PUBLISHED: 26 February 2015

Types of Reservoirs

The term “reservoir” can refer to a man-made or natural lake, as well as cisterns and subterranean reservoirs. In this section we focus only on man-made reservoirs. 

Man-made reservoirs are made when dams are constructed across rivers, or by enclosing an area that is filled with water. There are two main types of man-made reservoirs: impoundment and off-stream (also called off-river).

Reservoirs can vary in size and be as small as a pond and as big as a large lake [1]. There is so much variability when it comes to reservoirs – they can differ in size, shape and location. For this reason, it can be misleading to make blanket statements about reservoirs without “significant qualification as to their type” [2]

Depending on the purpose of a reservoir, operators will fill a completed reservoir with water, let water flow on through the dam and downstream, or leave the reservoir site empty until it is needed (e.g. a dry dam site for flood mitigation).  

WaterPortal Infographic Reservoirs page

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Impoundment Reservoir

An impoundment reservoir is formed when a dam is constructed across a river. Impoundment reservoirs are usually larger than off-river reservoirs and are the most common form of large reservoirs. 

Off-Stream Reservoirs

Off-stream reservoirs are reservoirs that are not on a river course. Rather, off-stream reservoirs are formed by partially or completely enclosed waterproof banks [3]

The embankments around an off-stream reservoir are usually made from concrete or clay. The size of an off-river reservoir will depend on how large of an area is excavated and how high the embankment is built.

Off-stream reservoirs are generally simple in shape and “virtually uniform in depth” [4] compared to impoundment reservoirs, which tend to have shallower shores and varying volumes and shapes.

Environmental Impact Assessment and Stakeholder Engagement

An environmental impact assessment (EIA) must be completed prior to construction beginning on a large or impactful reservoir and dam site. The EIA process is regulated at the federal government level by the Canadian Environmental Assessment Agency. 

The Alberta government also has its own process for completing an EIA. Alberta Environment and Sustainable Resource Development is responsible for the laws that are related to EIAs in Alberta (the Environmental Protection and Enhancement Act and the Water Act). At an inter-governmental level, the Canada-Alberta Agreement on Environmental Assessment Cooperation is an agreement between the federal government and the government of Alberta that streamlines the EIA process and ensures that the EIA meets the requirements set out by both levels of government [5].

An EIA may include, but is not necessarily limited to, an analysis of the following:

  • Whether the safety of navigation is impacted by the proposed structure;
  • The impact of the structure on migratory fish; and
  • The impact of the structure on endangered riparian species or fragile ecosystems.

Depending on the location of the dam and reservoir, stakeholders may need to be consulted before construction can begin. 


The construction of a reservoir and dam involves many steps that are taken carefully to ensure the dam is operational and meets all safety standards and environmental regulations. 

Diverting the River

Before construction can begin, the river must be diverted in order to make the building process easier. Water can be diverted through constructed channels on the surface alongside the river or through underground tunnels through the rock alongside the river. Both of these methods allow the water to travel downstream of the reservoir site and minimize the amount of water travelling to the construction zone.

 A temporary dam called a cofferdam is built above the main site of the permanent dam. The cofferdam is intended to protect the construction site in the event of a flood. 

On wide rivers, a cofferdam may be built on one side of the river to allow water to flow through half of the riverbed. The area behind the cofferdam will be drained and the first half of the main dam will be constructed. This side of the main dam will not be constructed to completion. There will still be some holes in this section of the dam. The cofferdam will then be removed and the water will flow through the holes in the incomplete main dam. 

Another set of cofferdams is built on the other side of the river and the rest of the main dam is fully constructed. In the final step, the original cofferdam will be reconstructed and the portion of the main dam behind the cofferdam is completed. Once the cofferdam is removed for the last time, the dam is complete and water is stored in the reservoir. 

In some cases, rather than constructing an on-stream dam, a dam is built off-stream in a topographical depression suitable for holding water (off-stream reservoir). Once the dam is completed, the river will be diverted to the off-stream storage site [6].

Preparing the Foundation of the Dam

A crucial step in the lifecycle of building a dam is preparing its foundation. This is the first step in construction for an off-stream dam. For an on-stream structure, the dam foundation is prepared after the river has been diverted.

Once the construction site is drained of water, the dam foundation is excavated. All loose soils and sediment are removed, roots and vegetation are grubbed and all water is removed from the site until bedrock is exposed.

If a dam is being built utilizing a valleywall on one or both sides, any blocks of the valley wall that are unstable are removed. Hundreds to thousands of cubic metres of unsuitable rock may have to be removed to reach bedrock with the appropriate strength, stiffness and permeability  characteristics required for construction.  

The bedrock geology will be surveyed for faults and cavities. Any cavities are filled with grout to increase the stability of the bedrock and to prevent water from leaking underneath the dam. Concrete may be used to fill larger openings such as surface cracks, fissures or irregular surfaces. The foundation surface of the dam will be moistened with some water and with the use of a roller it will be compacted to increase strength, stiffness and stability, while reducing permeability. 

Finally, if applicable, a mass concrete footing area for an intake tower (a tower used to control water supply for a hydroelectric plant) is constructed and spot rock bolts are installed below the intake tower [7].

Intake Towers 
Intake Towers” by Denver Aquino is licenced under CC BY 2.0. No changes were made to photograph. 

Building the Dam

Dam construction depends on the type of dam being built (see Table 1). Arch, gravity and buttress dams are built with concrete and are supported by steel. These dams are used for on-stream storage. These dams used the weight of their structure, abutments, anchors and a solid geological foundation to support them.

Embankment dams (either earthfill or rockfill) are made primarily of soil or rock found in or near the construction area to minimize transportation costs. Erosion is a major concern for embankment dams and continuous maintenance such as vegetation control is required. The requirements for the foundation are not as extensive for embankment dams as they are for concrete dams.

Filling the Reservoir

Once the dam structure is in place, the reservoir is almost ready to be filled. The land that will be underwater is first surveyed for anything that could potentially contaminate the water. Trash and debris are removed. Then, information signs will be placed around the reservoir and roads leading to the construction area will be barricaded. The reservoir can then be filled. 

For on-stream storage, water will have already begun collecting behind the dam. However, for off-stream storage, water must be diverted to the reservoir.

During the filling process the reservoir site is carefully monitored. Operators will watch for seepage of water through the dam and stay alert for mudslides or landslides, which can occur when the soil and new embankment areas around the filling reservoir are inundated and become wetter than normal. 

Water quality is monitored at the inflow and outflow of the reservoir and the reservoir water itself. Water quality can be affected by the reservoir since the water that was formerly flowing is now still. Over time, nutrients in the water can build-up and result in the growth of algae blooms (in Alberta, where most reservoirs are located closer to the mountains and inflows often have lower nutrient loads, algae bloom growth is less of a problem than it is in other jurisdictions). Chemicals can also become concentrated in the reservoir, making the water unsuitable for drinking. In Alberta, there are regulations for water quality in a reservoir. The Canadian Fisheries Act regulates the harmful alteration of fish habitat and any required compensation [8].

Testing Valves and Floodgates

Valves are placed in the dam so that water can pass through the dam and go downstream. Valves are tested to ensure that minimum required flows are met and that they can withstand the passage of higher flows if needed [9]. Spillways are common and are often required for safety purposes. A spillway is a part of a dam that allows water to automatically flow over the dam during a flood event.

In exceptional cases, when a dam needs to pass flows that are larger than what a valve can pass (e.g. during a flood event), a floodgate is used. These floodgates are tested periodically to ensure that safety standards are met. 

Monitoring the Dam

The dam will “settle” over time. Since a dam is such a large structure, its height will eventually decrease because its weight will compact it down. The structural integrity of the dam is monitored during this process to ensure that the dam is still functional and safe. 

Seepage is also monitored. This is particularly important because as the dam ages, tiny micro-fractures can form and the water can begin to pass through the dam wall. Once water begins to pass through, the hole will only get bigger and the problem will become greater.

The location of the dam is also monitored. The dam may shift slightly upstream or slightly downstream, which would indicate that the structure is unstable. It can also tilt to one side, which is another sign that the dam is unstable [10].

In addition to monitoring the dam structure, water quality is monitored upstream, in the reservoir and downstream of the dam. 


[1] J. Thornton, A. Steel & W. Rast Chapman, Deborah (Eds). (1992, 1996). Chapter 8 – Reservoirs. Retrieved from

[2] Ibid. 

[3] Ibid. 

[4] Ibid. 

[5] Canadian Environmental Assessment Agency. (2012). Canada-Alberta Agreement for Environmental Assessment Cooperation. Retrieved from:

[6] The British Dam Society. (2010). Construction sequence for the Three Gorges Dam. Retrieved from

[7]  United States Department of Agriculture – Natural Resources Conservation Services. (2012). Part 645 Construction Inspection National Engineering Handbook – Chapter 7: Foundation Preparation, Removal of Water, and Excavation. Retrieved from:

[8] Alberta Environment and Sustainable Resource Development. 2011. Water for Life – Healthy Aquatic Ecosystems. Page 8, Retrieved from

[9] Denver Water. 2014. “Cheeseman Dam Valve Testing.” Retrieved from:

[10] The British Dam Society. 2010. “Monitor Behaviour.” Retrieved from: