Just like it says, if steam engines don't become commercially effective until a few decades later, how extensive does the US canal transport network become? Where can, and more importantly, can't it reach?
The logic of canals is to connect waterways, so the great lakes and the great rivers are the potential here.
Not always. For instance, there are power canals that divert water from a river and channel it through turbines for industrial purposes.
Would you need much of that without steam? I know you have watermills etc, but you already had those for the mostpart
especially east of the Mississippi the USA has tremendous potential for river transport. There are lots of areas where canal interconnections are quite doable, a many connections that would be useful don't require dealing with substantial mountain ranges to make those connections. Using the Great Lakes you can connect to the Mississippi River system (through Chicago) or the Wisconsin River and thence to the Upper Mississippi (through Milwaukee area) that are easily as doable as the Erie Canal. The Mississippi is navigable up to Minneapolis/St Paul. Also, using the Ohio System you can then have some north/south canals to others. This can go around the flank of the Appalachians, canals through them would be a real issue.