Drainage basin hydrology and geomorphology

The **area of land drained by a river and its tributaries** — the catchment, bounded by the watershed.

The **boundary of a drainage basin** — the high ground separating one basin from the next.

Both water and energy **cross its boundary in and out** — rain enters; water leaves as discharge and evapotranspiration.

**Inputs** (precipitation), **stores**, **flows/transfers**, and **outputs** (evapotranspiration, river discharge).

**Interception**, **surface storage**, **soil water**, **groundwater** and **channel storage**.

**Infiltration**, **throughflow**, **overland flow**, **percolation** and **base flow**.

**Precipitation = evapotranspiration + run-off +/- change in storage.**

**Overland flow** is fastest; **throughflow** slower; **base flow** (groundwater) slowest.

Leaf surfaces have a **storage limit** — once full, no more rain can be intercepted and it passes to the ground.

Impermeable surfaces cut **infiltration** and storage; drains and bare ground raise **overland flow** — a faster response.

**Evapotranspiration** (to the air) and **river discharge** (to the sea).

Strength: it shows **interrelationships** and predicts discharge/land-use effects. Weakness: it is a **simplification** with fuzzy boundaries and patchy data.

The **volume of water passing a point per second**, measured in cumecs (m³/s).

A graph showing how a river's **discharge responds to a storm** over time (rainfall bars above, discharge curve below).

The **highest discharge** the river reaches after a storm.

The gap between **peak rainfall** and **peak discharge** — how fast the basin responds.

**Rising limb** = the steep climb as run-off reaches the river; **falling (recession) limb** = the gentler fall as it drains.

The steady background discharge from **groundwater** between storms.

A **short lag time + high peak** — impermeable, steep, urban, sparse vegetation; water reaches the river fast.

A **long lag time + lower peak** — permeable, gentle, vegetated, rural; water reaches the river slowly.

Impermeable surfaces stop infiltration and drains speed run-off → higher peak, shorter lag time.

Both **increase** — tributaries add water and the larger, smoother channel is more efficient (Bradshaw model).

Geology/permeability and relief/slope (also soil, vegetation, rainfall intensity, basin shape).

Two+ developed factors (peak + lag), an example, a weighing of their relative importance, and a clear judgement.

**Hydraulic action**, **abrasion** (corrasion), **attrition**, and **solution** (corrosion).

**Traction** (rolling), **saltation** (bouncing), **suspension** (held in flow), **solution** (dissolved).

When its **velocity falls** and it loses energy - heaviest load dropped first, finest last.

Soft rock under a hard caprock is eroded faster, drilling a **plunge pool**; the undercut lip collapses and the fall **retreats**, cutting a gorge.

The river's **load scrapes along the channel**, wearing the bed and banks wider and deeper (sandpaper effect).

V-shaped valley, gorge, interlocking spurs, rapids or potholes (any erosional feature).

Where the river meets the sea/lake its **velocity falls**, it drops its load (heaviest first) and **flocculation** clumps the clay - sediment builds up faster than waves remove it.

**Erosion** on the fast **outer bend** (river cliff); **deposition** on the slow **inner bend** (slip-off slope / point bar).

Erosion narrows a meander neck until the river breaks through at a flood; deposition then seals off the old loop as a crescent lake.

A **raised bank** of coarse sediment dropped first when a flood spills over the channel edge and slows.

Differences in **drop height/velocity**, **geology** (rock resistance), **discharge** (basin size/climate) and **load** (abrasion).

BOTH erosion AND deposition developed, a named river, a weighing of which dominates where/when, and a clear judgement.