Emigrating juveniles use the mainstems as migration corridors and as feeding areas that enable them to grow larger during migration to the ocean. Larger size when entering the ocean greatly increases survival in the marine environment. The percentage of smolts that survive downstream migration is influenced by depth of flow and cover from predation. In the migration corridor, instream cover is important for reducing predation. This is especially true for chinook smolts because in most cases they migrate downstream at a small size and are therefore highly vulnerable to predation. Large woody debris increases connectivity in migration corridors by providing cover and increasing food production. High water temperatures are potentially a barrier to downstream migration.

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