Apparently not all of them are trying to make it farther up river, some are already spawning - the female digs a trench (a "redd") and the male defends it. Once the trench is dug, the female deposits the eggs and the male fertilizes them. The most common type of salmon in the river is Chum - and this species does not particularly "pair up." One female's eggs can be fertilized by more than one male. (There are also Chinook and Coho salmon in the river and these do tend to pair). Once the eggs are fertilized the trench is covered over and the adults, well there is no sugarcoating here, they die. But don't be sad! There is a whole new generation of salmon growing beneath the gravel...
However there are birds - mostly sea gulls in my experience - hanging around the river trying to feast off the roe. They also will eat the dead salmon as well, but I did not see this happening. The up and coming generation of salmon is also threatened by trout in the river who have come to feed on the roe as well.
At night the bigger mammals come out - when the humans have stopped pacing the river banks taking copious amounts of photographs. The process of the salmon spawning is not just about the salmon themselves but the whole ecosystem that operates around it - fascinating stuff :)
The eggs hatch in march when the weather starts to warm. After they hatch (at this point they are "aelvins"), they stay beneath the gravel and feed off a yolk sac. Soon the Chum kiddies leave the gravel and head back to sea, but the Coho young stay in the stream for two more years.
If you have never seen salmon spawning and have the chance to - you must check it out!! You can hear about it and see pictures, but actually being there with all these fish that you never get to see normally is pretty unreal!
Thanks to the Goldstream Park website for the info.