It is a long, tiny brook basically (at least at the official filling spot) that comes out of the rocks in the woods and continues down into the trees. Was maybe 6-12" wide, no more than 8-10 inches deep at it's deepest spots. Some of these spots you could probably submerge a Nalgene enough to fill it but you would definitely be in the algae so you'd have to take care not to disturb the surrounding rocks. I had a Hiker Pro filter so I just dropped the end into a clear area at the base of a rock. I was going to take a photo of it but I completely forgot until I was half way back of the spur path. I've never used Liberty Spring so I can't compare the two. If you have a pump style filter it won't be a problem.
And as an aside, if you wind up in there I think I dropped a red Nalgene in a nylon carry pouch in that area. (The last place I definitely remember having it was on Davis Path near Boott Spur junction. I dropped it somewhere between there and my car at Pinkham Notch but I'm guessing it was at the spring because I was futzing around so much with my stuff.). Probably gone by now but if by chance you find it let me know. I don't care about the Nalgene so much but I love the carry sleeve it was in.
