Has anyone got a source on how one should calculate the length of a river? OK you have to define the start (that is hard enough), but then the end is difficult. My own web search shows that you should measure it to the mouth. However that leaves the question of the Estuary as an open one. Does the mouth start at a certain concentration of brine? Maybe it is the highest point of tidal waters? To illustrate the problem, the River Shannon is the longer river in Britain and Ireland if you include the estuary, but a fresh water measure gives that honour to the River Severn. Now you have no idea how much energy is going into this and I had a bad day yesterday (too much jet lag not enough coffee) and got my citations mixed up (the ultimate sin). So if anyone can help with a definitive reference to something on how you do this (my own web searches have failed) I would appreciate it.