Flow range for best boatability uncertain. Please help your fellow boaters with a comment or report.
Driving East toward Plain on the Chumstick Highway, turn right on River Road and then almost immediately left into the gravel parking lot. (If you cross the river on Chumstick Highway, you have gone too far). There's room for a dozen or so cars to park here: please preferentially park on the left side of the lot (until it's full), as that leaves room for vans with trailers to easily use the lot as a turnaround for classes.
A short gravel path from the lot leads to a set of cement stairs leading down to the river. During high flow (>5000 cfs) the bottom of the steps will be underwater. The stairs are about four feet across, and the shrubbery gets pretty close as you near the river, so (a) it can be a little challenging to carry a full soze river raft up or down these stairs and (b) from the river the stairs are nearly invisible until you are right on top of them.
Finding this access point from the river is a fascinating mix of simple and challenging. At the headwaters, you put in at the green girder bridge. This access point is river right, literally 20 feet downstream of the next bridge that you go under (Chumstick Highway). So it's easy to know where it should be, but can be maddeningly diffucult to spot before you pass it. Even though you know where it is, it can be easy to miss because there isn't a natural eddy at the access point.
If you're doing the whole float (from headwaters to Tumwater) with small kids, this can be a useful way-station, because there is usually a portapotty in the parking lot. Or at least there was the last time I parked there.