The layering between the fresh water from the tributaries and the salt water from the ocean is most distinct in highly stratified estuaries, although some sea water still mixes with the upper lense of fresh water. In this intermediate estuary type, mixing of fresh and salt water occurs at all depths. With this vertical mixing, salinity levels generally increase towards the estuary mouth, although the lower layer is always saltier than the upper layer. In this type of estuary, powerful mixing by tides tends to eliminate layering altogether. Salinity in these estuaries is a function of the tidal stage although tides are able to dominate salinity only in very small estuaries.
Rivers flow in a single direction, flushing out sediments and pollut ants.
Rather than quickly flushing water and pollutants through its system, an estuary often has a lengthy retention period. Consequently, water-borne pollutants, along with contaminated sediment, may remain in the estuary for a long time, magnifying their potential to affect the estuary's plants and animals adversely. Other factors also play a role in the hydrology of an estuary. The shape of the basin and the width of its mouth along with its depth, area, tidal range, surrounding topography, and regional climate combine to make each estuary unique. The Problems Like much of our nation's coastal waters, many of the more than 100 estuaries are also under siege.