or building dams; waterfalls attract tourism; and rivers/lakes are used for fishing, sailing, and water sports. However, flooding poses social (impact on society), economic (financial clean-up costs), and environmental impacts. Managing these impacts involves sustainability—supporting current needs without compromising future ones—through hard or soft engineering. Hard engineering includes constructing dams (effective and multi-purpose but cost millions), channel straightening (local advantage but causes downstream issues), embankments (; changes habitat), river dredging (; only short-term effective), and flood walls (). Soft engineering includes afforestation (slow to grow but creates habitats), peatland conservation (stores carbon and delays flood peaks), creating flood zones on low-value land (requires farmer compensation), and building wooden dams or ponds (cheap and may occur naturally via wildlife like otters).