Many estuaries in large river mouths support significant fisheries and high levels of commercial shipping. Most estuaries were created by sea level rise over the past 18,000 years, since the last ice age. During this time sea level has risen about 180 meters or 400 feet. Fjords are glaciated valley estuaries. Bar-built estuaries are separated from the ocean by sand bars.
Lagoons separating barrier islands from the mainland are examples.
Different types of water mixing within estuaries produce four different variations on this general pattern. These patterns generally progress from shallow to deeper estuaries. Vertically mixed estuaries are shallow, low-volume estuaries. Slightly stratified estuaries have two salinity layers with greater salinity at depth and true estuarine circulation. These layers are separated by a mixing zone.