ESTUARY., the mouth of a river where sea and fresh water meet and where tidal effects are conspicuous (Lat. aestuarium, a place reached by aestus, the tide) . An estuary may result from the long-continued joint action of river and tidal erosion confined to a narrow structural depression, or an estuary may be the drowned lower portion of a river-valley. Isobaths are drawn to show sea-depths and the isobathic chart of the Severn estuary, for example, shows a progressive deepening seawards by means of V-shaped lines which become blunter westwards until the last one is a mere indentation pointing up channel ; these lines may be indicative of the progress of the channel over the existing conti nental shelf. Where, as in the Severn and the Thames, the fresh water meets the sea gradually the water is thoroughly mixed, and there is very little change in salinity at high tide. The fresh water tends to flow over the denser salt water, but there is a continuous increase in salinity seawards, for the ebbing and flowing currents constantly mix the waters. On the other hand, where the river brings down a great quantity of fresh water into a narrow channel, the change of salinity from high to low water is very marked. Mill uses the Firth of Forth as an interesting example; here the landward half is a true estuary, but the seaward half has the water so thoroughly mixed that the salinity is almost uniform from surface to bottom, and increases but gradually towards the sea. In consequence, the Forth causes the sea to become slightly freshened throughout its whole depth for many miles from the land.