Category C - Tidal river lagoons
These occur where the mouth of a main river channel connects to shallow lagoons.
While the main river channel is mostly subtidal, the lagoons can have a significant area which is intertidal (i.e. between the low and high tide mark).
Most of the water in 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, although this flow tends to bypass the lagoons.
In the deeper, main part of the estuary, 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.
While the main river channel is well-flushed due to these processes, seawater remains trapped in the lagoons, where flushing is relatively poor.
Two-dimensional mixing due to wind, and the resuspension of bottom sediments due to waves, are both minor in the main river channel, but are greater in the lagoons to the shallower depths and the larger wind fetch. In lagoons, the resuspension of fine bottom sediments by waves produces coarser bottom sediment.
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.
Category C estuaries are representative of features commonly called tidal (lagoon) river mouths.