Just saw this tonight on the PortlandHikers.org, which is now OregonHikers.org... I'll need to find this! If you follow the link, there are some pictures.
I randomly stumbled across an old spring with obvious man-made improvements on descending the northeastern ridge from Wauneka Point. The spring is backed by an old mossy retaining wall (couple feet high, perhaps 20-30ft in length) similar to those seen occasionally along the Old Columbia River Hwy (OCRH).
There's a small basin - and, just below, half-hidden in the vegetation what looked like the remnants of a barrel-sized wooden structure surrounding a hole a several feet deep with water flowing through the bottom. (Nice place for the unwary to break a leg.)
It's located near the mouth of Moffett Creek and south perhaps 0.15 mi away from the bike trail. So probably not related to the OCRH, unless it was some old pullout or campground...or...?? Approx. loc: UTM 10 579849mE 5052521mN.
http://www.oregonhikers.org/forum/viewtopic.php?f=30&t=18816