Estuaries are the areas where rivers run into oceans. They often exist where the opening to the sea is somehow obstructed, for example by a sandbar or a lagoon (sandbars are ridges of sand built up by water; lagoons are shallow areas of water separated from the ocean by sandbars or coral). The water in estuaries is dominated by the flow of the tides. When tides are high, the ocean water washes through the estuary bringing with it sediments (particles of sand, silt, and gravel), nutrients, and organisms from the ocean. When the tide is low, the freshwater of the river floods the area, releasing its load into the estuary. Because estuaries exist where two different types of water come together and where the land meets the water, estuaries provide many different types of habitats for animals and plants. In addition, both the river and the ocean bring...