The Lake Sawyer / Black Diamond Natural Area open space is a combination of county- and city-owned open space and private timber company land with easements. Trails exist on both sides of route 169, though up until recently most were on the west side of the road. On the east side of 169, only one trail remains open to horses as of 2015: "Route 66", which is just a .8 mile out-and-back. However, the east side of the road is currently the only approved trailer parking, in a lot shared with dozens of bikers, making things a bit sticky. (Land to the south of the east parking lot belongs to Summit Ridge/New Life Church, who has allowed mountain bike clubs to build their own custom trails there -- horses are not permitted (and the trails aren't built to be safe for horses anyway). At the north end of the area, a working railroad runs east-west; riders should stay south of that to avoid the Henry's Ridge area, a mountain-bike-only area by informal agreement, though a connection to Ravensdale Ridge trails may be doable. Someday.)
So for now we'll focus on the west side of the road, assuming you either make it across 169 or park elsewhere. The network of trails that laces through this entire area has been a long-favorite haven of local mountain bicyclists, and vary from fairly wide and open (an old railway right-of-way bisects the area to the west of 169 in a north-south direction, the route of the eventual Green-to-Cedar River Trail) to narrow, low-clearance, and bumpy. Some are dotted with mountain-bike obstacles (there's usually a way around) rotting into the underbrush. And if you have a horse that tends to brush too close to tree-trunks, some of the trails in the northwest part of the area could be particularly painful to the kneecaps. To the south, the land is owned by Palmer Coking Coal Company and signed as such; see the note on the Danville-Georgetown Trails page about getting a permit from them to use trails on their private lands.
Land in this area has been changing hands a lot, and there are many proposed developments in the works: some are housing suburbs, and trails in those areas may be lost. Others are transfers of open space from one public agency to another, and may change how trails are managed. Of note:
Parts open, but maybe subject to future changes: read above
There are a variety of possible parking spots.
There are two official parking lots along Rte 169, one on the east side of the road, one on the west. The Rte 169 East parking lot has signed trailer spots around the outside edge; the end one has the most space, while some of the side spots could get tight with other vehicles parking in the center of the lot. This lot is VERY popular with bicyclists, and gets crowded late in the day. The West parking lot is inaccessible to trailers, having not enough room to turn around, but just south of it on the west side of the road is a wide spot you can park in if you don't mind being on the side of a highway.
A slightly better spot, though still small (2-3 trailers max with no other cars there) is the dead-end stub of 238th Ave SE, south of SE 288th St. From southbound 169, turn right onto 291st, which will quickly bend around and become SE 288th. (There is a small pullout on the south side of the road here that might also work to park a trailer in, if someone else hasn't gotten there first.) Turn left into 238th SE in about half a mile, and park in the round turn-around at the road-end.
Additional trailer parking is alongside Lake Sawyer Rd SE, near the Sawyer Woods Elementary School. From Rte 169 drive south toward Black Diamond and turn right onto Roberts Rd (at the white coal car triangle). Drive 1.25 miles, and go right onto Lake Sawyer Rd SE. Just short of a mile further, you will see a raised drive-through parking spot on the left shoulder of the road, before the intersection with SE 312th St. Park there, then cross the road and ride/lead eastward (right) to the end of 312th. HOWEVER: There is a locked gate, and cables around trees to the side that will make it tough to get a horse that isn't either small or agile through. There's a new parking lot access (cars only) for Lake Sawyer Park across from this parking spot that might also provide park trail access, but I haven't checked that out.
Length: many miles of trails
Surface: dirt, gravel
Share with: bicycles, hikers, maybe a train to the northeast
Rte 169 East parking lot: room for 5-6 trailers
if there aren't
many other people in the lot, but that's unlikely
side of Rte 169: 2? trailers, depending on how others are parked
238th dead-end: 3? trailers (be cautious of getting blocked in, or blocking others)
Lake Sawyer Rd SE: 3-4 trailers
Most of the area is Black Diamond Natural Area owned by King County. Around Lake Sawyer, particularly near the south end of the lake, the open space is a combination of King County open space land, future park land recently transferred from County ownership to the City of Black Diamond, and privately owned timber land that has county- and city-owned conservation easements in place.
Black Diamond parks website:
King County Parks' Black Diamond Open Space page:
Black Diamond Open Space
Also see the King County page above for some map links
(website down - hopefully temporary)
And finally, some of the trails actually appear on Google Maps if you go to "satellite" mode and zoom in. Harder to print and take with you, though.
Some of the trails in this area, especially the suspiciously flat and smoothly curved ones, follow the paths of long-gone narrow-gauge railways that served both the town of Black Diamond and the sawmill that used to exist on the northern shore of Lake Sawyer. Notable is the straight north-south route of the Columbia & Puget Sound Railway, originally a narrow-gauge line that ran from Seattle and Renton through Black Diamond to Franklin, and hauled coal and other products from many locations along the way. The rails were converted to standard gauge in 1897 -- modernization!