Category H - Drowned valleys, rias or fjords

Deep (10s of metres), narrow, elongated basins which are largely subtidal - the estuary bed is still below water at low tide.

Inputs from both the river and the tide during a tidal cycle make up very small proportions of the basin's total 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.

There is a strong longitudinal gradient, from head to mouth, in the water movement of the estuary. The river (and thus stratification) are driving forces in the headwaters (at the 'top' of the estuary), while the ocean (and thus vertical mixing) are driving forces near the estuary's entrance into the sea.

Category H estuaries are poorly flushed, with flushing becoming more poor in estuaries with multiple arms.

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, this sediment is generally fine sand or mud.

Category H estuaries are representative of features commonly called sounds, drowned valleys, rias or fjords.