Category B - Tidal river mouths
Elongated basins of simple shape and several to ten metres depth.
The majority of the estuarine area is subtidal - the estuary bed is still below water at low tide.
Most of the water entering the basin during any tidal cycle comes from river flow, rather than from tidal water coming in from the ocean. As a result, the movement of the estuary's water is dominated by river flow; there is a net export of water to the ocean and, as a consequence, the estuary is well-flushed.
On shorelines where there is littoral drift – the movement of sand and gravel along the shore due to wave driven currents –small sand bars or shoals form on the ocean side of the estuary entrance.
In deeper systems, a circulation pattern can develop where fresh water flowing out into the ocean is balanced by seawater flowing into the estuary beneath the fresh water. This can result in a salt wedge developing, where there is a sharp boundary between the fresher water and the wedged-shaped salty layer beneath it.
On coastal plains where there is a low gradient, seawater can travel a considerable distance up the estuary. Large river floods can in turn temporarily expel most of the estuary's seawater.
Two-dimensional mixing due to wind, and the resuspension of bottom sediment by wind waves, are both minor in this type of estuary because the length of water over which wind can blow (called 'wind fetch') is short and the and waves generated are small, and the depths too large, for there to be much stress put onto the estuary bed. As a result, bottom sediments tend to be muddy.
Category B estuaries are representative of features commonly called tidal river mouths.