An estuary is a partially enclosed coastal body of brackish  with one or more rivers or streams flowing into it, and with a free connection to the open . Estuaries form a transition zone between environments and maritime environments and are an example of an ecotone. Estuaries are subject both to influences such as tides, waves, and the influx of saline and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the column and in sediment, making estuaries among the most productive natural habitats in the world.