... The river flows from the east side of the ocean, and ends on the west side of the island, where the isthmus is for people to walk. ...
It is highly doubtful that such an island resulted from the standard map generation routine. That routine produces fairly standard "round" islands with little in the way of projections or indentations no matter the size.
Therefore, one has to assume that the "river" described is the result of editorial terraforming by Reignking.
When one uses the editor to terraform the island and introduces such things as rivers, bays and peninsulas -- one has to live with a really screwed-up pathfinding system. Caddet is almost correct, but he is talking about trips of way more than 100 tiles - and he overemphasises buildings. There is a rudimentary memory for trips of around 100 tiles or less; and roads do help dodging buildings.