Why would you expect a dead zone to start near the mouth of a river where the river flows into a body of water?

Dead zones often form near the mouths of rivers due to nutrient runoff, particularly nitrogen and phosphorus from agricultural and urban areas. When these nutrients enter the body of water, they can stimulate excessive algal blooms, which, upon decomposition, deplete oxygen levels in the water. This creates hypoxic conditions that are unsuitable for aquatic life, leading to the formation of a dead zone. Thus, the convergence of riverine nutrient inputs with larger water bodies often sets the stage for these low-oxygen areas.