Merge parallel segments of same polyline? Or other suggestions

Discussion created by batholith on Apr 29, 2013
Hello - up front apologies for the long winded question...
I'm working with NHD data, looking at calculating the length of large streams adjacent to parcels (polygon data), not internal to the parcel, only adjacent to it.  I've been working with a huge dataset, across the conterminous US, so NHD and property boundaries don't always match up --this has been a bit troublesome.  As certain flowline data can cut in and out of a boundary, it leads to gaps, etc.

For the most part, I'm only interested in "larger" streams so I've been using the "NHD area" polygons and have been selecting rivers within a certain buffer distance away from my parcel data.  This has worked OK, but then the problem comes to calculating the length that any given river from the selected "NHD area" features flow adjacent to any given parcel -since it's an area feature, no length. 

I can convert the NHD area to a polyline, but an issue has been that it creates a single polyline for both sides of the river segment; basically a single line that parallels itself, so the "merge divided road" tool or other similar tools that I know doesn't work.  Also, the "collapse dual lines to centerline" doesn't seem to work, and because some streams are wide AND highly sinuous, it wants to merge them in meanders, not just along the stream edges.

Does anyone know of a easy way to convert the NHD area data to a single line on which I can calculate length?

Thank you!