We've done it: We've gone off the charts!
Our Furuno chart plotter has a US and Canada coastal waters SD Card, and today we arrived at the limit of what coastal charts can display. At about mile 280 on the river, we reached the end of our world.
From now on our plotter will only display a blank grid with no chart on it. Are we going to fall off?
For the rest of our upriver navigation we will be using a combination of tablets running a couple of navigation software with proper charts going all the way inland as we want.