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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. 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”. 

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 …

Attendance reflects the reservoir’s allure

Summer lasted well into October this year, and it extended the busy season at the 850-acre Waterbury Reservoir.

Waterbury State Park often reported its parking lot was full on sunny days.

Little River State Park had more than 41,700 visitors — a 10 percent increase in overall use from 2016, boosted by new bike trails and upgraded campsites.

Waterbury State Park had just over 37,800 visitors to its day-use area, down 18 percent from 2016’s record 46,000. In total, nearly 80,000 visitors were logged in at the reservoir’s parks during the summer, not counting people who used access points that aren’t staffed.

The parks have five access points, several unstaffed, so those people and boats aren’t counted toward the total, but “anecdotally yes, we have seen an increase in usage, always dependent on weather,” said Chad Ummel, the reservoir’s “floating ranger.”

Summer was cold and rainy to start, and in June the reservoir had to close briefly when rainfall pushed the water level too high, but unexpected warmth in October boosted the overall numbers, Ummel said. The state parks actually extended their closing date to Oct. 10.

There was also an overflow effect: “When Lake Champlain has algae blooms, for instance, we do note that we receive more usage,” as when North Beach in Burlington had to be closed this summer, Ummel said.

Three years ago, the state government took over management of the remote campsites along the reservoir, and have been tracking the numbers of campers who visit. The sites are available only on a first-come, first-served basis.

“There was an initial transition because people have been camping on the reservoir for 80 years with virtually no regulation, no oversight, and there was a little bit pushback initially,” Ummel said. “But we found that most people have been quite appreciative for the state’s efforts, the composting toilets, just the presence on the water and overseeing has made for a more tranquil and enjoyable experience for most.”

Most campers seem to be Vermonters, Ummel said, but other New Englanders who have heard about the remote campsites often stay overnight.

Booming businesses

“Once it became summer, it became nice,” said Chuck Hughson, co-owner of Waterbury Sports.


The store opened two years ago in Waterbury, and Hughson says added bike paths in Little River State Park and Perry Hill have definitely attracted more people to mountain biking, he said; the numbers show in his bike-rental business.

“More people are going out with their friends and realizing they can do it too,” Hughson said.

There was no break between fishing and hunting season at the Fly Rod Shop in Stowe, manager Parker Wright noted. “Lots of people come in looking for gear saying ‘Hi, I’m going down to the reservoir,’” he said.

The shop was much busier in October than in years prior, probably due to the weather, Wright said.

There were also lots of people getting out on the water in boats, canoes, kayaks, or on stand-up paddleboards.

“Waterbury Reservoir continues to be a tremendous resource for people to go paddling on,” said Steve Brownlee, owner of Umiak Outdoor Outfitters.

“One of the benefits that we have seen is more people renting boats late in the season. We had terrific warm temperatures in the months of September and October. But the other big difference is the new regulations for keeping the water level high is something new in the last two summers,” Brownlee said.

At the end of 2015, the state decided the Waterbury Reservoir should be maintained at its summertime levels year-round, at about 590 feet above sea level.

In the past, the reservoir was drained every fall to between 540 and 560 feet above sea level, making room for spring runoff, then restored before summer. Now, water flow through the dam is managed to more closely match the natural flow of the Little River.

Exhausted after playing all day, visitors headed into town, and businesses in Waterbury continue to see growth.

“The bars and restaurants love the increased traffic from people enjoying the trails and reservoir,” said Alyssa Johnson, Waterbury’s economic development director. “The activities feed off each other well. There’s nothing better than good food and a cold beer after a day outside.”

“The Waterbury Reservoir is unique. It is a true multi-use body of water where nearly everything is allowed from innertubes to seaplanes,” Ummel said. “So it’s a fascinating scope of use where you get to see people on stand-up paddleboards, and jet ski circles around them. It’s an exciting body of water.”


Better amenities on the way for public at Waterbury Reservoir

Waterbury Reservoir and Dam. Photo courtesy Barry Solman ANR.

Vermont Business Magazine Eight months of work is underway to improve public access areas at the Waterbury Reservoir. The project will require temporary closures at some sites while work is underway, and will include improvements to parking lots, erosion prevention, and boat ramp upgrades. The locations include Waterbury Dam Boat Launch, Blush Hill Boat Launch and the Moscow Paddler Access. Green Mountain Power (GMP) will be doing the work as part of the company’s commitment to improving the Waterbury Dam and surrounding areas. GMP recently received a renewed license from the Federal Energy Regulation Commission to operate a hydropower generation facility at the Waterbury Dam.

The Department of Forests, Parks, and Recreation says the popularity of the Waterbury Reservoir is growing. “The improvements that Green Mountain Power will make over the next 8 months could not come at a better time,”said Susan Bulmer, Northeast State Parks Regional Manager.

 “We know what a valuable community resource this waterway is and we’re glad to work with the state to make these important upgrades,” said Jason Lisai, Green Mountain Power’s Director of Generation Operations.

As part of the Green Mountain Power improvements, a parking area, river access, and anglers’ trail were recently installed along Little River Road just downstream of Waterbury Dam. The trail was constructed as a collaborative effort with the Vermont state trail crew and GMP. This river access is currently open to the public for use.

All three locations will be finished for the 2019 summer season. Long-term maintenance and management will be performed by the Department of Forests, Parks, and Recreation.

Waterbury Reservoir is the ninth-largest waterbody in Vermont, created in the 1930s by the Civilian Conservation Corps as a flood control project. There are two State Parks and 30 remote campsites located on its shores and many people access the reservoir for boating, wildlife viewing, swimming, camping, and fishing each year. Waterbury Reservoir is surrounded almost completely by state land, managed primarily by the Department of Forests, Parks, and Recreation as part of the 44,444-acre Mount Mansfield State Forest.

Scheduled Closures:

Waterbury Dam Boat Launch

Closed: October 14 – November 8, 2018 and February 8 – February 18, 2019

Site improvements will include:

  • Installation of a new composting toilet
  • Installation of a concrete boat launch
  • Paving of the access road and parking area (may occur in Spring 2019 depending on weather conditions)
  • Erosion control and site drainage improvements
  • Traffic flow improvements

Blush Hill Boat Launch

Closed: November 9 – November 27, 2018 and February 19 – February 27, 2019

Site improvements will include:

  • Installation of a concrete boat launch
  • Re-grading the parking area
  • Guard rail installation
  • Erosion control and site drainage improvements