The vast majority of salmon are getting up, over, around, and through the four lower Snake River dams even as legal challenges and political battles swirl around them, according to the federal agency in charge of monitoring fish health. For every 100 young chinook and steelhead that head downstream and past the four dams every spring, about 75 survive. For each of the four dams, NOAA maintains a separate survival standard for juvenile salmon heading downstream. The agency wants 96% survival for yearling chinook and steelhead and 93% for "subyearling" chinook less than a year old. For adult fish swimming upstream, the survival rate is above 90%.