The drainage basin as an open system with inputs, stores, transfers and outputs, the components of the hydrological cycle and the hydrograph, and the processes of erosion, transport and deposition that create fluvial landforms.

What this key area is asking

The SQA wants you to treat a drainage basin as an open system, describe the inputs, stores, transfers and outputs of the hydrological cycle, read and interpret a storm hydrograph, and explain how erosion, transport and deposition produce the landforms of a river from upland source to lowland mouth. Marks at Higher come from linking a named process to a named landform, not from describing features in isolation.

The drainage basin as a system

The hydrological cycle within the basin has four working parts:

• Inputs: precipitation as rain, snow, hail or sleet.
• Stores: interception by vegetation, surface storage (puddles, lakes), soil moisture storage and groundwater storage in the bedrock.
• Transfers (flows): infiltration (water into the soil), percolation (deeper into rock), throughflow (laterally through soil), overland flow or surface runoff, and baseflow (slow seepage from groundwater that keeps rivers flowing in dry spells).
• Outputs: river discharge to the sea, evaporation, and transpiration from plants (together, evapotranspiration).

Because it is a system, change in one part forces change elsewhere. If deforestation removes the interception store, more rain reaches the surface quickly as overland flow, raising discharge and flood risk. This is the water balance in action: $P = Q + E \pm \Delta S$, where precipitation ($P$) equals discharge ($Q$) plus evapotranspiration ($E$), adjusted by change in storage ($\Delta S$).

Reading and interpreting a hydrograph

Fluvial processes and landforms

Three groups of processes shape the channel:

• Erosion: hydraulic action (force of water), abrasion (the load grinding the bed and banks), attrition (load fragments knocking together and shrinking) and solution (chemical dissolving of soluble rock such as limestone).
• Transport: traction (rolling large bedload), saltation (bouncing), suspension (fine material carried in the flow) and solution (dissolved load).
• Deposition: when the river loses energy through a lower gradient, reduced discharge or shallower water, it drops its load, the largest and heaviest material first.

In the upper course the river has steep gradients and erodes vertically, producing a V-shaped valley with interlocking spurs, and waterfalls and gorges where a hard rock band overlies softer rock. High Force on the River Tees, where Whin Sill dolerite caps softer limestone, is a classic example with a drop of about $21\ m$.

In the lower course the river erodes laterally and deposits, producing meanders, ox-bow lakes where a meander neck is cut through during a flood, and a wide floodplain built up by repeated overbank flooding and deposition of alluvium, often edged by raised levees of coarse sediment.

Examples in context

Example 1. The River Tees, north-east England. The Tees is a standard SQA-style case showing the full long profile. It rises on the impermeable, steep, high-rainfall slopes of Cross Fell in the Pennines, where the gradient produces a flashy regime and V-shaped valleys. At High Force the Whin Sill dolerite cap creates a $21\ m$ waterfall with a deep plunge pool and a gorge of recession upstream. Downstream near Yarm and Stockton the gradient flattens, the river meanders broadly across a wide floodplain, and large meanders and ox-bow lakes appear, with deposition building the floodplain. The contrast along one named river lets you compare upper and lower course processes in a single answer.

Example 2. An urban basin and the 2015 Carlisle floods. When Storm Desmond dropped over $340\ mm$ of rain in 24 hours on the impermeable, already-saturated catchment of the River Eden, the urbanised, low-infiltration ground of Carlisle produced a very flashy response. Discharge peaked far above channel capacity, lag time was short, and the river overtopped its banks. The event shows how land use and antecedent soil moisture interact with rock and slope to control hydrograph shape and flood risk.

Try this

Q1. Explain how a waterfall is formed. [4 marks]

• Cue. Hard rock over soft rock; the soft rock is eroded faster by hydraulic action and abrasion, undercutting the hard rock; a plunge pool deepens; the unsupported overhang collapses; repeated retreat upstream leaves a gorge.

Q2. Explain why an urban drainage basin tends to produce a flashy hydrograph. [3 marks]

• Cue. Impermeable tarmac and concrete reduce infiltration; drains and sewers move water quickly to the river; so lag time is short and peak discharge is high.