It depends on what type of salmon you're talking about.
Spring Chinook salmon will enter the mouth of the Columbia River in March and arrive in Idaho in late April - July.
Summer Chinook salmon will leave the ocean in April to early May and arrive in the Salmon River in late May to July.
Sockeye salmon leave the ocean in late May and early June and reach the upper Salmon River in late August.
Fall Chinook salmon leave the ocean in late July to early August and arrive in the Snake River in late August to early September.