MRC PONTIAC & PONTIAC – Local residents have been outraged to see photos of dry riverbeds upstream when property owners in MRC Pontiac and Pontiac have been dealing with record flood levels. There have also been accusations of water control
MRC PONTIAC & PONTIAC – Local residents have been outraged to see photos of dry riverbeds upstream when property owners in MRC Pontiac and Pontiac have been dealing with record flood levels. There have also been accusations of water control
mismanagement by the Ottawa River regulation board. Many of the pictures originated from Deux-Rivieres (ON), a community upstream from the Rapides-des-Joachims dam/reservoir, showing large expanses of dry land surrounding docks.
Michael Sarich, senior water resources engineer with the Ottawa River Regulation Planning Board (ORRPBB), attempted to explain the phenomenon, which he says isn’t as easy as putting more water in some places and taking it away from others.
“You don’t want anybody to be making those kinds of decisions – saying ‘hold water there and it’s going to hurt those upstream, but will help downstream residents’– so the [dam’s] operational mandate is to let the river return to its natural state,” he told the Journal, explaining the entire process further.
Seven northern reservoirs can influence the Ottawa River’s level between Rolphton and Fort-Coulonge. Six of them have significant storage capacity while the southernmost, in Rapides-des-Joachims, does not. Reservoirs are emptied over the winter to prepare for the spring melt when they hold back as much northern water as possible to mitigate flooding downstream. Once they fill to maximum levels, water is slowly released. Spring flooding generally has two peaks; the first when natural runoff accumulating in the river reaches a maximum and the second when northern reservoirs start releasing water.
Regular dams, unlike reservoirs, have minimal capacity and are unable to significantly change the flow of water, so, in times of increased volume, they adjust their gates to allow water to pass through at the same rate it arrives. In some areas, this returns the river to what it was like before the dams existed. Low water levels will occur in stretches where water can move through quickly because of the geomorphology of the river.
“[The dams] are mandated to ensure their operations will in no way make flooding
any worse than what would have occurred under natural conditions. Because water levels are artificially raised above dams, there’s an effect upstream, so as flow increases, operators lower their water levels to ensure any flooding occurring upstream is no worse than if the dam never existed,” explained Sarich.
Due to natural characteristics of the river (narrowing, obstructions, etc.), some dams can have far-reaching backwater effects. Backwater occurs when the flow slows due to reaching some impediment, preventing it from moving through as quickly as it is arrives; it begins to pile up when there is high flow and creates very high water levels upstream.
Picture two pieces of pipe the same length: one 3” in diameter that reduces to 1” and another that remains 3”. If you empty the same amount of water into each pipe, water will flow through the pipe that remains 3” without a problem, but will move through slower and backup in the one that reduces to 1”. The same effect happens at certain dams, such as in Bryson and Rapides-des-Joachims.
“Consequently, as 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 before the generating station was built,” says the ORRPB website. Trying to hold water back would flood upstream communities and possibly damage infrastructure. In short, lower water levels help combat serious backups.
The Rapides-des-Joachims facility’s backwater effect can reach as far as Mattawa (100 km away), so this small reservoir is kept at half capacity in the spring to give some relief to downstream communities, while at the same time preventing serious backups. Once the probability of flooding is reduced in Mattawa, this reservoir is then filled to normal levels. “It’s been the same operations since 1950,” stressed Sarich, indicating nothing has changed this year for ORRPB.
Pictures of docks surrounded by dry land in Deux Rivieres, part of the Rapides dam/reservoir, is a matter of perspective, Sarich claimed. “It’s not a dry reservoir, it’s a very small dry portion and the amount of water that could be stored there is very small. If you used that storage over a couple days, you’d provide almost no relief downstream, but you’d definitely raise flood levels in Mattawa by half a meter. It’s the same at every locality; Rapides, Bryson, Chenaux and Chats Falls,” he concluded.
The ORRPB was established by the Canadian, Québec, and Ontario governments in 1983 in order to “ensure integrated management of the principal reservoirs of the Ottawa River Basin,”
to provide protection against flooding, and maintain the interests of users, notably hydro producers. According to Sarich and Francis Labbé, Hydro Québec Media Advisor, high volumes of rain and snow are to blame for the severe flooding this year and not much more can be done by the managers to prevent the problem. However, many regions, including Mansfield and the MRC Pontiac, have called for independent inquiries into the river management system.
