salmon

How and where does salmon spawning occur? A female salmon returns to her natal river or river of birth to spawn. Salmon, sea trout and brown trout often use spate rivers for spawning. A fast flowing, hilly or mountain river subject to floods or high water is described as a spate river. The flood level rises quickly after heavy rain but drops just as fast. While water levels in a spate river may fluctuate very quickly, it may, at the same time, have some deep holding pools with good gravely areas for salmon to spawn. A salmon travels upstream to the source of a river where the water is shallow and fast flowing and where the riverbed is strewn with clean, loose gravel. After selecting an area in the stream, she excavates a hole in the gravel with her tail and nose, sometimes helped by the male. The gravel is piled high, often reaching the surface of the water, and always downstream of the excavation. Then the female deposits her ova (up to 5,000 eggs) in the deepest part of the excavation. The male, who is alongside throughout, fertilises the eggs as they are deposited from the female. When spawning is complete, the hen buries the ova (fertilised eggs). Then all of the excavation is fully covered, forming a mound of approximately 40cm by 40cm, depending on the size of the fish. The mound of gravel or egg nest made by a trout or salmon in a stream is called a “redd”. A small number of females and males (about 5%) may go back to sea a second time and return to spawn again. Salmon that return to sea after spawning are called spent fish or kelts and some even return a third time. This completes the life cycle of a salmon

I guess they know where is safe or home and that is why they return back there.