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 most are on the west side of the road. On the east side of 169, the land to the south belongs to Summit Ridge/New Life Church, who has allowed mountain bike clubs to build their own custom trails there -- horses are requested to stay off of the area (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. (Riders should also be aware that trains may pass by on that track at any time of day, and be prepared.)
The network of trails that laces through this 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) to narrow, low-clearance, and bumpy. Some are dotted with mountain-bike obstacles (there's usually a way around). 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.
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. In particular, an irregularly-shaped chunk south and east of SE 312th is owned by the City of Black Diamond, which has plans to develop it into a park with ballfields, picnic and lake-shore facilities, pathways, and currently, no horse parking or access from the Lk Sawyer Rd parking spot (below). However, a recent King County map shows a plan to develop an equestrian parking area at the Lake Sawyer Road location to service the future-planned Green River trail... so open space access from there may also be in the works. Of important note is private land signed "No Trespassing," owned by Palmer Coking Coal Company at the southern end of the trail area; see the note on the Danville-Georgetown Trails page about getting a permit from them to use trails on their private lands.
Open, but maybe subject to future changes: read above
There are a variety of possible parking spots. Many bicyclists park along the side of Rte 169 north of Black Diamond (south of the intersection with SE 291th St). It doesn't take many cars, though, to prevent enough space for a horse trailer there.
Rumor has it that King County will soon be building a parking area here that can accommodate trailers. Some space has been roughed out, but isn't accessible yet.
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 would 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.
Length: many miles of trails
Surface: dirt, gravel
Share with: bicycles, hikers, maybe a train to the northeast
Parking for:
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:
http://www.ci.blackdiamond.wa.us/Depts/NaturalResources/nat_resource_home.html
Map:
http://www.switchbacks.com/maps/PDFs/Lake_Sawyer.pdf
Also: http://www.frcv.org/content/maps/lakesawyer.pdf
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.