This is a bird that has always given me trouble from a photographic standpoint. They usually are pretty hard to see, preferring thick forested streams with poor light and once they notice they are being watched they fly around non-stop. However, this particular bird was obviously an exception and allowed prolonged open views all the while singing and pumping his rump up and down like a lunatic. These photos were taken at the NC Arboretum at Chapel Hill.
The literature is rife with inconsistencies on how to differentiate the Louisiana from the Northern Waterthrush. Some people look at the coloration, but this is not the best because Northern Waterthrushes can be white or yellowish underneath. Some folks say look at the contrast between the belly/flanks coloration and the supercilium: on the Louisiana they are consistently the same (above bird is a good example) where the Northern Waterthrush can contrast quite a bit. Other folks say to look at the thickness of the streaks. Yet other folks point to the throat streaking: the Louisiana has a nice bare throat where the Northern has a streaky one. Other folks rely on habitat: the Louisiana prefers fast moving streams where a Northern is happy in a fetid pool. If you have a musical ear the song differences or even the "chink" calls can distinguish them. However, today I noticed one I did not use before: the Louisiana has brighter pink legs where the Northern has darker legs.
The other thing to consider is that Louisiana come back earlier in the Spring than the Northerns do. Also Louisiana Waterthrushes breed in NC and Northerns just pass through.