Introduction to Water Bodies

Water covers approximately 71% of the Earth's surface, with the global ocean holding about 97% of all Earth's water. Understanding water bodies is fundamental in physical geography. This study guide covers the characteristics of oceans, seas, ocean currents, lakes, and the dynamic actions of running water (rivers) as outlined in the syllabus.

1. Oceans and Seas

World Distribution of Oceans

There are five major oceans in order of size:

• The Pacific Ocean: The largest and deepest ocean, home to the Mariana Trench.
• The Atlantic Ocean: The second-largest, S-shaped ocean, bisected by the Mid-Atlantic Ridge.
• The Indian Ocean: Located south of Asia, bounded by Africa and Australia.
• The Southern Ocean: Enclosing the Antarctic continent.
• The Arctic Ocean: The smallest and shallowest ocean, located around the North Pole.

Seas are smaller divisions of oceans, partially enclosed by land (e.g., the Mediterranean Sea, Caribbean Sea, and South China Sea).

Salinity of Ocean Water

Salinity refers to the degree of saltiness or concentration of dissolved salts in water, measured in parts per thousand (‰). The average salinity of ocean water is 35‰ (or 35 grams of salt per 1000 grams of water).

• Factors increasing salinity: High rates of evaporation and freezing of polar ice (which leaves salt behind).
• Factors decreasing salinity: Large inflows of freshwater from major rivers, heavy rainfall (precipitation), and melting glaciers.

Uses of Oceans and Seas

• Transportation and global trade routes.
• Source of marine foods (fish, shellfish) and mineral resources (offshore petroleum and gas).
• Climate regulation through heat absorption and distribution.
• Tourism and recreational opportunities.

2. Ocean Currents

An ocean current is a continuous, directed movement of seawater generated by forces acting upon this mean flow, such as breaking waves, wind, the Coriolis effect, temperature, and salinity differences.

Types of Ocean Currents

• Warm Currents: Flow from equatorial regions toward polar regions. They bring warm temperatures to higher latitudes (e.g., the Gulf Stream, Kuroshio Current, and Brazilian Current).
• Cold Currents: Flow from polar regions toward the equator. They bring cold water to lower latitudes (e.g., the Benguela Current, Canary Current, Labrador Current, and Humboldt/Peru Current).

Causes of Ocean Currents

• Prevailing Winds: The planetary winds (such as trade winds and westerlies) drag the surface water along.
• Coriolis Force: The rotation of the Earth deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Temperature Differences: Cold water is denser and sinks, while warm water is lighter and rises, driving deep ocean thermohaline circulation.
• Salinity Differences: High-salinity water is denser and sinks below fresher water.
• Configuration of Coastlines: Landmasses deflect and redirect ocean currents.

Effects of Ocean Currents

• Climate Influence: Warm currents warm adjacent coastal areas and increase precipitation (e.g., the North Atlantic Drift warms Western Europe). Cold currents cool coastal areas and cause fog and arid conditions (deserts) onshore (e.g., the Namib Desert is influenced by the Benguela Current).
• Fishing Grounds: Where warm and cold currents meet, plankton thrive, creating rich fishing grounds (e.g., the meeting of the Gulf Stream and Labrador Current near Newfoundland).
• Navigation: Sailing with a current speeds up travel, while sailing against it slows progress. Cold currents also carry dangerous icebergs into shipping lanes.

3. Lakes

A lake is a large body of water surrounded by land. Lakes can be freshwater or saline.

Types of Lakes

• Tectonic/Rift Valley Lakes: Formed by tectonic movements resulting in crustal fracturing. They are typically long, narrow, and very deep (e.g., Lake Tanganyika, Lake Nyasa, and Lake Baikal).
• Volcanic Lakes: Formed in craters of dormant volcanoes or calderas (e.g., Lake Bosumtwi in Ghana, Crater Lake in Oregon).
• Glacial Lakes: Formed by glacial erosion or deposition (e.g., the Great Lakes of North America).
• Depositional Lakes: Formed by barrier deposition, such as lagoons or oxbow lakes (e.g., Lake Chad, which is also a remnant of an ancient inland sea).
• Man-made/Artificial Lakes: Created by damming rivers for hydroelectric power (e.g., Lake Kainji in Nigeria, Lake Volta in Ghana).

Uses of Lakes

• Domestic and industrial water supply.
• Irrigation for agricultural development.
• Generation of Hydro-Electric Power (HEP).
• Transportation, fishing, and recreation.

4. Rivers: Action of Running Water

Running water is the most powerful agent of denudation (erosion and landscape modification) on land.

Processes of River Erosion

• Corrasion (Abrasion): The mechanical scraping of the river bed and banks by rock fragments carried by the water.
• Attrition: The wearing down of the load particles themselves as they collide with one another.
• Solution (Corrosion): The chemical dissolution of soluble rocks (like limestone) by water.
• Hydraulic Action: The physical force of water dislodging and sweeping away loose materials.

Processes of River Transportation

• Traction: Heavy boulders rolling along the river bed.
• Saltation: Sand and pebbles bouncing along the bed.
• Suspension: Fine silt and clay carried within the body of the water.
• Solution: Dissolved minerals transported in the water.

Stages of a River Course and Associated Landforms

A river profile is divided into three main stages:

1. Upper (Youthful) Stage: Characterized by steep gradients, vertical erosion, and high energy. Key landforms include V-shaped valleys, gorges, rapids, waterfalls, and potholes.
2. Middle (Mature) Stage: Characterized by lateral erosion and a gentler gradient. Key landforms include U-shaped/wider valleys, meanders, and river cliffs.
3. Lower (Senile) Stage: Characterized by low energy, minimal erosion, and extensive deposition. Key landforms include oxbow lakes, levees, floodplains, and deltas.