River Basins and the Water Cycle A river basin is an area of land that is drained of its water by a river system. They can be any size from a couple of miles width to the size of a large country. A river basin will contain all the elements that make up a river system from its source until it reaches the sea. The boundary of the river basin is called a watershed. Typically, a watershed is formed by the crest of a hill or a mountain. Any rain that falls onto one side of a hill will fall into one water basin. Any rain that falls on the other side should end up in a different water basin system. Rivers can have many sources. They usually start in higher land areas where the rain falls on the hills or on the mountains. Sometimes, especially in mountainous areas, the water could actually fall as snow. The source would then be the snow as it melts. It is also possible for water to be pushed up from under the ground in what are called springs. The source of the water will form small streams of water as it flows from high land to lower land. These streams will form into tributaries which in turn will flow into larger and larger rivers. Finally, you will have one major river for the basin that will flow into the sea at the mouth of the river. Water in the sea is heated by the sun and evaporates. As it rises, the cooler air condenses the vapour and turns it into clouds. The wind in weather systems then move these clouds around and often back over land. These clouds can release the water as rain, hail, sleet or snow. The cycle then starts again. Fresh Water The water in the sea contains a great deal of salt. This makes it all but unusable for human consumption. Fortunately, the salt is too heavy to be picked up by evaporation. Therefore, when the rain falls the water is fresh and is able to be used by humans. Apart from the obvious uses of water as something to drink and clean with, it can also be used by agriculture and industry. Irrigation systems allow rivers to be used to water fields so that they can grow crops. Industry often uses water as a coolant or as a cleansing agent. Flowing water can even be used as a source of power. Hydro-electric dams collect water behind them and then releases that water to run through huge turbines. They are effectively giant water wheels. As these wheels turn, they generate electricity. Of course, rivers can also support eco-systems of their own. Fish, amphibians, insects and many mammals depend on the flow of fresh water to provide their habitat and source of food. Some humans also depend on these fish stocks for their own livelihoods and as a food source. All Water % Sea Water 97.2% Ice and Snow 2.1% Underground 0.8% Rivers and Lakes 0.2% Fresh water is surprisingly scarce in the world. Most of the world's water is contained in her oceans. But, as has already been pointed out, this is salt water and so of little use to humans. Only 2.8% of the world's total quantity of water is fresh water. Of this, the vast majority of fresh water is stored as ice or snow. A significant amount of the world's water also seeps underground as groundwater. Only 0.1% of the world's total water stocks flows as surface water in rivers and in lakes. To make matters worse, this surface water is often polluted by humans. Pesticides and fertilisers used in farming is washed in to the river systems. Factories often pump out dirty water after they have finished using it for cooling or cleaning with. Nature also affects where the rain falls. Prevailing weather patterns and ecological systems affect the rainfall. There are places in the world that have too much rain and other places that do not have enough. Rainfall also varies from year to year. Some places depend on the amount of rainfall that they receive. If for some reason the same quantity of rain does not fall in a particular year or for a number of years the result can be a severe drought. If there is too much rain the result could be a flood. So, for a number of reasons, fresh water is a precious commodity. Humans have tried a number of ways of getting access to fresh water. Rivers are the most obvious example but there are other ways too. One way is to take water from lakes through pipelines and aquaducts. Digging wells is a way of getting access to groundwater. It is also possible for groundwater to be forced out of the ground as a spring. Finally, humans have tried to build their own lakes (reservoirs) to store water through the use of dams. The Aswan Dam Dams bring advantages and disadvantages. On the plus side, they can allow humans to plan where to have their water supply rather than relying on where nature will put one for them. Dams can allow the exact flow of a river to be controlled. This means that it can help to compensate for too much or too little rain. Additionally, the newly created lake or reservoir can provide a habitat for wildlife that depend on water. And, as has already been mentioned, dams can also be equipped with turbines to generate electricity with. It was for these reasons that a huge dam was built in the middle of the Sahara desert by President Nasser of Egypt in 1970. Egypt has depended on the flow of the River Nile for thousands of years. By building this new dam, the Egyptian government could control the flow of water and so prevent the floods or droughts which occurred from time to time. Unfortunately, there were a number of environmental side-effects that have become more and more of a problem as time progresses. The main problem is that little silt is now being deposited along the banks and in the delta of the Nile. Silt is soil which is rich in nutrients and minerals and which flows naturally along a river. With the building of the dam, the silt cannot pass the artificial lake created behind the Aswan Dam. This means that the nutritious silt cannot get to the farmers downriver and it also means that the lake itself is filling up with silt. The lack of a natural fertiliser has meant that Egyptian farmers have had to start buying chemical fertilisers to do the job of the silt for them. This has another knock-on effect of meaning that more chemicals are being washed into the River Nile and polluting it. Pollution and the lack of nutrients in the water is also affecting the fish stocks in the river which means that there are fewer and fewer fish to catch. One other unforeseen environmental impact is the spread of a disease called Bilharzia. This is a disease that is spread by water snails. In the past, the water snails were washed along the River Nile and out into the sea. Now, they cannot get past the Aswan Dam. They now spread Bilharzia to the people living around Lake Nasser. Disrupting the natural flow of water systems can bring benefits to a society, but it can also bring problems. In fact, it can bring many unforeseen problems. It is important that planners try to anticipate the impact that they are going to have on the local environment. Otherwise, they may end up doing more harm than good. Floods Floods occur when the channels that water use are overwhelmed. Rivers break their banks and the water spreads out into the surrounding areas. There are many reasons for these channels to be overwhelmed either due to natural reasons or to man-made ones (or probably a combination of both). Heavy rainfall is the most obvious cause of floods. Existing river channels can only carry a certain amount of water, they need time to be able to carry water away to the sea. If there is a particularly heavy rainfall, or a series of heavy rains in a short period of time, the banks of the rivers can burst and cause flooding to the surrounding countryside. Existing rivers can also flood if they silt up. This means that silt (soil) has been washed down the river and collects in a channel somewhere along the course of the river. If too much silt collects in this channel, the water is displaced into the surrounding countryside. Although, this is often after a heavy rainfall somewhere up the river system. Vegetation (particularly trees) can act as a natural sponge to soak up excess water or to at least slow its flow. All plants require water and will try to hold on to as much of it as they can. Their leaves and roots will all try to catch water. These same leaves and roots will also act as a brake. The leaves will stop the water from hitting the ground too hard. This will mean that it is more difficult for top soil to be washed away as silt. Roots will slow down the flow of groundwater and so will give the river systems more time to carry away the existing surface water. Reeds and grasses in marshlands and swamps do a similar job. If the trees and vegetation are removed at all, the likelihood of a flood down river will increase greatly. Dams are a man-made way of trying to prevent flooding. However, we have already seen that these can also affect the environmental system in other unforeseen ways. Besides, dams are very expensive and time-consuming to build. They can also be politically unpopular as they force some people off their land and out of their homes.

The mouth is where the river meets the sea Images River Related Images Worksheets Available Example Student Work Examples of Pieces of Work Learning Tasks River Vocabulary Exercise How water journeys to the sea Exercise River Basins Exercise Fresh Water Exercise The Aswan Dam Exercise Floods Exercise Rivers Crossword Online Resources Earth Observatory This NASA site has pictures of rivers from satellites. It is a fantastic resource. For example, it has lots of before and after shots of flooded rivers. Watershed Game Can you answer the questions in this quiz Digital Brain This site explains about rivers and floods very clearly Water Cycle Grabber 2 This site has some interesting and surprising facts for you to learn Rivers Links You are spoilt for choice for information about rivers. Floods This site shows some good pictures and has a good analysis of what causes and prevents floods. BBC Guide to Rivers and Water This is a GCSE revision site. However, you should know some of the vocabulary and ideas presented here. Glossary Aswan Dam A huge dam built on the River Nile in Egypt basin An area of land that is drained by a river and its tributaries. Bilharzia A disease that is spread by water snails channel This is a course which water runs along. Sometimes it means a deep course. deforestation The cutting down or burning of trees to clear land delta A large body of sediment deposited at a river's mouth, generally triangular in shape drought This is a long spell of dry weather flash flood When a flood happens quickly and unexpectedly groundwater Water that seeps underground and moves back to the sea in a different cycle irrigation This is where man-made channels are used to water farm land mouth Where a river meets the sea overgrazing when there are too many animals eating too much of the grass saquia In Egypt, this is an animal powered wheel to carry water into irrigation channels sediment This is the material that is washed down along rivers. It can be soil, nutrients, minerals, etc... silt Sediment that has washed down a river and collects in a channel or river mouth shaduf In Egypt, this is a long pole with a bucket on the end. It is used to move water into irrigation channels. soil exhaustion When the land has been used too much and has lost many of its nutrients and minerals source This is where the river or stream begins tributary This is a stream or a small river that flows into a main river watershed This is the boundary between different water basins. Resources Available CD-ROM Rivers You can find more about rivers on this CD-ROM CD-ROM Test for Success KS3 Geography This is a general geography CD-ROM title Reference Books Dorling Kindersley Rivers and Lakes Ganeri, Anita Raging Rivers 1977 Guy, John Earth Alert! Rivers 1988 Royston, Angela Rivers and Streams Taylor Rivers and Oceans

---

---