The Northern Waterthrush has densely streaked underparts that are also found in the Louisiana Waterthrush and Ovenbird. The Louisiana Waterthrush usually has a whiter, wider supercilium, and an unspotted throat. Some Northerns are distinctly yellowish on the underparts-Louisianas are always white (but can have 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 a pale 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.