Overlaping is meaningless if it is done underwater. Or at water level and in the same horizontal plain. The overlap needs to have some height. 4 inches in height is like a foot overlap with only a 2 inch rise.
Why is that? Don't really understand, if I overlap a foot in length why would it matter? I guess you must be right cause I was having issues even with a foot of overlap, but I suppose not properly since gravel remained on top of where the stream liner was going in. I had removed the liner yesterday and just buried the filter, so no more stream, not until maybe spring when I do it properly.
But yes, I assume I would have to remove all gravel from that area, so that overlap is on the actual liner, and then there would be a big height overlap, enough I assume not to wick water out, but this is from what I've watched on bigger lined streams. pretty interesting how such a small thing can cause something that cannot be very easily fixed. at least it was not for me.