How large can Canada get Realistically compared to OTL.

Here's a nice one by Nugax...

DathisCanada1830_Provisional_by_IainFluff.png
 
Dathi's CanadaWank (YACW) explores this question and produced this map.

EDIT - Ninja'd! See the post above mine.
 
Here's a nice one by Nugax...

DathisCanada1830_Provisional_by_IainFluff.png

Other than renaming 'Oregon' to 'Columbia', the exclusion of the Mississippi River Basin, and the inclusion of Ellesmere Island, that's probably as big as Canada could get without it turning into 'British USA'.

We can probably grab some of the Alaskan Panhandle too.
 
Top