Parking for this trail is at the end of Salmon River Road about 0.7 miles past the bridge/TH for Salmon River Trail #742
. The last stretch of road is dirt but you should be able to get around the potholes in any car. Many of the cars parked at the trailhead are going to be people enjoying the river, not people hiking to the top of Salmon Butte.
This trail has a very gentle and steady grade compared to other 3,000 foot climbs in the region. It is also very smooth - few rocks or roots - so the going is surprisingly easy. Finally, it's gets very little use considering it's proximity to Portland.
The clearly signed trail begins where the road is blocked off to vehicles and descends a very steep ~15 feet down to the river before almost immediately climbing back up to the closed-off road. You can just start off on the road to skip this section if you want. The road crosses South Fork Salmon River on a bridge and then quickly transitions to dirt singletrack. You'll follow the old road for about 1.5 miles but it's very grown-in and covered with wildflowers so it isn't too "roady" while still very clear to follow. There is some impressive old growth along the river and again after a logged area at the end of the road.
At about 2,750 feet and over 2.5 miles from the trailhead the fist view opens up on a clear bend in the trail with some great snack-time-rocks. You can see the South Fork valley and Sheepshead Rock. There won't be another clear viewpoint for about 2.5 more miles. At about 3.75 miles the trail appears to head out into a clearing because so many people miss the first true switchback here. There are only a few more switchbacks as the slope of Salmon Butte is gradual enough to not need many.
Finally, at about 5 miles the trail curves around the ridge of Salmon Butte, enters tons of rhododendrons, and a few views to Hood briefly open up. The last mile of the trail is the loveliest thanks to the rhododendrons and other wildflowers. 3/4 of a mile from the summit the trail merges with an old road that bends around to the south side of Salmon Butte before the final ascent push.
From the top you can expect to see St. Helens, Mount Rainier, Mount Adams, Mount Hood, Mount Jefferson, and a glimpse of the Three Sisters. It's large enough for a few groups to hang out.