It establishes a gradient between fresh water and salt water. In some places, the gradient moves quickly from fresh to marine habitats, while in others the influence of large rivers ensures that true marine habitats are found much farther offshore. The freshwater-saltwater gradient varies seasonally, but the mixing results in high biological productivity and a diversity of species. In fact, estuaries rival tropical rainforests and coral reefs in their diversity and productivity. Estuaries on the north Gulf of Mexico feature a common range of fish species, but each estuary has its particular mix of these as well.
Estuary systems involve far more than just water.
All of these habitats form the drainage basins that connect freshwater sources with coastal estuaries. Louisiana's coast features some of the largest and most productive estuaries in the world. We have both broad bay and narrower channel estuaries, and two major river deltas. Our coast is in fact a continuous series of estuaries. While each is different, they all add immeasurably to the richness of our coast.