index > Today's Earth, Our Future > 1. Water on our planet > An Important Place Where the River Meets the Sea
 
An Important Place Where the River Meets the SeaAs many of us know, seawater is salty and river water is fresh. So what about the water where the river meets the sea? This meeting place is called a "river mouth." It is the place where river water and sea water intermix, acting as a bridge between the sea and the river. Fish can be roughly divided into two types: those that live in the sea and those that live in rivers. But there are some fish that live in both types of water. Some fish grow up in the river and swim to the sea to lay eggs. Others, such as salmon, Japanese trout, eel, and the three-spined stickleback grow up in the sea and then swim upriver to lay eggs. As you can see, the river mouth is an important place for these kinds of fish. Their survival depends on it. Have you ever gone digging for clams at the beach? River currents form the kind of area where clams love to live. When the river current is strong, the river can carry sand and mud downstream, but when the river reaches the sea, the current speed slows down. As a result, sand and mud pile up at the river mouth and make a large, flat area. When the tide goes out, these flat areas, called "tidal flats," appear. They are created from "the meeting of the river and the sea."
Tidal flats have bountiful nutrition carried in by the rivers. They are uncovered and exposed to air as the tide goes out twice a day. There is plenty of oxygen, so the tidal flats are very comfortable places for small creatures to live. Tidal flats are home to many sandworms and clams. The tidal flats are also popular gathering places for the many birds that feed on these creatures too! As you can see, tidal flats are important not only to us humans, but also to fish and migratory birds. When the tide is out, we humans go digging for clams. Meanwhile, when tidal flats are covered with seawater, fish gather to lay eggs, and it is a place for young fish to grow. For migratory birds that come from other countries, tidal flats play the role of gas stations. Birds that have traveled for hundreds of miles to reach these areas rest their tired bodies and eat so they can "be refueled" and take off again. So, river mouths and tidal flats are vital to many living things. But, humans have destroyed many of these areas. For example, wastewater from houses flows into these areas. Another example is that sea water is blocked from entering and mixing in the river mouth so that fresh water can pumped up for agricultural use. Finally, tidal flats are especially vulnerable to being filled with soil to make more land for expanding farmlands or industrial sites. In Japan, about 40 percent of the tidal flats have been filled and buried in this way in just the past 50 years. Migratory birds must be suffering hardships. They fly for hundreds of miles to the tidal flats to look for food or to rest, only find that the gas station that was supposed to be there has disappeared! Today, there are movements to preserve river mouths and tidal flats across Japan. We hope you will support these movements to protect migratory birds and fish. In Japan, there are several tidal flats, even near big cities like Tokyo and Nagoya. Why not go there and feel the sea breeze as you watch the beauty of the migratory birds taking flight? You will also be able to find many hermit crabs and other tiny crabs at your feet!
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   1. Water on our planet
   
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