The Planning Board publishes the most frequently asked questions (FAQs) to provide residents of the Ottawa River Basin and those living downstream of the river in the Montreal Archipelago with information on how integrated management of the principal reservoirs within the Ottawa River Basin is implemented, limitations of flow regulation, understanding the river’s flow regime, and other topics of interest.
These FAQs represent questions that have commonly been asked or address areas where the Planning Board feels additional information would be of interest to the public. Please send your comments, suggestions, and questions to the Ottawa River Regulation Secretariat.
In 1983 Canada, Ontario and Québec created the Ottawa River Regulation Planning Board (the Planning Board) to ensure the integrated management of the flows from the principal reservoirs of the Ottawa River basin. The goal of this integrated management is to minimize the impacts of floods and droughts along the Ottawa River and its tributaries, particularly in the Montreal Region, while maintaining the beneficial water uses such as hydro-electric energy production and preserving statutory or environmental levels and discharges in respect of other interests.
Under the 1983 Agreement Respecting the Ottawa River Basin Regulation, the governments also established two other entities that report to the Planning Board, namely the Ottawa River Regulating Committee and the Ottawa River Regulation Secretariat, which act respectively as the operational arm and working arm of the Planning Board.
The Planning Board consists of seven members who represent Canada (3 members), Ontario (2 members), and Quebec (2 members). The member agencies that make up the Planning Board are:
- Ministère de l’Environnement et de la Lutte contre les changements climatiques du Québec
- Ministry of Natural Resources and Forestry of Ontario
- Ontario Power Generation
- Environment and Climate Change Canada
- Public Services and Procurement Canada
- Canadian Coast Guard
The Planning Board also ensures that relevant hydrological information for example forecasts of river flows and levels along the Ottawa River is made available to the public and government organizations, especially provincial agencies given that the preparation and issuance of flood-related messages along the Ottawa River are a provincial responsibility.
There are currently thirteen principal reservoirs within the basin that are subject to integrated management under the policies of the Ottawa River Planning Board (the Planning Board). These reservoirs, which have usable capacities of more than 200 million cubic metres, are owned and operated by the four agencies that comprise the Ottawa River Regulating Committee (the Regulating Committee). The term “integrated management” means integrating the reservoir decision-making processes of these four agencies into one common process that provides a common information and decision-support system. This exchange of information via the Regulating Committee allows operators to see the effects of their flow regulation decisions at the reservoirs on downstream river flows and levels and to make adjustments on a day-to-day basis, aiming to minimize the impacts of floods and droughts.
It should be noted that the Planning Board role is often misconstrued to be that of a “control board,” which it is not. The Planning Board does not have legal authority over the decisions of the operators of the principal reservoirs. Each operator remains responsible for the operational strategies and decisions at its facilities.
The Ottawa River Regulating Committee (the Regulating Committee) is responsible to the Ottawa River Regulation Planning Board (the Planning Board) for the integrated management of the principal reservoirs. It is comprised of the four agencies that manage the principal reservoirs within the basin, namely Hydro-Québec, Ministère de l’Environnement et de la Lutte contre les changements climatiques du Québec, Ontario Power Generation, and Public Services and Procurement Canada. The members are responsible for the day-to-day operations of the principal reservoirs and of other dams along the river and tributaries.
The Regulating Committee, a consensus based unit, establishes agreement on appropriate regulation strategies that are in line with the regulation policies and criteria adopted by the Planning Board. The work of the Committee includes agreeing on actual river conditions, forecast river conditions, and operational strategies such as increasing or decreasing the control over flows at the principal reservoirs. It is also responsible for releasing relevant information to the public and other organizations.
The Ministry of Natural Resources and Forestry of Ontario is an associate member of the Regulating Committee. It contributes important hydrometeorological information and plays a key role is disseminating information in Ontario.
The Regulating Committee may recommend new regulation and operational practices and procedures to the Planning Board when needed.
In order to provide integrated management of the principal reservoirs there is a continuous flow of information between the agencies of the Ottawa River Regulating Committee (the Regulating Committee). The Ottawa River Regulation Secretariat (the Secretariat) acts as a data processing centre and assists the Regulating Committee in all aspects of its work.
During the spring freshet, or other unusually high flow events, there is a daily exchange of data between the agencies of the Regulating Committee and the Secretariat. The data received from the agencies includes real-time data of water levels and flows collected from remote sensing equipment located throughout the basin, results of snow surveys, as well as declared operations at their structures for a period of ten days. The Secretariat combines this data with forecasted inflows for all tributaries and other points in the basin as input to a flow routing models that predict water levels and flows at different points throughout the basin. The model results are evaluated by the Regulating Committee and Secretariat and discussed at a conference call during critical periods to confirm or change operational strategy in order to minimize the impacts of flooding. This process is repeated every day during high water events and once a week at other times.
Flood reduction measures are undertaken annually by members of the Ottawa River Regulating Committee (the Regulating Committee) in preparation for the spring runoff. Typically this involves emptying the principal reservoirs during the winter period with reservoirs being at their lowest levels before the spring snow melt begins. This available storage volume is then used as the spring melt progresses to reduce downstream flows. The integrated reservoir management that is achieved through the Planning Board mandate optimizes the use of this storage and allows dam managers to coordinate operations between multiple reservoirs, on a daily basis, in order to minimize flood impacts along the Ottawa River, its major tributaries and in the Montreal region.
It should be noted that it is not possible to hold back the entire runoff volume since runoff, during the spring freshet, generally exceeds the capacity of the reservoirs to store water. Except for years when there is little snow and precipitation, most of the principal reservoirs must release the bulk of the runoff draining into them. In addition, the runoff from a significant part of the basin cannot be retained as approximately two thirds of the basin does not have significant storage capacity to mitigate flows.
The Regulating Committee, supported by the Ottawa River Regulation Secretariat, makes relevant hydrological forecasts resulting from integrated management available to the public and government organizations, especially provincial agencies given that the preparation of flood messaging along the Ottawa River and tributaries are a provincial responsibility.
The Ottawa River Regulating Committee works closely at all times with provincial agencies responsible for issuing flood-related messages.
In Ontario, Conservation Authorities and District Offices of the Ministry of Natural Resources and Forestry issue flood-related messages and information to municipalities and other key agencies involved in flood preparedness and response. Current flood-related messages can be viewed on-line on the individual conservation authority websites and on the provincial flood webpage: https://www.ontario.ca/flooding.
In Quebec, the Centre des opérations gouvernementales du Québec and the Sécurité civile collaborate with municipalities to protect residents. They are informed of relevant hydrological forecasts by the Ottawa River Regulation Secretariat (for the main stem of the river), and member agencies of the Regulating Committee depending on the tributary or facility of interest. Monitoring of flood conditions by Quebec can be viewed at https://geoegl.msp.gouv.qc.ca/adnv2 .
The Ottawa River Regulation Planning Board uses its website (www.ottawariver.ca) as the main tool for making hydrological forecasts available to the public. Current and forecast conditions on the Ottawa River along with conditions at the major reservoirs in the system are available on the website. A general four-day forecast is also provided at key locations within the basin during the spring freshet period or other high water events.
The Regulating Committee issues a news release when the spring freshet season is starting in the Ottawa River basin and when it anticipates that river conditions on the main stem of the Ottawa River (between the Lake Timiskaming outlet down to the Montreal region) could exceed significant flood levels.
When abnormal river conditions are expected and that there is no risk of exceeding flood level thresholds, or when special operations for a specific structure or river reach are planned, the Regulating Committee or one of its members informs residents by posting a news bulletin on the website.
Residents can follow the Ottawa River Regulation Planning Board Twitter feed (www.twitter.com/ORRPB) to be notified when news releases are issued, when daily or the general four-day forecasts begin on the website, or when news bulletins are posted on the website.
In spring, the amount of runoff increases causing the watercourses to swell, which naturally increases the water levels throughout the river system. However, in some areas upstream and in close proximity to “run-of-the-river” hydroelectric stations, the opposite can happen where operators lower water levels to limit upstream flooding.
It should be noted that, due to the natural characteristics of some areas of the river, the water level upstream of a hydroelectric facility can create a backwater effect over several kilometres. In some cases this effect can extend upwards of more than 50 km. Consequently, as the river flow increases, some stations must lower their water levels to limit flooding in communities located far upstream by maintaining water levels as close as possible to natural conditions, i.e. conditions before the generating station was built.
During this period when levels are kept low, the flow of water through the facility is the same as the flow of the river. Attempting to hold back any water and maintaining higher water levels upstream of the dam could pose serious risks to shoreline residents and to the run-of-the-river facilities that have a limited reservoir capacity.
Flood impact reduction during the spring freshet and other high flow events is achieved mainly by storing the spring runoff in the thirteen principal reservoirs that are subject to integrated management under the 1983 Agreement. Seven of these reservoirs can influence the flows of the Ottawa River between Rolphton and Fort Coulonge. The six northernmost (from Lake Timiskaming and further north in the Abitibi area) have a significant storage capacity and are the first to be utilized in the regulation strategy to minimize flood impacts. The timing of the refill at these locations is optimized to mitigate flooding all along the Ottawa River below Lake Timiskaming. These measures aim at reducing northern discharges while local uncontrolled tributary rivers such as the Petawawa and the Coulonge reach their peak and start to recede.
The Des Joachims facility, which includes the seventh reservoir, can create a backwater effect all the way upstream to Mattawa (similarly to facilities described in Q5). This is why the spring refill strategy at Des Joachims consists of two stages. The purpose of the two stage refill is to balance the risk of flooding in upstream and downstream areas.
During the first stage of the refill, the water is brought up only about halfway to the top of the reservoir. This enables low water levels to be maintained upstream of the facility to limit the amount of backwater effect created and assist with the mitigation of downstream flooding by using some reservoir storage. These low (“natural”) levels are sometimes remembered only by elders who have known the river before Des Joachims was constructed in 1950.
The second stage of the refill, when the rest of the reservoir is filled, is completed once the probability of upstream flooding in Mattawa is reduced. It is only then that dam operators return to normal operating levels.
Residents along Lac Coulonge will observe similar flood mitigation strategies with the lowering of water levels above the Rocher Fendu and Bryson facilities. At these facilities, water levels are maintained at a lower elevation during spring freshet when required in order to minimize upstream levels.
The Ministère de l’Environnement et de la Lutte contre les changements climatiques du Québec has a website with current and forecast water levels and flows available for the Montreal area at the following URL:
- Forecasts for the Montreal Archipelago by MELCC (only available during the spring; in French only)
At certain times of the year, the Ottawa River has a considerable effect on the flows of the St. Lawrence River in the vicinity of the Montreal archipelago. This is why the Ottawa River Regulating Committee liaises and shares relevant flow forecasts of the Ottawa River with the Great Lakes ‐ St. Lawrence Regulation Office, who is responsible for carrying out the day-to-day regulation activities for the International Lake Ontario – St. Lawrence River Board.
Plan 2014 has no effect on flows and levels in the Ottawa River upstream of the Carillon dam because of specific reasons:
- Existing river characteristics: Under all water level conditions, even flooding in the Montreal Archipelago (Lake St. Louis), it is physically impossible for water to back up in to the Ottawa River upstream of the Carillon Dam. The difference in the water elevations upstream of the Carillon dam and in the St. Lawrence River is too large. The difference in water elevations along the Ottawa River is illustrated in this figure.
- Large portion of the basin flows are uncontrolled: The excess water from precipitation or spring melt from the central and southern parts of the Ottawa River basin, representing roughly 60% of the basin area, is uncontrolled. This means that even if dam operators wanted to hold back the amount of water flowing in the river, they could not as facilities located in those parts of the basin have no available reservoir storage. It is true that water levels along much of the main stem of the river are regulated during most of the year; however, during the spring flood period, flows are naturally so high that the dams are operated as run-of-river facilities, passing all arriving flow, and could not be subject to operational restrictions dictated by Plan 2014 on the St Lawrence River. More information on the management of water at dams is provided in the Frequently Asked Questions 2019 (Questions B, D, E and I).
- Carillon Dam does not regulate upstream levels during flooding: Outside of the spring flood period the dam regulates the water level in the upstream river reach between Hull/Ottawa and Carillon with the observed difference in water elevation between Hull/Ottawa and Carillon being approximately 50 cm. During the 2017/2019 spring floods, the difference between water elevations in Hull/Ottawa and Carillon was approximately 450 cm (i.e. 4.5 m or roughly 15 feet). During large flood events, the amount of water flowing in the river combined with the effects of the narrows at Grenville (aka the Long Sault rapids), located approximately 20 km above the Carillon dam, control the upstream levels. The Carillon dam is operated as a run-of-river facility, allowing all arriving upstream flows to pass, in order to ensure that upstream river levels are prevented from rising above levels that would occur naturally under the same flood flow conditions.
Answer: Because of reduced evapotranspiration during the fall, soils remain wet longer after it rains and cannot absorb as much rainfall as in the summer. Consequently, when it rains, more water runs off (runoff) and flows to streams and the Ottawa River, which causes water levels to increase.
Explanation: During the fall, as days become shorter and temperatures become cooler, the vegetation becomes dormant: trees shed their leaves and vegetation stops growing. With lessened canopy, vegetation is not intercepting rainfall as much or taking up the water contained in the soil. This natural phenomenon leads to less water being evaporated from surfaces and less transpiration whereby plants convert soil water into vapour. Such reduced evapotranspiration rates mean that the soil remains saturated with water from rain over a longer period. Rain water that fails to seep into the soil (which is sometimes referred to as ‘excess rainfall’) runs off the land and into lakes and streams. This runoff quickly follows streams’ courses to reach the Ottawa River. In the fall, in most areas along the Ottawa River, the larger the volume of water flowing into the river, the higher the water levels will be. It is common to see water levels rise rapidly when heavy rainfall occurs over a significant part of the watershed.
Normal Flow Range of the Petawawa River in m3/s
This greater influence of rainfall on runoff during the fall leads to greater river flow variability and overall volume, as shown in the figure. The figure presents the normal flow range of the Petawawa River, a natural river that is not regulated by control structures. The green band depicted on the figure is defined as the normal range of observed flow values and consists of the flows observed in the river 50% of the time over the past 30 years. The median value (illustrated as percentile 50%) is a measure of the middle conditions, while percentiles 25% and 75% define the normal boundary. A wider band means that flows vary more at this time of year. On the other hand, a narrower band means that the conditions are less variable.
Operationally, principal reservoirs located in the head-water areas of the basin are full at this time of the year. All excess rainfall that results in increased river flows must necessarily pass through the reservoirs system as there is no available storage to retain that water.
On the main stem of the Ottawa River, increased river flows do not require significant changes in operations at hydroelectric facilities. In the fall, water levels at hydroelectric facilities may still be in the higher end of their normal operating ranges to satisfy recreational commitments. These water levels are not associated with any flood risk. It is important to note that river conditions are always monitored closely. Should forecasts call for a possible risk of flooding, which is rare in the fall period, the levels at the main stem facilities are adjusted to make way for the natural runoff. For more information on these operations, refer to FAQ D – How are facilities located between Lake Timiskaming and the Deux-Montagnes area managed during periods of high flows?’.
 A percentile is a type of statistical representation of a value in terms of percentages. For example, the 25th percentile is the value below which 25% of historical flows are found, and above which 75% are found.
Answer: Several factors influence the extent and magnitude of the spring flood, the main factors being (1) the recharge state of the water table at the end of winter, (2) the amount of water contained in the snow on the ground before the start of spring melt, (3) the speed of melting of the snow cover and (4) the presence or absence of rains during the melting period.
Explanation: It is important to state that these ‘factors’ are elements that contribute to flooding. They should not be considered ‘indicators’ of future floods, as accurate knowledge of important factors, such as rainfall, are not known until a few days before they occur.
The extent of the spring flood is determined by (i) the volume of water from the melting of the snow cover throughout the watershed, and (ii) the rainfall that will be received during the melting period. While it is impossible to know how much rain will be received more than a few days in advance, certain factors that influence the melting snow volume may occur in the months leading up to the spring flood.
The first factor is the degree of soil saturation. If the late fall and early winter rains are heavy and the soil is soggy during the winter frost, the soil will have a high degree of saturation, which will decrease the soil’s ability to absorb water from melting snow cover in the spring. Important winter thaws can also have the same effect. In other words, soil saturation helps to increase the volume of water in the river, by increasing the base flow.
The second factor is the amount of water in the snow (also known as ‘snow water equivalent’) just before the start of melting. It is often believed that total depth is to be closely monitored, but it is rather the snow water equivalent that really matters. A layer of snow that has diminished and become dense following thaw-freeze cycles may contain more water than a thick blanket of fresh snow. Depending on late winter weather conditions, a significant portion of the snow cover may also disappear by sublimation (similarly to snow disappearing from roofs even though it’s too cold to melt). In addition, mild weather periods in January or February may turn some of the snow cover into runoff that will leave the basin during the winter period. The condition of the snow cover at the beginning of March normally gives a good first indication of the volume of water to come.
The magnitude of the spring flood (or peak flow) is determined by the precise time when maximum water from melting snow and rain moves through the river. The main factors influencing the size of the peak are (i) the speed at which the snow cover melts, (ii) the presence or absence of weather systems with heavy rainfall during the melting period, and (iii) the duration and trajectory of these systems. Because the basin is very large, the rainfall runoff from the northern part of the basin can take up to two weeks to flow through the watershed. For this reason, the exact sequencing of significant weather events can have a cumulative effect on peak flows, for example heavy rain in the northern part of the basin, followed by heavy rain in the southern part of the basin a week later. The 2017 flood is a very good example of this phenomenon where two consecutive systems affected the basin. Unfortunately, these rare systems can be forecast only a few days in advance.
For more information on this topic, see our FAQ – Flood of 2019: A – What causes flooding?
2019 Flooding FAQ
Answer: Flooding occurs when the volume of water flowing in a river or stream exceeds the capacity of the channel. Snowmelt runoff floods are the most common type of flooding in Canada. Government of Canada – Snowmelt Runoff ( https://www.canada.ca/en/environment-climate-change/services/water-overview/quantity/causes-of-flooding.html#snowmelt ).
Explanation: Numerous factors affect the volume of water flowing in a river and, therefore, the potential for flooding. The most important factors are the amount and type of precipitation, the nature and condition of the drainage basin (or watershed), and climate. During the Canadian winter, most of the precipitation is simply stored as snow or ice on the ground. During the spring melt, large quantities of water are released and can combine with rainfall, which can lead to heavy spring flow and flooding. This is called freshet.
In spring when the soil is saturated and still partly frozen, the rapid melting of snow means that there’s little opportunity for the water to be absorbed. A large portion of the water contained in the snow ends up in the river. In addition, when rainfall occurs along with the snow melt, a larger portion of the water contained in the snow combines with rainfall and runs off on the ground surface to low lying areas and streams.
Overall, a land area twice the size of New Brunswick drains to the Ottawa River. As an example, assuming that 50 mm of rainfall were to be received over much of the watershed and that half of this rainfall would run off to the river, this would represent an additional volume of 3,650 million cubic metres of water into the river. That’s more water than needed to fill 1,000,000 olympic-sized swimming pools.
(Figure credit: Environment and Climate Change Canada)
Answer: In years where spring runoff is significant, it is not possible to prevent flooding.
Explanation: The primary way to prevent or reduce flooding is to store spring melt or runoff in the principal reservoirs. By storing or keeping water in these reservoirs, the volume of water (or river flow) is reduced downstream. The principal reservoirs in the Ottawa River basin are large enough to hold on to approximately 40% of the average spring runoff. By using the principal reservoirs in this way, it is possible to prevent significant downstream flooding in most years.
Flooding occurs on the main stem of the Ottawa River when:
- The spring runoff greatly exceeds the size of the principal reservoirs — Each year, the principal reservoirs are emptied over the course of winter and then filled during the spring period of several weeks and sometimes months. Once full, principal reservoirs must pass what flow they receive to downstream areas.
- There is significant spring runoff in areas where there are no principal reservoirs — Facilities on the Ottawa River in the central and southern parts of the watershed have very little ability to store spring runoff. During periods of high flows these facilities operate as run-of-river dams. The storage volumes in the central and southern parts of the watershed are very small compared to the overall flow volume during the freshet period. For instance, if the dams at those facilities were to be completely closed when the freshet is at its peak, they would fill up and worsen upstream flooding within hours. To significantly reduce downstream flooding, a facility needs to be able to store water upstream over a period of several days (sometimes weeks) depending on river conditions.
Carillon Dam located in the southern portion of the Ottawa River on April 25, 2019
Answer: Given the multi-jurisdictional nature of the Ottawa River (shared between Ontario, Québec and Canada), there is no one agency responsible for the regulation of water. A group of agencies, each responsible for their own operations, comprise the Ottawa River Regulating Committee and work together with the purpose of integrating the management of the flow from the thirteen principal reservoirs in the Ottawa River basin. During the spring, the goal of this management is to minimize the impacts of flooding as much as possible.
Explanation: The Ottawa River Regulation Planning Board is not a control board. It is a board created by governments for the purpose of ensuring that the flows from the principal reservoirs are managed in a coordinated way by the four agencies that operate them. These agencies are Ontario Power Generation, Hydro-Québec, the government of Canada and the government of Quebec. The Ottawa River Regulation Secretariat, composed of two staff engineers, support the work of the Regulating Committee and the Planning Board and communicate with the media, public and government agencies on water flow and level conditions on the Ottawa River.
Each of the four agencies remains responsible for managing its facilities and principal reservoirs. Water management requirements are subject to provincial and federal legislation and regulations. These water management requirements, which consist of respecting statutory and environmental levels and discharges do not apply to every part of the river nor every possible situation. For instance, in river sections between Lake Timiskaming and the Deux-Montagnes area, the peak water levels reached in flood-prone areas during the freshet are the result of natural temperatures, precipitation and local natural restrictions in the river. These peak levels may exceed statutory maximum levels in river sections that are subject to such regulation if river flows become high enough.
 Public Services and Procurement Canada
 Ministère de l’Environnement et de la Lutte contre les changements climatiques of Québec
Answer: Facilities on the Ottawa River between Lake Timiskaming and its outlet at Deux-Montagnes are considered to operate as run-of-river dams (or facilities) during high flows (refer to the glossary).
Explanation: These facilities cannot reduce the levels and flows in the river given that they have minimal storage ability. At these facilities, before high flow periods occur, the level at the facility is adjusted to ensure that upstream, naturally occurring, flooding will not be made any worse by the presence of the facility as the river flow increases further. Once the level is adjusted, the facility passes any water that arrives at the dam, which is typically described as “passing inflow”. When passing inflows, operators increase the flow through a facility as the arriving river flow increases. A flow increase through a facility is achieved by removing logs from log sluices or opening control gates. Similarly, the flow through a facility is decreased when the flow of the river recedes. A flow decrease through a facility is achieved by replacing logs in log sluices or lowering gates deeper into the water. However, these flow changes do not change the fact that further upstream from the facility the water levels are controlled by the natural features of the river and not the facility. (Refer to Question E).
During a period of high flows that cause flooding, the main stem of the river is considered unregulated because water cannot be stored in this section of the river to further reduce the flows and water levels.
Answer: Operators of Carillon, Chats Falls and Bryson would reduce levels in river reaches located upstream of the facility where there is flooding if they could. However, it is not physically possible to do so. Further opening of gates and log sluices would not make water levels go down. In the flooded area upstream of these facilities, the water levels are determined by the high river flows and the natural constrictions on the river and other features that are upstream from the facility. During high flow periods when flooding is occurring, operators of these three facilities lower the water level just upstream of their facility. The lower level at the facility transfers control of water levels in the flooded area to the natural occurring physical constriction (such as rapids) of the river that existed prior to development.
Explanation: In the spring, water levels on the river increase naturally because the volume of water that flows into the river is increased by the natural runoff from snow melt and rainfall. The natural constrictions on the river and other features such as islands restrict the flow of water. Under natural flow conditions in a river the water level will increase as the flow increases.
There is no way to force water flow through the river system. It must flow out of the river section naturally, at a speed that is governed by the physical characteristics of the river (e.g. slope of the river bottom, natural narrows such as rapids, width of the river, river-bed and bank type such as rocks or sand). As long as the level at the facility is low enough to maintain control at the upstream natural control section then any further flow increases at the facility will not result in a change of water level on the upstream side of the natural control. If the facility were to reduce the flow, the level would rise and the facility would eventually regain control. The high level at the facility would then result in the level upstream of the natural control point rising higher than would occur prior to development. That is part of why these facilities do not reduce the river flow during periods of high water. Instead, they “pass inflows” as described in Question D.
Answer: Lowering water levels sooner would not change the degree of flooding in the spring.
Explanation: At the principal reservoirs, water levels are lowered progressively from approximately mid-December to the end of March. The Des Joachims reservoir, which is the smallest of the thirteen principal reservoirs, requires only one month to empty. Des Joachims is emptied every year during the month of March. Water that was stored in the principal reservoirs flows out of the river before natural runoff could cause flooding to downstream locations or local communities. People can follow the emptying and filling of the principal reservoirs every year on the Planning Board’s website.
This strategy allows the flows from the principal reservoirs in the Ottawa River to decline to a minimum at the end of March, prior to the beginning of the usual snow melt period. The remaining flow in the river, just before the spring starts, is the total of all of the natural tributaries (Petawawa River, Rouge River, etc.) in the system and is termed the base flow. This base flow varies naturally depending on overwinter conditions and thaw cycles.
Facilities that are operated like run-of-river dams during periods of high flows have small amounts of upstream storage and it requires only a few days to lower the level of the water that is held back above their dams (that is in the river sections immediately upstream of the facility). In the spring, operators follow river condition forecasts very closely in order to prevent higher water levels above their facilities. Facilities that operate as run-of-river during period of high flows lower water levels just before the snow melt runoff causes river flows to increase significantly. This can be observed above some of the run-of-river facilities on the river by the low levels seen just above the facilities. Lowering water levels at the run-of-river facilities in February when the river flows are low would result in low water levels for an extended period of time, possibly a period of ten weeks, and would not reduce the flooding during the spring melt.
Answer: The principal reservoirs help reduce flooding in downstream locations by storing large volumes of water and thus reducing flows in the lower river sections, thereby mitigating the downstream flooding. Run-of-river dam facilities located along the length of the Ottawa River do not cause flooding or make naturally occurring flooding any worse (refer to FAQ 8 and 9).
Explanation: During the 2019 spring freshet, the regulation strategy consisted of reducing water flow from the principal reservoirs while southern tributary rivers such as the Petawawa, Dumoine, Coulonge, Mississippi, Petite Nation and Rouge rivers reached their peak and started to recede. It is estimated that by optimizing the use of storage during this year’s flooding that peak water levels along the main stem of the river were reduced by a minimum of 40 cm in all locations.
More detailed analysis that provides specific values for reductions in water levels will be provided in the Summary of the 2019 Spring Flood document that will be prepared by the Regulating Committee.
Answer: No. Hydropower generation is more efficient with a high water level upstream of the dam and low water levels downstream of it. During a flooding event, the opposite occurs where upstream water levels on the river are lowered to prevent flooding and downstream water levels are naturally high due to high river flows. Also, the freshet period is not associated with high electrical demand as in the winter or in the summer during heat waves.
Explanation: Flood mitigation and public safety are the main focus of water management activities by Hydro-Québec and Ontario Power Generation during the spring melt. Flood conditions are inefficient for producing hydro-power and result in spill through gates and log sluices. Most water does not pass through the turbines and does not produce electricity, providing no benefit to Hydro-Québec or Ontario Power Generation.
Answer: No. All communities located downstream of the principal reservoirs benefit from the water that is stored and therefore prevented from flowing downstream during flooding.
Explanation: Flood impacts in the Montréal Archipelago are mitigated by holding (or storing) spring runoff in the principal reservoirs to reduce the peak of flood flows. Since the major reservoirs are located primarily in the northern sectors of the Ottawa River basin and with water flowing north to south, all river sections benefit from water storage during the same flood event. Also, with peak flows occurring at different locations along the length of the river within two to three days of one another the benefits of the use of reservoir storage to reduce peak levels is applicable to the whole length of the river. For instance, on the upper Ottawa River, the town of Mattawa benefits from the presence of the six principal reservoirs located in the Abitibi-Timiskaming area, with flood flows being lower than would have occurred without the presence of the principal reservoirs. Similarly, downstream residents in communities such as Constance Bay on Lac Deschenes benefit from eight principal reservoirs storing water. With two principal reservoirs located on the Gatineau River and three located on the Lievre River that feed into the Ottawa River downstream of Lake Deschenes, the lower reach of the river between Lake Deschenes and the Hawkesbury – Grenville area benefits from the thirteen principal reservoirs, in the same way that the Montréal region eventually does.
Answer: Two major rainfall events from Colorado and Texas merged to bring 20 to 50 mm of precipitation over much of the Ottawa River Basin, at a time when spring snow melt runoff had already caused water levels to exceed major flood levels in most flood-prone areas along the Ottawa River. These weather events were the first of several that would hit the watershed and increase flood levels.
Explanation: Unfortunately, it is not possible to forecast precisely the quantity of precipitation that storm systems will bring more than a few days before they occur. Three days before the two major rainfall events arrived, on April 16, 2019, the Regulating Committee published a News Release to caution residents that based on the forecast rainfall and temperatures, levels and flows along the Ottawa River between Lac Coulonge and the Montréal Region could reach conditions observed during the first peak of the 2017 flood. However, two days later and one day before the rain began, weather forecasts were calling for much more rain. This is why on April 18t, the Regulating Committee issued a further News Release informing residents of an increased risk of flooding, with river conditions possibly as severe as in May 2017 to be expected.
Answer: Yes. The Ottawa River Basin was hit by heavy rainfall when the melting of a deep snowpack was already causing historic floods on the tributaries that feed into the Ottawa River.
Explanation: The water contained in the snowpack was approximately twice that of a normal year in early spring. The spring was late in coming with below average temperatures with snow still accumulating over much of the watershed. With little sunshine and cooler than normal temperatures, there was little reduction in the snow pack during the early spring period. April was very wet with approximately twice as much rain as normal with a sequence of heavy rainfall events through April and in to May, which added additional runoff to streams that were already swollen by significant snow melt volume.
Many tributaries (Petawawa, Coulonge, Madawaska, Petite Nation and Rouge rivers)located in the central portion of the watershed had record breaking flow rates which fed exceptional quantities of spring runoff into the Ottawa River, causing the first peak to occur between April 29th and May 1st. Other tributaries such as the Mississippi and Bonnechere rivers experienced significant flooding impacting communities along the length of their shores. Many smaller tributaries experienced unprecedented high flows that washed away portions of roads and culverts.
Answer: Meteorological conditions vary from day to day and from year to year. Spring floods are affected by multiple factors, and no two are alike. Some years may appear to have similar meteorological conditions; however, different weather patterns, over different sectors of the basin can make a big difference in the degree of flooding experienced in some locations.
Explanation: Two years that have apparently similar weather characteristics may have very different spring floods. For example in the Lac Coulonge area, flooding occurred in 1985 and did not occur in 1984. Yet, both years had similar total precipitation. Where exactly precipitation occurs and if it is concentrated over a short or longer period of time can affect a particular location in different ways. In the case of flooding in the Lac Coulonge area, one such analysis would reveal that the peak flow rate of the Coulonge River in 1985 was about twice as large as that in 1984. Flooding on natural, unregulated tributaries, is an indication that the received precipitation was of sufficient intensity to cause downstream flooding.
Another example is the depth of the snowpack, which is used as an indicator of the amount of water available to melt in spring. Generally, the water contained in the snowpack is only an indicator of the possible river conditions during the spring given that a large portion of the snow could be lost to the atmosphere.
Answer: Weather conditions, the natural characteristics of the downstream river such as narrows and rapids, and a town’s geographic locations compared to the principal reservoirs in the north govern the timing of and the degree to which different towns experience flooding. Also, the particular areas which are receiving rain can impact where flooding is the worst.
Answer: Flooding is not expected to happen every other year. On any given year, there is 5% chance of having a medium flooding event (for example a 1:20 yr flood) and there is a 1% change of having an exceptional flood event (for example a 1:100 yr flood). However, it is possible for large flood events to be clustered, like events that occurred in the 70s, rather than evenly distributed in time.
Explanation: Flows in the Ottawa River vary from year to year because the weather conditions over the watershed such as precipitation and temperatures change from year to year. The sequence of precipitation and temperatures that give rise to flooding is determined by nature. This is why the risk of flooding is often described as a probability of occurrence such as the 5% chance or 1% chance of flooding on any given year.
Some will remember the flooding that occurred in the 1970’s. Serious flooding happened in 1974 and 1976 in many areas. Yet, flows over the next two decades were generally within the normal range and did not cause serious flooding. Natural variability is unfortunately easily forgotten and this is part of why exceptional floods are so often unexpected. Significant flooding along the Ottawa River occurred in the 1920’s, the 1950’s, the 1970’s and in the 2010’s. Flooding will occur again, we just don’t know when.
Answer: Québec and Ontario agencies involved in the management of the principal reservoirs on the Ottawa River flows communicate daily throughout the spring flood season to assess together current and forecast river conditions and what actions may be required to minimize flood impacts.
Explanation: Agencies with principal reservoirs within the Ottawa River basin make up the Regulating Committee (refer to Question C). As part of the Committee’s work, operators of all principal reservoirs assess together current river conditions, forecast river conditions, operational strategies such as increasing or decreasing flows at the principal reservoirs and relevant information to governmental agencies and the public on expected river conditions.
In 2019, fifty conference calls were held during the spring freshet by the Committee.
Answer: Analyses and decisions pertaining to water management strategy are done by qualified and experienced technical staff. However, many processes are automated, for example the collection and transmission of hydrological data and the operation of some water flow control structures, which is under constant human supervision. In addition, various technologies are used to provide guidance and decision making support, for example meteorological and hydrological forecasting models. Modelling and forecasts are ultimately only used as input to the decision making process for the operation of the principal reservoirs.
Answer: Following the 2017 spring flooding, the Planning Board undertook several actions that involved communications. Given the importance of timely flow forecasts in protecting against flooding, it consulted with provincial agencies that are responsible for flood-related messaging to residents. Improvements to pathways of communication were subsequently identified and implemented. The Planning Board also heard the public’s desire for better access to river flow conditions and initiated a revamping of its website. In addition, the Planning Board and its member agencies, gave multiple outreach presentations to municipal officials in Ontario and Québec to raise awareness on the limits of regulation and ensure emergency management coordinators were aware of available forecast information.
Looking ahead, the Planning Board recognizes that communications have evolved tremendously in the last few years with social media. To adapt to this new context, it will assess what new ways can be used to communicate water management strategies rapidly and efficiently, especially during major hydrological events.