Very shallow basins (several metres depth). They are often elongated and run parallel to the shore.
In general, the estuary is not affected by ocean tides or swell waves because the entrance to the sea is barred shut most of the time.
As a result, these estuaries have no intertidal area – the area between low and high tide – and are not flushed well. Floods can sometimes open an entrance for several days or weeks each year, allowing some exchange with the ocean (flushing) to take place. Their entrances are generally narrow, and close when ocean waves build up sand and gravel in the entrance that can't be flushed away by tides and the river itself
Category A estuaries are characterized by muddy bottom sediment. Inputs from rivers may be small and brief, and there is two-dimensional circulation and mixing. Due to the shallowness of the estuary, the resuspension of bottom sediments by small, wind-generated waves is an important part of the estuary's sedimentation processes.
Category A estuaries are representative of features commonly called coastal lakes.