The east side of the Gore Range has access issues to National Forest land because of numerous private properties that block access. The two closest, legal trailheads are the Brush Creek to the north of Slate Creek and the Rock Creek, to the south of Slate Creek. Reaching the Brush Creek TH requires 4WD with good clearance. The Rock Creek TH can be reached by passenger vehicles. Mileage to Upper Slate Lake from the Brush Creek TH is 10.8. Mileage from the Rock Creek TH is about a half mile shorter. However, you do more minor up and down on the Gore Range trail from Rock Creek requiring a little more overall elevation gain that you do if coming in from Brush Creek.
Apparently, it's also possible to come into Slate Creek from a closer access along Boulder Creek. We met a couple here who had hiked in from Boulder Creek, a supposedly closed to public access trail, because the road passes through private property. There is no official trailhead there but according to some sources, you could drive up that road, drop off your packs, drive back down and park your car away from private property, then one person who drove back down could hike back up or ride a bike and stash it somewhere near the trail beginning. Use this at your own risk, which could involve having your vehicle towed or your bike stolen. A friend of ours had his bike cables cut. The ride is about 2.7 miles up with almost 1,000 feet of gain - a steep sucker!
Our proposed approach then has you starting from the Brush Creek TH. From the trailhead, the trail takes out following an old, unused diversion ditch that contours west back toward the Gore Range Trail, about .65 mile (15 minutes) in. At the intersection, turn left (south) and begin hiking the Gore Range trail. At this intersection, you immediately drop down and cross a flowing drainage, which is Brush Creek. There’s no bridge. Walking south on good trail, you will come to the next intersection with a sign post in about one mile. Continue past this trail that goes to Lost Lake and keep walking generally SE on the Gore Range Trail. Shortly after the Lost Lake trail turnoff, lose a little elevation to cross another fork of Brush Creek, then regain a little elevation to begin a mile long contour to Squaw Creek, where you will lose about 200 feet in elevation to cross this creek. On the other side, regain nearly 200 feet in elevation to cross a ridge at just above 9,000 ft., then drop some to cross Hay Camp Creek and a trail that follows that drainage. Watch carefully to remain on the Gore range trail here. There's one brief section where grasses may obscure the trail. From the crossing of Hay Camp Creek, the Gore Range trail gains over 300 feet in elevation to cross another ridge as the trail turns more to the south and then drops to Slate Creek where a wooden sign marks the trail intersection for the Slate Creek Trail and the Gore Range Trail. There are the remains of an old cabin nearby. At this intersection, you've covered 5.3 miles and have walked through miles of sadly, beetle-kill forest. Intersection coordinates are: N 39° 45' 47.48" W 106° 11' 40.30". 9075 ft. elevation. The devastation is amazing and portions of the forest are almost completely gone as of our visit in 2020, but along Slate Creek at this intersection, trees were still in good shape. We saw no good campsites until you come to where Slate Creek & the Gore Range Trail meet. From the trail intersection, walk south on the Gore Trail just a short distance toward Slate Creek and you'll see a very nice campsite location on your right, nestled in among conifers. Water is easily accessible here and there are numerous flat spots that could accommodate several tents. If coming from Rock Creek, there is no official bridge across Slate Creek. There are just some logs to balance your way across.
From the Gore Range trail, the Slate Creek trail initially passes through open meadows and small stands of aspen for the first mile and a half and gains minimal elevation. In fact, at the trail intersection, the elevation is just below 9,100 feet, which is only slightly higher than your beginning elevation at the Brush Creek TH. At some beaver ponds at 9,200 ft., nearly two miles up the drainage, you first begin to gain the elevation needed to get to Upper Slate Lake at 10,860 ft. So you have about 1,600 feet of gain in the last 2 miles. After those beaver ponds, the trail passes through some nicer forest and then crosses some marshy areas just before it begins the steeper ascent. That last forested area before the climb begins is your last chance for a campsite until arriving at Upper Slate Lake.
We took a break in the vicinity of the turnoff for the main Slate Lake at 9893 ft. We could not find an easy access across the creek here to get to that lower lake, other than to wade or dare to cross the rushing stream on fallen logs. The USGS map makes it appear that the main trail crosses the stream here then crosses back over after the lake. This is no longer the case. The trail remains on the north side of the stream. Continue on from there, to arrive at Upper Slate Lake with initially some more steep gain and several switchbacks that don't appear on the maps clearly before the trail begins to level out some, then does a minor drop as it nears the lake. Two things to note about this trail. First of all, older maps show the trail as delivering one to the lake outlet. The trail has since been re-routed so as to bring you out at a small peninsula that juts into the lake on the SE side, about midway along that shore. This peninsula is a rock outcrop, covered in trees with numerous small to large camp spots. It is at a point where the lake pinches down to its narrowest. The other shore is literally a stone’s throw away.
The second thing to note about the Slate trail is that the USGS Squaw Creek quad does not show a trail heading up the Slate drainage from the Gore-Slate intersection described earlier. The other two quads, (Willow Lakes & Vail East) however, do show this trail. In addition, the Trails Illustrated map for Green Mtn. Reservoir shows things correctly as well. On that map, if you’re coming in from the south along the Gore Range trail, you drop down and cross Slate Creek, gain a little elevation and then come to the Slate Creek trail intersection. It appears however, that an older branch of the trail on the south side of Slate Creek can still be followed to a crossing of Slate Creek and then joining in with the main trail on the north side later on.
If you want to break up the backpack in, then there is an excellent campsite area on the north side of Slate Creek, where it meets the Gore Range Trail. See coordinates below. There's enough room here for multiple tents. There's also another campsite up along the Slate Creek trail just before the trail enters a forested section and before it starts making strong elevation gain. This may be a horse-packers camp. The turn off for this will be the only clear trail you see taking off to the right as you're heading upstream. Coordinates for the turnoff are: N 39° 44' 51.94" W 106° 13' 09.31".