EISPIRATEN

So, years ago I went on a hike that I want to revisit now but I can't remember the name of the trail or the location. Hopefully, someone here can help.

-I believe the trail was in the San Gabriel/ Monrovia/ Altadena area
-the trail followed alongside a river to the right side of the trail
-the parking lot was quite small, fitting maybe 5 or 6 cars on a shoulder along the road
-the trail started off paved, with concrete or asphalt I believe, and was pretty wide maybe 10 feet across
-i remember seeing bicyclists, as well as hikers
-the trail was pretty flat with very little elevation gain, and I think the round trip distance was ~11 miles, in and out
-after a few miles pavement ended and the trail was dirt covered and narrowed to maybe 5 feet across

West Fork San Gabriel river from highway 39.

West Fork is paved all the way to Cogswell Dam and the round trip is more than 14 miles, but the rest of the description fits. Something about the change to dirt is making me think the West Fork isn't what he's looking for.

Altadena? Maybe it's the Arroyo Seco road entering near the JPL gate? Small parking area. Lots of bicycles. Paved road turning to dirt. Starts out with the creek on the left, but then it's on the right for a while before the road turns to dirt. But the trail crosses the creek lots of times so I'm not sure that fits the description.

That's a good guess, I think.