What is a B-run steelhead?

In Oregon, Science Friday by Nick Chambers

Fisheries managers predict this will be a very bad year for returning B-run steelhead in the Snake River. The Snake, with its headwaters straddling the Teton Range on the Idaho-Wyoming border, is the largest tributary to the Columbia and its intact habitat and steelhead runs are vital to the overall health of Columbia River steelhead populations. Despite the high quality …