These are coastal coves where river and sea water mix. The environment here is also shaped by the river sediments deposited in the slow-flowing conditions.
On the Estonian coast, only the Matsalu Bay can be considered an estuary, where the salinity increases more or less evenly – from the freshwater estuary of the Kasari River to six per mille in the Väinameri Sea. The Matsalu Bay is extremely shallow – in most of it, the water depth does not reach three metres, and in the eastern part, it is even less than a metre. The water here is quite cloudy (transparency less than 1.5 m), and the transparency usually deteriorates further towards the end of the summer, as the rivers carry organic particles to the bay and the nutrient-rich bay develops mass phytoplankton.