If memory serves me right, the mountains are a result of the last big ice age. Everything north was completely covered in a glacier. So the river beeing older does not seem unlikely.
This is one of my favorite spots on the map. The mountainrange is dividing a big part of the north from southern region called Westphalia. Only one river breaks through it allowing for an easy way across the mountains. The citys name at that spot is Porta Westfalica which is Latin and means "gate to Westphalia". I think this visualization illustrates the reason behind the name very well.
Being pretty young and just getting used to making estimates, this was a wonderful read!
Having to guess how long a project will take is still very scary for me. Is there any way to get over the fear of others disagreing? I always feel like I'm about to embarrass myself by either estimating to high or to low...