Rivers

Steep, fast and fiercely erosive

• Steep gradient and fast, turbulent flow
• Narrow V-shaped valleys and deep gorges
• Rapids, waterfalls and plunge pools
• Vertical (downward) erosion dominates

In its youth a river has abundant energy and little water, so it cuts downward into bedrock rather than sideways. Interlocking spurs, potholes and cascading waterfalls are signatures of this high-energy zone.

Widening, winding and gathering strength

• Gentler gradient, larger volume of water
• Meanders begin to swing across the valley
• Lateral (sideways) erosion widens the channel
• River cliffs and slip-off slopes form

As tributaries add water, the river balances erosion and deposition. It begins to snake in graceful meanders, cutting the outer bank while depositing sand on the inner bank, steadily broadening its floodplain.

Broad, slow and richly depositional

• Very gentle gradient, wide slow channel
• Extensive flat floodplains and levees
• Large looping meanders and oxbow lakes
• Deposition dominates — deltas and estuaries

With maximum water but minimal slope, the tired river drops its load, building fertile floodplains and fanning into deltas where it meets the sea. These plains are among Earth’s most productive farmland.

Flowing all year round

• Continuous flow throughout the year
• Fed by reliable rainfall or snowmelt
• Support permanent ecosystems and cities
• Groundwater keeps them flowing in dry spells

Perennial rivers never run dry because their catchments store enough water — in aquifers, snowfields or glaciers — to sustain flow even through droughts. Most of the world’s great rivers are perennial.

Rivers that come and go

• Dry beds (wadis) for much of the year
• Sudden, powerful flash floods when it rains
• Common in arid and monsoon climates
• Braided channels of sand and gravel

In deserts and monsoon lands, rivers may flow for only weeks at a time. Their dry channels — wadis or arroyos — can transform into raging torrents within minutes of a distant downpour.