Drainage basins make good borders in mountains while straight borders are better in flatlands. Straight borders are usually drawn by white men - thousands of miles away - who never walked the land.
For example, the border between upstate Maine and Quebec was defined as the height of land (separating the asking Lawrence River drainage from American rivers) back when canoes were the primary means of transportation.
When it came time to draw borders between New Hampshire, Vermont, upstate New York and Quebec, they just drew straight lines through lumpy, forested, largely unihabited wilderness. That border was supposed to follow the 45th parallel, but a surveying error out the international border a few miles farther north.
West of Quebec, the border follows the middle of the Great Lakes.
From the Great Lakes - all the way to the Pacific Ocean - the international border follows the 49th parallel. The 49th parallel was chosen to give Canada a port at the mouth of the Fraser River.
On the Prairies, straight provincial borders were drawn across unsettled lands ..... er lands that had been promised to railway tycoons. Ottawa expected those pesky nomadic natives and half-native Metis farmers to quietly step aside. We all know how well that went! See the book "Clearing the Plains."
The only topographical border - in the prairie provinces - is that with British Columbia. The southern part of that provincial border follows the foothills of the Rocky Mountains.