As parts of Fredericton remain under water and southern communities along the St. John River prepare for possibly the worst flood in 45 years, many ask what role the Mactaquac dam has in the flooding.

The dam, opened in 1968, is operated by NB Power and is a key part of the system that regulates the flow of water into the lower basin of the St. John River.

Some residents have questioned why the 96-kilometre-long headpond is so low, and why the utility can't do more to control the amount of water getting into the river system.

Hydro engineer Philip Gilks said the headpond is lower than normal, and there is a reason for it.

"Our operators have an operating curve that they must follow," he said. "If the flows exceed 80,000 (cubic feet per second), that's about the capacity of the plant, then they have to start spilling through the gates."

Gilks said it's at that point the operators have to draw the headpond down.

"What that does is create a slope on the pond — water needs to run downhill," he said. "It seems counter-intuitive that the pond is so low, but if you think about it that way, if we don't create the slope on the pond, then the water can't exit the river."

Follow guidelines

Gilks said the decision about whether to release water is based on guidelines developed when the dam was built.

"They've been updated, reviewed and modified slightly," he said. "As the flows increase, we have to create that slope on the pond."

Gilks said he understands why people get upset when they see the headpond so low, but he said it is what must be done to emulate the natural flow of the river.

The hydro engineer said it may seem like a big headpond to most, but the reality is when the St. John River is swollen it's not.

Gilks said the headpond is too small to hold back the river. (CBC)

"The Mactaquac is a big dam, but the pond is not, and if we were to hold the river back, it would only be a matter of hours, a few hours, until the pond would be full and we would have to release."

If that happened, he said, it wouldn't be a natural flow of the river and NB Power would be scrambling to manage it.

"It really isn't a practical solution," he said. "We really can't hold the river back. It's just too much water coming down for the size of the headpond."

Gilks said another reason to keep the headpond low is to give operators time to react if the water comes faster than predicted.

"It's unfortunate that the flooding is happening, but none of us can stop that," he said. "We live on the river. That's just what we have to live with. All we can do is manage it the best we can to minimize the impact."