After the Lower Siouxon #130D, continue past the parking area returning to the creek. At around four miles, you'll come to an area where a small creek flows across the trail on a wide rocky bed. There is a bridge crossing Siouxon Creek to the left that leads up to Huffman Peak on Chinook Creek #130A (an epic ride listed here as Siouxon/Huffman Peak Loop), but continue straight past the bridge another two miles until the trail crosses another creek.
You can continue across the creek and up the trail to its terminus at the National Forest Development Road 58. However, that section isn't always clear so we generally turn around at this creek crossing.