Estuary may be glacial river valley whose mouth has been flooded by the rising sea. When sea level dropped because so much water had been locked up on land in form of glacial ice which caused that drop , then rivers cut valleys downward to sea level. When glaciers melted, sea level rose again and deep valleys and their river mouths were filled with ocean water, forming estuaries.
Shallow sunlit water of estuary is fed by both river and sea (tides).It loses its delicate balance if either one is too much (spring rains too heavy, too much fresh water - storms at sea , too much sea water).
