There are some charts available in the archives. I'm sure your brother-in-law, being the historian that he is, could tell you.
One of those things we had in the harbour the other day showed an old chart of where the river came down. It didn't show any flow, but it showed the depths of the river. The latest one we have doesn't give any depths, but I know an old-timer was telling me that at one time, just upriver from where our harbour is in Ladner, there was 60 feet of water. Now there might be 10 feet on high tide. That channel used to come right from the main river and come right through Ladner. There are maps that show it, but I don't know where you would get the water flow answer.