The remaining flows are then sent downstream. The reservoir is then ready to supply water to downstream users for another winter. Given that a dam is a block in a river, it will accumulate all sorts of things behind it primarily sediment. However, debris, trees, rocks and other objects can also be collected behind a dam during high flow events such as floods. In order to know what to remove from a reservoir, bathymetric surveys can be completed to map the bottom of the reservoir.
These surveys might reveal that there is a large buildup of sediment in one specific area or throughout the bottom of the reservoir. If the reservoir is found to have a large accumulation of sediment, operators may investigate dredging as an option to remove the sediment. A cost-benefit analysis is completed before dredging begins to make sure that the benefits outweigh the costs. Another method used to remove sediment is to open up reservoir gates at the bottom of the dam.
Operators can periodically open these gates to release sediment downstream. Whether dredging or gates are used to remove sediment is dependent on how quickly materials gather behind the dam. In some places there is a high chance of sedimentation, while in others it is not a problem.
The type of sediment that accumulates also plays a role. In Alberta, a large portion of sediment is sand and gravel. If silt and mud were collecting instead, reservoirs would need to be dredged frequently. No large reservoirs in Alberta have been dredged. When you look at a river, you can see water flowing downstream. However, there is more than just water flowing in a river. Sometimes large items, such as logs, float down the river.
There are also pieces of rocks, sand and soils called sediment that move downstream with the river. Usually sediment that is carried down a river is very small or fine-grained. Sediment can range in size from sand, to silt to mud to gravel. During high-stream events such as floods, sediment can be larger and coarser-grained than normal.
Sediment can get as big as gravel-sized particles e. Worldwide, as much as 13 billion tonnes of sediment travels from rivers to seas every year, while two to five billion tonnes of sediment are trapped in reservoirs [1].
The sediment that flows in a river can come from the landscape of the watershed or the river itself. Sediment is created during the processes of weathering and erosion.
Weathering is a natural process that breaks rock down into smaller and smaller particles. There are two types of weathering: physical and chemical. Physical weathering includes breaking rocks down through water, ice, wind and temperature change. Water collects in cracks in rocks. When the temperature goes down in the winter, the water freezes and expands, making the crack even larger.
In the spring when the ice thaws the water flows out of the crack and takes sediment with it. Chemical weathering refers to alterations in the rock on a chemical level. For example, acidic rainwater falling on rock can cause the rock to dissolve. Rusting is also a form of chemical weathering where the oxygen in the air weakens the rock. Plants can also cause physical and chemical weathering. A shrub with deep roots might crack through rock, causing physical weathering.
The same shrub might produce acidic compounds which dissolve the rock. Erosion is different than weathering. While weathering is the process of a rock breaking down where the rock sits, erosion is the process where sediment is broken off a rock and is carried to another location.
When the reservoir gets too full, the floodwater can be passed downstream over a spillway. Sometimes, floodgates are used on top of spillways, and they can be fully or partly opened to control the amount of water let out into the river downstream. Plants will grow naturally on fertile soil that is watered by rain. However, in regions of the world where the climate is very dry for some seasons, the soil becomes so dry that it restricts the growth of vegetation.
This problem can be overcome by irrigation, a man-made system for watering the land. Irrigation water can be stored in reservoirs during the rainy season, then in the drier seasons it can be released from the reservoir and distributed over the land through a system of canals.
Usually, the water flows under the influence of gravity to the areas requiring it, or the water can be pumped out of the canals onto the land. Rio Cobre Dam and its main irrigation canal, in Jamaica. The construction of a dam across a river forms a reservoir that raises the water level upstream , stores the water, and slows down its rate of flow. This improves the navigation conditions upstream of the dam for ships and boats.
Dangerous areas of rocks and sandbanks, previously in shallow water, become well covered, and rapids in the river disappear. Also, water from the reservoir can be released into the river downstream during the drier seasons of the year to make sure that it is deep enough for navigation all the year round. A dam forms a barrier to any shipping, so navigation locks similar to the canal locks in Britain can be constructed to one side of the dam, or special lifts can be built into the dam for raising and lowering vessels.
Model of the Three Gorges Dam with ship lift and ship lock. A ship lift and a navigation lock are being provided in the Three Gorges Dam which is currently being constructed across the Yangtze River in China.
The ship lock will have two channels each 1. Use these resources to teach your students how humans modify the physical environment and the compounding impacts those changes have. Individuals, communities, and countries depend on a variety of different resources to help them thrive: electricity, timber, oil, water, and food to name a few.
Because these basic resources are such a large part of our daily lives, it is important that we manage them responsibly to ensure future generations have what they need. Human civilization heavily impacts the environment and the rich natural resources we depend on.
All communities face the challenge of managing resources responsibly, not only for themselves, but for the sake of the world around them.
Learn more about how individuals and communities can manage their resources to support themselves and the world around them. Freshwater is a precious resource on the Earth's surface. It is also home to many diverse fish, plant, and crustacean species. The habitats that freshwater ecosystems provide consist of lakes, rivers, ponds, wetlands, streams, and springs. Use these classroom resources to help students explore and learn about these places. Students investigate the interconnectedness of Earth's water reservoirs by learning about the hydrosphere.
They then discuss ways in which humans are connected to and alter those reservoirs, especially the ocean. A dam is a structure built across a stream or river to hold water back. Dams can be used to store water, control flooding, and generate electricity. Hydroelectric energy is power made by moving water. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Reservoirs can be located below ground, at ground level like this one or above the ground as water towers. Photograph by Krzysztof Synoradzki, My Shot.
Also called a service reservoir. Also called a cistern. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. Last Updated Jan. Media If a media asset is downloadable, a download button appears in the corner of the media viewer.
Text Text on this page is printable and can be used according to our Terms of Service. Interactives Any interactives on this page can only be played while you are visiting our website. Related Resources. Human Modification of the Environment.
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