The Oak Grove Fork of the Clackamas River has atleast two major waterfalls. Unfortuantely, the hydro project that dammed Lake Harriet has diverted nearly 90% of the river's flow, leaving the river at only a trickle most of the time.
Barrier Falls - 20 ft
Crack Falls - 12 ft
Just a small distance below Harriet Lake Dam is a very unique falls. The whole river flows into a crack in theground. Warning, PGE has built a building at the very top of the falls that has security cameras and a very large water pipe coming out of it. Some days this pipe is gushing water, some days its shut off. Either way, do not approach this falls from the west, as the road is posted off limits. There's a nice ridge to hike down from FR57 to the falls on, plus you have to be on the east side of the river to get down into the crack anyway.
Lynn Falls - 25 ft
Along FR4630 a little before you reach Lake Harriet from Ripplebrook, You'll come to a wide sharp corner with a waterfall tucked in behind the hill.
Right next to the Shellrock Lake trailhead is the outlet creek from Hideaway Lake. Right next to the parking lot is a very photogenic falls. It requires a little bit of a scramble, but its very worth it.
View Clackamas River Waterfalls in a larger map