Slightly Stratified Estuaries
In slightly stratified or partially mixed estuaries, saltwater and freshwater mix at all depths; however, the lower layers of water typically remain saltier than the upper layers. Salinity is greatest at the mouth of the estuary and decreases as one moves upstream. Very deep estuaries, such as Puget Sound in Washington State and San Francisco Bay in California, are examples of slightly stratified estuaries (Ross, 1995). Even though Puget Sound is classified as a fjord in terms of its geology, it does not exhibit the characteristics of a fjord when classified by water circulation.
In this animation, you can see the slight stratification of the blue-colored freshwater flowing from the river on the right hand side of the image, and the green- colored seawater from the ocean on the left hand side of the image flowing beneath it.