The Oregonian —
The freshwater plume that discharges from the Columbia River into the Pacific Ocean around Astoria is an ever-changing phenomenon. Near the Columbia River Estuary at the mouth of the river it may be mostly freshwater; as it expands into the Pacific it becomes brackish; in places it might be a 50-50 mix of fresh- and saltwater; eventually ocean water overtakes it completely.
During heavy flow periods in spring the plume can extend 100 miles offshore at its outermost edge and as far south as Newport. In fall, it’s a more localized feature around the mouth of the river and can spread like a narrow jet along the Washington coast less than 10 miles from shore.