This article presents a discussion of the middle estuary. This part of an estuary is difficult to define. An arbitrary working definition has been accepted by many persons in the field: the characteristics of the middle estuary correspond most closely with portions of the estuary having average salinities between 5-18 ppt. The middle estuary is important because the greatest area of many North American estuaries of this type, and it is this coast estuaries seem to have significant middle salinity regions.