The Louisiana Waterthrush has densely streaked underparts that are also found in the Ovenbird and Northern Waterthrush. The Northern Waterthrush has a yellowish supercilium and a spotted throat. Some Northerns are distinctly yellowish on the underparts while Louisianas are alway white underneath (sometimes with buffy flanks). The songs of the two are quite different and are the best way to separate them. The Ovenbird does not pump its tail and lacks the white supercilium instead having an eye ring.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.