Bodies of Water: Oceans, Lakes, Rivers, Glaciers - Second Grade Science
When viewed from outer space, Earth glows as a brilliant blue marble because water blankets more than 70 percent of our planet's surface. Water is the lifeblood of our world, shaping the rocky continents, regulating climate and temperature, and sustaining every living creature. Earth's water is distributed across diverse bodies of water that differ in size, whether they are salty or fresh, whether their water flows or rests still, and whether they exist as liquid or solid glacial ice.
Earth's Water: Saltwater vs. Freshwater
The vast majority of Earth's water is salty, leaving only a tiny fraction available as freshwater for drinking and agriculture.
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| EARTH'S TOTAL WATER DISTRIBUTION |
| |
| SALTWATER (97%) |
| [=======================================================] |
| Oceans and Seas (Too salty for land plants, animals, or drinking) |
| |
| FRESHWATER (3%) |
| [==] |
| - 2% Locked in Solid Glaciers and Polar Ice Sheets |
| - Less than 1% Available in Liquid Rivers, Lakes, and Aquifers! |
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Oceans: The Vast Saltwater Giants
Oceans are enormous interconnected basins of saltwater that surround our continents. Earth has one continuous world ocean divided into five major geographic names: the Pacific, Atlantic, Indian, Southern, and Arctic oceans.
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| OCEAN CHARACTERISTICS |
| |
| - IMMENSE SIZE: The Pacific Ocean alone covers more area than all |
| dry land on Earth combined! |
| - SALINITY: Contains roughly 35 grams of dissolved salts (mostly |
| sodium chloride) per liter of water. |
| - TIDES AND CURRENTS: Ocean water is constantly moved by lunar |
| gravity (tides) and planetary wind belts (currents). |
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Why is the Ocean Salty?
Over billions of years, rain has fallen onto continental rocks. Rain contains tiny amounts of dissolved acids that break down minerals. Rivers carry these trace minerals into the sea. When ocean water evaporates into clouds, only pure freshwater turns to vapor, leaving the salts behind to accumulate over time.
Rivers and Streams: Flowing Freshwater
A river is a natural flowing ribbon of freshwater that moves continuously downhill across the land, powered by gravity, toward an ocean, lake, or another river.
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| THE PATH OF A RIVER |
| |
| [ SOURCE / HEADWATERS ] High mountain snowmelt & springs |
| \ |
| \ Rushing rapids, steep canyons |
| \ |
| [ MEANDER ] S-shaped curves across flat valleys |
| \ |
| [ MOUTH & DELTA ] Empties into ocean/lake! |
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The River's Journey
Source: High in mountain peaks, melting snowpack or bubbling underground springs form tiny bubbling rivulets called headwaters. Tributaries: Small mountain streams merge together into wider, deeper rivers. Meanders: As the river flows onto flat plains, it slows down and carves wide, looping curves called meanders. Mouth and Delta: The river empties into an ocean or lake. As the current slows, it drops rich silt, creating a fan-shaped landform called a delta.
Lakes and Ponds: Standing Freshwater
Lakes and ponds are inland bodies of water surrounded entirely by land on all sides. Unlike rivers, lake water does not rush swiftly in a continuous direction.
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| LAKES VERSUS PONDS |
| |
| PONDS LAKES |
| - Small and shallow - Large and deep |
| - Sunlight reaches the bottom - Cold, dark bottom layers |
| - Water plants grow across base - Carved by glaciers/tectonics |
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How Lakes Form
Many large freshwater lakes, such as the Great Lakes of North America, were gouged out thousands of years ago by massive crawling glaciers that melted as the climate warmed. Other lakes fill dormant volcanic craters (calderas) or form when tectonic faults slip.
Glaciers: Rivers of Moving Ice
A glacier is a massive, persistent river of densely packed ice and snow that slowly creeps downhill over land under its own immense weight.
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| ANATOMY OF A GLACIER |
| |
| Zone of Accumulation (Heavy snowfall compresses to ice) |
| .------------------------. |
| / THICK BLUE ICE SHEET \ |
| / \ |
| ============' '============ Ground |
| Creeps inches per day, gouging U-shaped valleys and grinding rocks! |
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Storing Earth's Freshwater
Glaciers store nearly 69 percent of all the freshwater on our planet! Valley glaciers crawl down alpine peaks, carving steep mountainsides into broad, U-shaped valleys. Continental glaciers (ice sheets) cover vast expanses of Greenland and Antarctica up to two miles thick. When glaciers reach coastal waters, huge slabs break off with a thunderous roar—a process called calving—to become floating icebergs.
Chapter Review Exercises
Question 1: What percentage of Earth's surface is covered by water, and why is most of it unusable for human drinking?
Question 2: Explain the journey of a river from its high-mountain source down to its mouth where it empties into the sea.
Question 3: How does a standing freshwater pond differ from a deep freshwater lake?
Question 4: What is a glacier, and why are glaciers vital reservoirs for the planet's freshwater supply?
Solutions and Explanations
Solution 1: Over 70 percent of Earth's surface is covered by water. However, roughly 97 percent of that water is in salty oceans, which contains too much dissolved mineral salt for humans, land animals, or agricultural crops to drink safely.
Solution 2: A river begins at its high-mountain source from melting snow or natural underground springs. It rushes downhill as swift streams that merge into a wider river, carves loops called meanders across flat land, and finally slows at its mouth, depositing silt into a delta as it empties into a lake or ocean.
Solution 3: A pond is relatively small and shallow enough that sunlight can penetrate all the way to the muddy bottom, allowing rooted aquatic plants to grow throughout. A lake is much larger and deeper, possessing deep, cold bottom zones where sunlight cannot reach.
Solution 4: A glacier is a massive, slow-moving river of solid ice formed from compressed snow that creeps downhill over land. Glaciers are vital because they lock up roughly two-thirds of all the freshwater on Earth, releasing cold meltwater into rivers during dry summer months.