I tried the _testCrvSelfIntersection on this curve, but I think there must still be one because when I try the _planarsrf command the surface created is transparent. lake champlain long and skinny.3dm (10.2 MB)
No, they are not not self intersecting. I think the issue is that the curves are so complex, the mesher is having trouble with the default meshing settings for the resulting surface. You have one dupe curve in there as well.
I deleted the surface and the dupe curve and remade the surface using PlanarSrf. I used the following display mesh settings and the surface showed up fine.