From Parkman's Plain.

After you register at the Park Maintenance building, go up to Hwy 71 and drive to the east.  After a number of twists in the road, the Parkman's Plain parking area is on your left, with a sign that declares "Lasalle and Tonti Canyons".

Follow the trail north, descend the Owl Canyon stairs (no crampons) to the river's edge, and head back west along the river trail.  Finally arrive at the mouth of Lasalle and Tonti Canyons, where a large arched bridge crosses the creek.  Head up the left side of the canyon, staying in the left (east) fork.