Mixing of Columbia River and Ocean Waters in Summer

Mixing between Columbia River water and the adjacent ocean water is a two-stage process controlled primarily by high river discharge. The first-stage mixing, of fresh water of the river and upwelled deeper ocean water, occurs within the estuary and yields low-salinity water. The second-stage mixing, of seaward-flowing low-saline water and deeper ocean water, occurs primarily in the transition area within 20 km of the river, and adds additional nitrate and phosphate. During summer the river is the dominant contributor of dissolved silicate. Because of summer photosynthetic depletion, the river contributes virtually no nitrate; the deep ocean water contributes most of the phosphate and nearly all the nitrate. In'the oceanic area, the low-salinity surface water lies above the surface ocean water which inhibits further upward nutrient transfer by mixing. Nitrate is depleted within 30 km of the river. The lack of nitrate therefore limits photo synthetic activity in the surface ocean layers.