The gentle (north) side covers about a 12 mile out and back with several trail options which can be used to extend the ride about 3 miles. It's mostly swoopy double track and a short fireroad piece, interlaced with sections of fun single track that climbs gently away from Greenwaters Park out to Salmon Creek Falls campground that marks the turn around point. There are a few steeper climbs and decents, but generally everything is ridable by anyone.
On the opposite side of the creek it's a different animal. The trail rolls over gnarled tree roots, up bridges that are built on slants, and has a couple switchbacks. The degree of climbing/decending on this side is much higher also. Basically, an awesome trail for anyone wanting to improve their skills. This side begins at Greenwaters Park but instead of crossing Salmon Creek, you continue on the southern side of the creek.