estuarian UK /ˌɛs.tjʊˈɛəɹi.ən/US /ˌɛs.t͡ʃuˈɛɹi.ən/ adjective1 •Related to or characteristic of an estuary.We would expect estuarian signatures to fall between the 87 Sr/ 86 Sr signature of the Amazon mouth and the global marine signature except very near the mouth of the Tocantins. — 2015 formsmore estuarian · most estuarian