5.1 Rivers
The area from which a river and its tributaries collect water is its drainage basin; basins are separated by watersheds (divides). The volume of water passing a point each second is the discharge, measured in m³/s. Its seasonal pattern is the river regime: rivers in eastern Anatolia fed by snowmelt peak in late spring, while Mediterranean rivers shrink greatly in the summer drought.
In its upper course, where the gradient is steep, a river erodes, carving V-shaped valleys and canyons. In its lower course, where the gradient is gentle, it deposits, forming meanders and floodplains. Where it meets the sea, a delta forms if currents and tides can't carry away the sediment: the Bafra delta of the Kızılırmak, the Çarşamba delta of the Yeşilırmak and the Çukurova delta of the Seyhan and Ceyhan are examples. Where the coast is deep or tidal currents are strong, an estuary may form instead.
FormulaDischarge (m³/s) = cross-sectional area (m²) × velocity (m/s)
5.2 Soils
Soil forms from mineral fragments produced by the physical and chemical weathering of bedrock, mixed with decayed remains of living things (humus), water and air. Climate, parent rock, relief, organisms and time all shape it; forming 1 cm of soil can take hundreds of years. A mature soil shows horizons from top to bottom: A (humus-rich topsoil), B (where washed-down materials collect) and C (weathered parent rock).
Soils formed over large areas mainly by climate are zonal soils: in the humid tropics, heavy leaching gives red laterite rich in iron and aluminium; in the taiga, acidic, leached podzols; on semi-arid steppes, humus-rich, highly fertile chernozems and brown steppe soils. Where parent rock dominates, intrazonal soils form (terra rossa on Mediterranean limestone); where soil hasn't had time to mature, azonal soils (alluvium) are found.
5.3 Biomes
A biome is a large ecosystem of similar plant and animal communities developing under a similar climate. Tropical rainforests lie near the Equator, hot and wet all year; they hold a very large share of the world's species, yet their soils are surprisingly poor because most nutrients are locked up in plants. Savannas occupy tropical regions with a distinct dry season, and deserts lie near the tropics and deep inside continents.
Mid-latitudes have Mediterranean maquis, deciduous forests and steppes; high latitudes have coniferous taiga and tundra with its frozen ground (permafrost). Altitude acts like latitude: climbing from a mountain's foot to its summit, biomes change in order as if travelling from the Equator to the pole. The change from dense forest to alpine meadows in the Kaçkar Mountains is a fine example.
Key terms
- Discharge
- The volume of river water passing a point each second.
- River regime
- How a river's discharge changes through the year.
- Zonal soil
- A soil shaped mainly by climate.
- Biome
- A large community of plants and animals shaped by a similar climate.
End-of-unit problems
Solve each one yourself first, then open the solution.
At a gauging point a river is 25 m wide and 1.6 m deep on average, flowing at 1.5 m/s. What is its discharge? How many m³ of water does it carry in a day?
Show the solution
Cross-section = 25 × 1.6 = 40 m². Discharge = 40 × 1.5 = 60 m³/s. A day is 86,400 s: 60 × 86,400 = 5,184,000 m³ (about 5.2 million m³).
Tropical rainforests are among Earth's most productive ecosystems, yet when cleared for farmland the soil becomes infertile within a few years. Explain why.
Show the solution
In a hot, wet climate, weathering and leaching are intense, so nutrients wash out of the soil quickly (laterite). In the forest, nutrients cycle mainly through living plants and fast-decaying leaf litter. Clearing the forest breaks this cycle, rain washes away what's left, and the soil soon becomes poor.
Compare the months of highest and lowest discharge for a snowmelt-fed river and a rain-fed river in a Mediterranean climate. For which is a dam more critical for irrigation?
Show the solution
Snow-fed river: highest in late spring to early summer (snowmelt), lowest in winter (water is stored as snow). Mediterranean river: highest in winter (the wet season), lowest in summer. Since irrigation demand peaks in summer, exactly when the Mediterranean river shrinks, a dam storing winter water is far more critical there.
Why are chernozem soils found in the world's "breadbasket" regions (Ukraine, Kazakhstan, the North American prairies)?
Show the solution
Semi-arid steppes get little rain, so nutrients aren't washed away. Dense grasses that die back each year add plenty of humus, and cold winters slow decay so it builds up. The result is a deep, dark, very fertile soil — ideal for cereals such as wheat.