Category G - Fjords or sounds
Very deep (up to 100s of metres), narrow, elongated basins which are largely subtidal.
Category G estuaries are characterised by sills at the mouth and along the length of the estuary that were formed as debris pushed up in front of glaciers.
Inputs from both the river and the tide during a tidal cycle make up very small proportions of the basin's total water volume.
Water movement is controlled primarily by thermohaline forcing – in other words, water circulation is maintained by the large difference in density which are a result of the difference in salinity between freshwater and seawater. Because of this, the water circulation pattern is characterised by freshwater flowing out of the estuary at the surface, which is balanced by seawater flowing into the estuary underneath the freshwater. In some cases, wind can change this circulation pattern, and both two- and three-dimensional wind-driven circulation can become a major force in some cases; however, wind is not responsible for mean circulation over extended periods of time.
As a result, these estuaries are poorly flushed, particularly those with multiple arms. The very deep basin, as well as the partitioning which the sills provide, means that flushing only happens in a relatively thin layer of freshwater moving over the top of a 'dead zone' of seawater.
The resuspension of estuary bottom sediment by ocean swell or wind waves isn't an important process, due again to the large depth of the basin. As a result, the bottom sediment is generally fine sand or mud.
Category G estuaries are representative of features commonly called fjords or sounds.