I find the path that the river takes possible, but somewhat implausible. The way that it flows now would require a peninsula where the center is lower than the edges, which is unusual. And you do not depict mountains along the western edge like you do along the eastern. That implies that the western edge is relatively flat, so it would it would be much more believable that a river or rivers that originate in the mountains would simply flow straight west or southwest to the sea.