So I hope I'm not posting somewhere people... er, aren't anymore.

I should have read the thread before I started.
With the Group / Clipping Mask deal, I just made... groups in the layers area. I figured it was just for ease of organization and somehow I managed to stumble my whole way to the rivers step. *cough* I'm glad I've saved a lot of stages, because I'm afraid I know the answer to my own question.

Nothing happens when I select the Land Mask to use the brush to stroke the river path. I recall earlier times when erasing/adding on the Land Mask did nothing, but I kind of brute forced my way through. Is there any way to fix it now? I'm not a TOTAL PS newb, but a lot of this stuff is beyond what I usually do.