Upon their return in the beginning of the winter season, coho salmon may have to wait in the estuary to re-acclimatize to the fresh water conditions, and possibly to await access into their streams. For example, heavy rains and tidal movement may be needed to raise water levels so that the fish can swim across beaches into their creeks. Coho will spend most of their time in the deep pools while migrating. This allows them to avoid the predatory dangers of the shallow, quick moving riffles. Their average size upon return is 4.0 kg, and a length of about 50 cm.
Males are usually larger than females.
By turning on her side and flapping her tail up and down, the female uses the water to push gravel around, and clears a depression in the gravel called a redd. Once the redd is deep enough, the female prepares to lay her eggs. The dominant male, who until now defended her, gets very close to her and releases sperm into the gravel nest. Other males may also deposit sperm into the nest. Once the act is finished, the female will immediately cover up the fertilized eggs to minimize predation.