OK, let’s see if I get this because it is rather complicated. The rainbow trout (oncorhynchus mykiss) is actually native to the Pacific basin. BUT it has been introduced elsewhere since 1875, often for sports fishing and is now all over the place. And where they have been introduced they have done no good to native species. Consequently the rainbow trout is included in the list of 100 globally invasive species.
The steelhead trout is actually an anadromous (and that means sea-going) form of the coastal rainbow trout. The freshwater forms that have been introduced into the Great Lakes migrate into tributaries to spawn and THEY are also called steelhead. Can that be right? Further research is required.
So the freshwater river going guys are smaller between .5-2.3 kg but lake dwelling and sea-going ones can be as big as 9kg.

The freshwater ones, as we might find in the Ottawa river, usually spawn where there is a bed of fine gravel. The female lays 2000-3000 eggs , which fall into spaces between the gravel. Meanwhile the male is swimming along behind, fertilizing the eggs. The female immediately covers them with gravel. They take 4-7 weeks to hatch. They live to be about 10-11 years.
And they really are beautiful, especially the breeding males with their reddish- pink stripe along the lateral line.