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Every year, we work with government agencies, scientists, municipalities and most importantly, nature, to predict conditions and manage our river systems for the safety of people and the environment, even through these most challenging of times.

As warmer temperatures set in, OPG’s operations staff across the province are already busy managing the impacts of Freshet, the annual spring runoff that occurs when snow melts and spring rains fall.

Freshet is an important but often misunderstood annual phenomenon that affects Ontario’s hydroelectric operations and the people who live on or use the affected river systems.

Predicting water levels can be very difficult, and our employees must balance many factors including public safety, limited reservoir storage capacity, environmental stewardship and local water management plans and agreements, to determine allowable limits for managing the holding or release of water from our many dams on rivers across Ontario.

On large river systems like those in Ontario, spring Freshet can take up to several weeks. Regardless of water levels, please be safe around rivers and dams as water levels can change in an instant. Learn more at opg.com/watersafety

What is Freshet?

 
 
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Fast facts

393,390

km2 of land area within OPG's watershed (estimated)

241 dams

on 24 river systems across Ontario

27.6 billion

m3 in OPG's total storage capacity

653 billion

m3 of water managed by OPG in 2019

Ontario's watersheds

Find out what OPG is doing to manage water in each regional watershed.

 

Our Eastern Ontario regional watershed is home to 15 hydroelectric stations situated on six river systems including the Ottawa, Madawaska, St. Lawrence, Mississippi, Trent and Rideau.

Regional watershed partners

As we work to predict conditions and manage river systems for the safety of people and the environment, we work with many partners and agencies in this region including:

Challenges

River width and depth

Areas where the river narrows or becomes shallow (e.g., rapids) can cause water to backup upstream of these narrows when water flows are high. High flows at natural river restrictions can require special water management strategies for flood mitigation at nearby dams.

Water management plans

We work with our watershed partners and the Ministry of Natural Resources and Forestry (MNRF) on a balanced approach to water management on the Madawaska River that considers the various interests of many users.

This work produces the Madawaska River Water Management Plan, which includes operational plans for each waterpower facility or dam; provisions for management of water levels and flows; stakeholder communication; and environmental programming.

River system data

Madawaska River

Mississippi River

Ottawa River

Public consultation

For those in the region unable to attend our recent live telephone town hall sessions, you can find recordings here:

Along with our operations at the Sir Adam Beck Complex on the Niagara River, OPG operates 20 stations in central Ontario including facilities on the Muskoka, Severn, Beaver, and Musquash rivers.

Regional watershed partners

As we work to predict conditions and manage river systems for the safety of people and the environment, we work with many partners and agencies in this region including:

Challenges

Limited water storage capacity

Our generating stations on the Muskoka River are considered “run-of-the-river” as they have a limited water storage capability and limited ability to influence river flows. We operate these facilities in coordination with operations at the MNRF controlled structure to manage appropriate flow conditions. Existing structures have specific limits in terms of flow passage and water retention capability.

Natural river geography

The Muskoka River System at Bala poses a challenge due to a particular river constriction. The river has but a single outlet for the upper two thirds of the entire Muskoka Watershed. As flows increase, the levels increase naturally due to the restriction along the Bala Reach (Moon Chutes). The Moon River from Bala diverges into the Moon and Musquash rivers. Our Ragged Rapids and Big Eddy stations are located on the Musquash River. As flows increase at Bala, it becomes difficult to pull water down the Musquash River, which leads to lower water levels in the Upper Musquash.

The Eugenia GS is fed from Lake Eugenia Reservoir, which is managed to an annual winter drawdown, and spring fill-up plan. Many factors are reviewed to determine the yearly plan such as snow pack, site conditions, and weather forecast.

Water management plans

We work with local waterpower companies and the Ministry of Natural Resources on a balanced approach to water management on the Muskoka River that considers the various interests of many users.

This work produces the Muskoka River Water Management Plan, which prescribes the compliance water levels and flows for our generating stations operating in the Muskoka watershed.

River system data

Beaver River

Muskoka River

Severn River

Our Northeast Operations (NEO) involve a region approximately 100,000 km2, stretching from North of Kapuskasing to south of North Bay. NEO has 21 generating stations and 22 control dams across seven river systems include the Mattagami, Abitibi, Montreal, Matabitchuan, South, Sturgeon, and Wanapitei rivers. We operate in many site communities including Smooth Rock Falls, Timmins, Dymond, North Bay and Sudbury.

Regional watershed partners

As we work to predict conditions and manage river systems for the safety of people and the environment, we work with many partners and agencies in this region including:

Challenges

The water management planning process and strategies for water management takes into account many factors including:

  • Natural geographic restrictions
  • Community-specific requirements (e.g., notifications when water levels may impact water tanks or other facilities)
  • Interests of residents and stakeholders both up and downstream of our facilities

We do our best to share information and decisions openly.

Water management plans

During freshet, communication with our regional partners is open and frequent to ensure all operators collaborate to maintain water levels and flows, mitigate flooding, and adhere to Water Management Plans implemented by the regulator.

River system data

Abitibi River

Matabitchuan River

Mattagami River

Montreal River

South River

Sturgeon River

Wanapitei River

Public consultation

For those in the region unable to attend our recent live telephone town hall session, you can find a recording here:

Our Northwest Operations has 11 hydroelectric stations that provide a clean, low-cost, renewable and reliable source of power to Ontarians year-round. The stations are fully automated and controlled remotely from our Northwest Control Centre in Thunder Bay.

The river systems in this region involve the Aguasabon, Nipigon, Kaministiquia, English and Winnipeg rivers.

Regional watershed partners

As we work to predict conditions and manage river systems for the safety of people and the environment, we work with many partners and agencies in this region including:

Water management plans

Our operations follow established water management plans to manage water levels and flows while balancing environmental, social and economic objectives, and various interests within the watershed.

The Aguasabon, Nipigon and Kaministiquia river systems in this region have water management plans regulated by the MNRF. We developed these plans in consultation with MNRF, stakeholders and the public.

The English and Winnipeg River systems are regulated by the Lake of the Woods Control Board.

River system data

Aguasabon River

Kaministikwia River

Nipigon River

Public consultation

For those in the region unable to attend our recent live telephone town hall session, you can find a recording here:

Flood warning and emergency management are the responsibility of the Province of Ontario and local municipal authorities. To get updates about flood warnings and watershed conditions, visit the Ontario Flood Map or  your local conservation authority's website.

For information related to specific actions you can take to protect yourself and your property during a high water event, please contact your local municipal authority. 

For general information on preparing for flooding, please visit the Government of Ontario Floods website and the Government of Canada's Get Prepared website.

Freshet timeline

Project Status: OngoingStart Date: Early springExpected Completion: Early summer
Typical annual activities
Click the ( i ) under each activity for details.
Spring
Snow surveys
January - End of snow
Freshet timeline
Snow surveys
January - End of snow

Snow surveys provide regular updates of actual snow conditions throughout the province.

Our snow surveys occur on the 1st and 15th of every month during the winter and spring seasons. Across the province, crews return to the same sites to measure snow depth and snow water equivalent, which is the amount of water stored in the snowpack.

Taking surveys at the same places allows for a year-over-year comparison. We share this data with the Ministry of Natural Resources and Forestry, Conservation Authorities and water managers across Ontario.

Begin drawdown
January - March
Freshet timeline
Begin drawdown
January - March

Where applicable, we conduct drawdown every winter according to provincially mandated water management plans. Dates depend on reservoir size and location.

Drawdown is the process of lowering a reservoir’s operating level from the summer maximum to the target winter water level, to prepare for spring snowmelt and rain. This process can take months for large reservoirs, and days or weeks for smaller ones.

Pre-Freshet preparation exercises
February - March
Freshet timeline
Pre-Freshet preparation exercises
February - March

We conduct annual training, equipment tests and communication protocols to ensure everything and everyone is ready and available for the Freshet period.

Outreach communication with local partners
January - June
Freshet timeline
Outreach communication with local partners
January - June

It takes a community. Every year we meet with partner agencies, other river operators and local leadership to prepare for Freshet.

We’re not the only presence on many Ontario river systems. Throughout the year, we regularly communicate with other dam owners and regulators, such as the Ontario Ministry of Natural Resources and Forestry.

As we prepare for Freshet, we also ensure we have open lines of communication with municipal agencies, conservation authorities and political leaders as appropriate throughout the Province.

Reach drawdown targets
March
Freshet timeline
Reach drawdown targets
March

Prior to Freshet, all reservoirs will meet their target water levels.

Drawdown targets are set based on the capacity of the reservoir, environmental considerations and the needs of local stakeholders. Since we cannot predict months in advance what the spring will hold, drawdown targets are set for the same minimum range every year.

Begin enhanced monitoring
March - June
Freshet timeline
Begin enhanced monitoring
March - June

Water conditions tend to change more rapidly in the spring. At this time, we increase our monitoring of weather forecasts and watershed conditions.

Freshet commences
March - April
Freshet timeline
Freshet commences
March - April

The timing of Freshet varies year-to-year and by latitude, and can begin any time from the beginning of March to the end of May.

Scheduled conditions and modelling calls
January - June
Freshet timeline
Scheduled conditions and modelling calls
January - June

We participate in scheduled conditions/modeling calls with other agencies (ORRPB, MNRF, etc.). These inter-agency coordination calls can occur multiple times per day if conditions warrant.

Begin reservoir refill
March - May
Freshet timeline
Begin reservoir refill
March - May

Refill begins during Freshet based on weather and watershed conditions.

We use the extra water available during Freshet to raise reservoir water levels from winter minimums to the summer operating levels. The exact date that refill begins and the rate of refill are strategic decisions based on the weather forecast, snow conditions and existing river flows.

Monitor and adjust
March - June
Freshet timeline
Monitor and adjust
March - June

Monitor conditions and revise water management strategy daily or sub – daily as conditions warrant.

Complete reservoir refill
April - June
Freshet timeline
Complete reservoir refill
April - June

Reservoirs water levels reach their prescribed summer operating range by the target date, usually the Victoria Day long weekend.

Freshet concludes
June
Freshet timeline
Freshet concludes
June

We typically Freshet-related activities in June.

Lessons learned
Throughout the year
Freshet timeline
Lessons learned
Throughout the year

Every year is different and there are always things to learn. We’re always careful to incorporate new lessons learned into our operational practices for the following years.

Public meetings
June - July
Freshet timeline
Public meetings
June - July

We hold public meetings with representative stakeholder groups throughout the Province to provide updates on the past seasonal Freshet.

Summer

Frequently asked questions

We maintain a comprehensive dam safety program that ensures our flow control equipment, including spillway gates and log lifters, are well-maintained and routinely tested in advance of spring Freshet and throughout the year.

Where there are storage reservoirs on the river system, we lower water levels throughout the winter, referred to as “drawdown”, to make space for additional Freshet water. Our goal is for our reservoirs to be at their lowest level before Freshet begins. We carry out these drawdowns in compliance with provincially mandated Water Management Plans, required under Section 23 of the Lakes and Rivers Improvement Act for all provincial waterpower facilities.

The water we store in our reservoirs helps to mitigate flooding risks.

Unfortunately, some of our smaller hydropower dams in Ontario, such as those on the South River at Eliott Chute, Bingham Chute and Nipissing, have very little storage capacity. Stations like this are commonly referred to as run-of-the-river, because they essentially pass their water either through the generating units or through the sluiceway, or a combination of the two.

Dams can have many benefits including flood control, hydropower generation and water supply but the primary purpose of our dams is to produce of clean, reliable electricity.

The best way dams can help mitigate flooding is by storing excess water runoff (snow melt and rain) in a reservoir (if there is one). Each winter we draw down our reservoirs to their minimum operating level, and then refill them with runoff water during the Freshet. Filling reservoirs temporarily removes some water from the river system by placing it into storage. We can then slowly release the stored water later in the year when water levels are lower. In this way, reservoirs can buffer the flow of the river by shaving or flattening the peak water levels normally occur under natural conditions.

But once a reservoir is full, it must pass any further water it receives downstream. A reservoir’s ability to buffer water flows depends on its’ size (storage capacity) and how full it is (available storage), relative to the volume of water in the river at the time. Under extreme conditions, even large reservoirs may become quickly filled and have little ability to mitigate Freshet high waters.

Some dams, known as run-of-river, have virtually no ability to reduce flooding. These facilities have essentially no reservoir storage and must allow all incoming water to pass through them, especially during high flows.

Refilling a reservoir during Freshet takes a delicate strategy. We rigorously monitor watershed conditions and weather forecasts to assess how to use our available storage most effectively. We must also manage our reservoirs according to the geographical reality of their location, while accounting for local restrictions, reservoir size or ice conditions on the river.

We conduct snow surveys at a number of locations, measuring snow depth and water content, as an indicator of how much moisture could drain into a specific catchment area.

But knowing how much water could enter the river system doesn’t tell us exactly how much water actually will enter river systems.

We must consider the many other factors that influence water levels including temperature (daily highs and nightly lows), sublimation, vegetation and precipitation, and make decisions weeks or even months in advance using the best data we have available to us.

We can only make decisions about water management involving our own facilities, and we must make these decisions in accordance with limits set in established Water Management Plans (WMP) or other applicable regulatory frameworks. We regularly communicate water conditions at our facilities to the Ontario Ministry of Natural Resources and Forestry (MNRF) and other dam operators, reporting water levels, flow and expected future conditions.

In the event of a flood, we operate our facilities according to the WMP if possible, and notify the MNRF or conservation authorities so they can issue flood messaging to the public. We also provide information to local emergency management authorities so they can coordinate their response. A flood event at our dams is rarely an emergency because of our rigorous dam safety design considerations that comply with the Lakes and Rivers Improvement Act.

Floodplain mapping and defining potential inundated areas is the responsibility of Conservation Authorities, MNRF and municipalities. We share data and contribute to the community of knowledge in this area.

Predicting water levels can be very difficult, especially if there are many freeze-thaw cycles during the winter and early spring. Excessive snow doesn’t always cause higher water levels, as periods of warm, sunny weather may cause the snow to sublimate (go directly into the atmosphere), reducing water levels in the watershed. Meanwhile sudden and unpredictable spring storms can significantly increase these water levels.

We can measure the water content in snow and the size of the drainage basin, but it’s a challenge to predict exactly how much of that water will enter the river system or how quickly.

We make our decisions based on the best information we have available at that time but forecasts are not 100% reliable.

No.

Hydroelectric generation is most efficient when there are high water levels upstream of a dam and low water levels downstream of it. During a flooding event the opposite occurs, where upstream water levels on the river are intentionally lowered to prevent flooding, while downstream water levels are naturally high due to high river flows. 

It’s important to keep in mind, extremely high water levels are inefficient for generating megawatts as water must be passed though log sluices and gates rather than the turbines that generate power.

Flooding occurs when the volume of water flowing in a river or stream exceeds the capacity of the channel. Snow melt runoff floods are the most common type of flooding in our province.

Meteorological conditions vary from day-to-day and year-to-year. Spring floods are affected by multiple factors, and no two Freshets are alike. Some years may appear to have similar overall meteorological conditions but differing local weather patterns over different sections of the basin can make a big difference in the degree of flooding experienced.

Reference: Ottawa River Regulation Planning Board FAQ

Weather conditions (rainfall in particular), the natural characteristics of the downstream river such as narrows and rapids, and a town’s geographic locations compared to the principal reservoirs, can all impact both the timing of, and degree to which, different towns experience flooding.

We don’t expect flooding to happen every other year. On any given year, there is 5% chance of having a medium flooding event (e.g., a 1:20 year flood) and a 1% chance of an exceptional flood event (e.g., a 1:100 year flood). However, it’s possible for large flooding events to cluster rather than being evenly distributed in time.

Reference: Ottawa River Regulation Planning Board FAQ

Glossary of terms

Drainage basin

An area of land that channels rainfall and snowmelt into a body of water or stream.

Freshet

A large, seasonal increase of water discharged into a watershed due to snow melt and/or rainfall.

Reservoir

An area upstream of a dam where water is or can be stored for a long period of time (several weeks and sometimes months). A large reservoir can regulate (or alter) the flows in the downstream river section.

Run-of-river dam

A type of hydroelectric facility with little to no reservoir for storing water. This type of facility has little ability to regulate (or alter) the flows in the downstream river.

Runoff

The excess water, from precipitation or spring melt, which isn’t retained in the ground and flows into the surrounding streams.

Storage

The available capacity or space (volume) within a reservoir to storage or hold water.

Tributary

A stream or river that flows into a larger lake or river, such as the Petawawa and Gatineau rivers.

Watershed

Area of land that channels rainfall and snowmelt into a body of water or stream.

The physical effects of climate change are noticeable across Ontario. Operating a fleet of 66 hydroelectric generating stations and 241 control dams across the Province, we’re acutely aware of the potential for climate change to impact our facilities and their operations. Despite these impacts, we have an ongoing commitment to manage our rivers systems for the safety of people and the environment.

For more than a decade, we’ve been studying the impacts of climate change on the spring Freshet and water availability on a year-by-year basis. With our partners at the Ouranos Consortium on Regional Climatology and Adaptation to Climate Change, we explore challenging questions to help us learn how to mitigate potential climate change impacts on the safe operation of our facilities, and to understand changes in operations that may result from climate change.

Our findings align with Canada’s Changing Climate Report on how climate change will alter the spring Freshet in Ontario, including earlier peak spring water flows and higher winter water flows. We’ve recorded overall warming trends and anticipate increased precipitation in the years ahead. We expect the greatest precipitation increases during the winter and spring months, although we also expect more frequent and earlier melting.

We will continue to access world-class expertise to help us understand and adapt to the impacts of climate change during Freshet and year-round, and we’ll continue to engage communities, stakeholders and regulators to develop local operational solutions to Freshet changes. Our safe and flexible dams provide options for Ontario to resist the impacts of climate change.