The Louisiana Waterthrush is very similar to the Northern Waterthrush, but there are some differences.
The chest and belly of the Northern Waterthrush are more washed yellow and are more densely streaked, whereas the Louisiana Waterthrush has more white underparts and less dark streaks.
In addition, the Louisiana Waterthrush has a heavier bill and a nearly pure white eyebrow, while the Northern Waterthrush's eyebrow is usually somewhat buffy.
The eyebrow of the Louisiana Waterthrush is wider at the rear.
Perhaps the most diagnostic morphological feature is the throat, which is streaked in the Northern Waterthrush but plain in the Louisiana Waterthrush (clearly visible on the last photo).

Robert K. Rees Memorial Park, New Port Richey, Florida, USA, 1 April 2022