The Garmin 430 displays a very much simplified representation of the river, it's certainly not accurate enough and in no way designed to be used in that much detail.
e.g. check out the display here:
Garmin 430 Online Training - Buttonology
On that screen, under the range 5.0nm, the word "overzoom" appears. In practical terms, this means it is pointless zooming in further because the "map" has no more detail to show.