Parks Canada officials say the grizzly bluff-charged a hiker who had come upon the bear while it was feeding on a carcass.
They say the bear ran out of a bush and came within five meters of the hiker, who was not injured.
Conservation specialists have put up closure signs and tape to clearly mark the prohibited area.
The trails will remain off limits until the bear wanders off.
Anyone caught disobeying the closure faces a fine of up to $25,000.
Parks Canada is reminding people to be particularly vigilant as grizzly bears have recently come out of hibernation. This means they are very hungry and focused on finding food.
Surprise bear encounters may happen at any time and can lead to defensive behaviour by the animals.