So is it presumed that that dead chum swam into Lafarge on their own?
Although I have fished the lake many times, I checked the City of Coquitlam's watershed map to confirm that their is no natural drainage from the lake. Remember it is an old gravel pit... Typically water flows into these, not out. However, I do recall that there is a overflow/storm drain in the SE corner of the lake, which I presume leads to a trib of the Coquitlam River (maybe Maple Creek?).
So is the thinking that the chum salmon migrated up a Coquitlam River trib and then through the underground storm water system to the lake. If I recall, the overflow infrastructure at the lake is heavily screened off so there if no easy way for a salmon to make the transition from storm water system to open lake...