The city website suggests access via:

Boul. Maurice-Duplessis (east of CEGEP Marie-Victorin)
Boul. Maurice-Duplessis (west of boul. Hippolyte-Lafontaine)
Henri-Bourassa (west of autoroute 25)
Corner of Perras and Ozias-Leduc

stream tunnel stream
This small park is not a very accessible spot. There is nowhere to park a car. Take the 48 bus from Henri-Bourassa metro station and get off at the corner of Ozias-Leduc and Perras.

What is not obvious is how to actually enter the park at all, at any point. You’re on Perras here, with traffic moving to and from autoroute 25, and there are no sidewalks on the park side, and it's confusing because the park is initially mixed up with the grounds of a hospital. The actual entrance is inside the hospital gate and off to the left. Your one clue is that all entrances to the park are marked with gateposts like the ones shown at left.

As is obvious on the map below, the park is not wide. It’s basically a trail going from nowhere in particular to nowhere else in particular, and you’re never completely away from the sound of the autoroute. But you do get some birdsong, and the trees are nice. There are few mosquitoes.

The northern section is a gravelled footpath among trees. After traversing this, you come out of another such gateway onto Maurice-Duplessis. It’s not obvious how to continue. Cross the road and there’s just a fence on the other side. Then you’ll notice another of those gates next to the entrance to CÉGEP Marie-Victorin.

The southern half of the trail has a little more character. It starts in a little climb above the school’s sports fields, and eventually turns west along the Train de l’Est railway line. Actually seeing the stream after which the park is named is not so easy, although you may hear it. The path is away from it on the western side.

The atmosphere is nicest along the section with the train tracks above the trail on the left, with a tunnel of trees for quite a long way. It’s cool and pleasant under the trees, and no trains go past on weekends.

Here you come to a decision: do you go under the tracks via this painted portal, or do you keep going rightwards and exit onto Henri-Bourassa? If you take the portal you emerge on Henri-Bourassa further east and if you can cross there you could work your way through several parking lots and car dealerships down into Anjou sur le Lac, whose semi-artificial lake is probably the source of the Ruisseau. Or you can exit a little further west along Henri-Bourassa and walk to Langelier, where the 33 bus passes fairly frequently.

For most Montrealers it’s quite a long way to go for a fairly small park. There are no conveniences at all in or near the area, but you won’t be walking far enough to need any. A water bottle is a good idea: there are no drinking fountains. I think if you’re considering this part of town then you might bite the bullet and go all the way to the Parc-nature de la Pointe-aux-Prairies, which is a lot bigger, but twice as far for someone living in central Montreal. On the other hand, Île-de-la-Visitation park is closer, but on nice weekend days it tends to get crowded. The Ruisseau-de-Montigny park doesn't seem like the kind of place to draw crowds.