Why is the water near the Mississippi River delta less salty than the open Gulf of Mexico?

The water near the Mississippi River delta is less salty than the open Gulf of Mexico because of the massive influx of freshwater discharged by the river into the gulf.1

While the open gulf maintains a salinity level comparable to the North Atlantic—approximately 36 parts per thousand—the coastal waters near the delta experience significant fluctuations. During periods of high river flow, the large volume of freshwater dilutes the seawater, resulting in salinity levels as low as 14 to 20 parts per thousand extending 20 to 30 miles offshore. This mixture of freshwater and saltwater is commonly referred to as brackish water.123