Structures on the shore can also impede the flow of sand and cause beach erosion and other problems. Many beach areas, particularly barrier islands, are naturally variable places that can change radically in a very short time during storms. People just should not build in these places. In the watershed of San Francisco Bay, in the last century hydraulic mining for gold in the Sierra Nevada brought an enormous quantity of silt down to the estuary, and a lot of it is still here. The bathymetry of the bay was changed drastically, and a lot of contaminated sediments were laid down and are still here.
Wetlands have a large number of functions in healthy ecosystems.
They act as filters of sediment and contaminants, nursery areas for a lot of estuarine fish, a food source for many fish and birds, and shelter for a wide variety of animals, not to mention aesthetic value to humans. Obviously, when wetlands are displaced by farms, houses, or airports, these functions are no longer performed. Although many of the remaining wetlands are protected, others are still being destroyed. This is a particular problem in estuaries because of the intense human activities. In my opinion, introducing a new species to an estuary is nearly always a bad idea.