Coasts, coral reefs and rivers
In this chapter:
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Coasts are where the ocean and the land meet
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Coral reefs are usually located just off the coastline in tropical areas
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The Great Barrier Reef is the largest coral reef system in the world
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River water comes from underground springs and rainfall
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Rivers create landforms such as valleys, estuaries and deltas
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What happens near the source of a river can have effects on areas downstream
Coasts
Coasts are where the ocean and the land meet. They are made up of various landforms including beaches, bays, cliffs, headlands, estuaries and deltas. (Refer Image 1) These landforms are created by erosion. Waves, which form due to strong winds over the open sea, crash against rocks that make up the shoreline. As the waves crash, they pick up sand and rocks, which rub against the shoreline and break it up further. This process is called abrasion.
In some places, the rock is weak and is crushed to small pieces. In other areas, the rock is stronger and able to resist erosion. These strong areas of rock become headlands, which have steep, rocky cliffs. In between headlands, there are bays, which have sloping beaches. Generally, sandy beaches are flatter while pebbly beaches are steeper. Pebbly beaches are usually found in areas that do not have much shelter from headlands because waves bring in pebbles and carry away the sand.
Tides also affect the coastline. Tides are the result of the gravitational pull of the moon and sun on the water in the ocean. Tides change the depth of the water, areas that are under water during high tide are exposed during low tide. Coasts are also home to many animals, such as sea birds, crustaceans, fish and sand worms. Many of these animals are adapted to live both in the ocean and on dry land. Short grasses that have adapted to high levels of salt in the soil also grow along the coast.
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Coral reefs
Coral reefs are usually located just off the coastline in tropical areas. They are completely under water and form in tropical areas with shallow waters that sunlight can penetrate. Coral reefs consist of many plants and animals. There are so many that they often have to compete for space. Even coral is a by-product of biotic processes, or processes that happen due to living organisms. Coral is made by organisms called polyps. As polyps die, they leave a limestone sediment behind which becomes the coral we see in our reefs. This process proceeds very slowly, so a piece of coral existing today could have been in the process of being made for hundreds of years.
Coral reefs are also home to many other sea plants and animals including fish, crustaceans, sea anemones and sponges and they are an important habitat for fish and shellfish breeding. Together they all create a vibrant ecosystem. (Refer Image 2). The relationships between animals in coral reefs are so complex that scientists still do not fully understand them.
An example of a coral reef is the Great Barrier Reef, which exists off the coast of north-eastern Australia. It is the largest coral reef system in the world and consists of 2900 distinct reefs and 940 islands. Scientists believe it may hold up to 400 species of coral, 4000 species of molluscs and 1500 species of fish. While coral reefs are located mostly in tropical areas, other types of reefs exist in more temperate areas.
Rivers
River water comes from underground springs and rainfall. Rain that falls in the area surrounding a river runs over or through the soil (in the form of groundwater and springs) to the river. This area is called a river's drainage basin. Springs can create small streams, called tributaries, that join to make a larger river. This water is then supplemented by rainwater. Rivers are home to animals such as freshwater fish, eels and insects. Rivers are very important to the land's ecosystem because they provide a place for animals to find water. They are very important to human beings because they give us water for domestic and industrial uses as well as shipping channels. Rivers also provide fertile soil which allows plants to grow, both in their natural form and on farms and grazing areas. As rivers move across the land, they cut channels called valleys into the land. Rivers are located on every continent except Antarctica.
Deltas
When a river cuts through the coast to enter the ocean, the land around it can be changed in several different ways. One thing it can do is create a delta. Deltas are deposits of sand and silt that build up as a very large river enters the ocean at the river mouth. When the river reaches the flatter ground along the coast, it slows down and deposits its load, or the materials that it is carrying. As the sand and silt are deposited, the river breaks up into many different channels, causing a half-circle of land to jut out into the ocean. Deltas are usually fan-shaped, but sometimes deltas with fewer channels can have a shape more like a bird's foot.
Estuaries
A river can also end in an estuary. As the river flows through the land, it cuts a channel into the surrounding rock and soil. If the sea level drops (usually due to an ice age) the river will cut a deeper channel to reach the lower water. When the sea level rises again, this deeper channel is filled in with seawater, creating an estuary. (Refer Image 3) The water in an estuary is affected by both saltwater coming from the sea and fresh water coming from the river. Soil is deposited in estuaries by the river as it loses energy.
Rainfall and human activity along the river can affect areas downstream, or closer to the river's mouth. Heavy rainfall near the river's source, for example, can cause flooding downstream. Pollutants introduced into the river travel along to the mouth, where they can be deposited along with silt and soil.
