According to the geomorphology and physiography classification scheme for estuaries proposed by Pritchard (1967) and presented in Kennish (1986), Great Lakes estuaries can be classified as Type A: drowned river valleys or rias. The termination of Wisconsinian glaciation, which resulted in a eustatic rise in sea level of somewhat over 100 m and the formation of such Atlantic coast estuaries as Chesapeake Bay and Delaware Bay, also resulted in isostatic rebound in the northern Great Lakes region and the formation of drowned mouths of many tributaries to the lake. Based on analysis of literature and field observations, it is proposed that the term freshwater or Great Lakes estuary be applied to the drowned portion of streams entering the Great Lakes. The common denominator in virtually all definitions of estuaries is morphology, particularly the basins morphometry and the hydraulic relation of the water level contained therein to the entering stream. Where the land mass and the stream profile rise above a selected datum, the estuary begins and the river debouches. This concept can be useful in modeling water quality and physical attributes in connection with biotic, physiochemical, sedimentary and contemporary geological processes. It can also be applied in the Great Lakes as a practical way of identifying the margins of the lake, thus clarifying ownership disputes and clouded issues where demarkation of lake and river arise. © 1990 Estuarine Research Federation.
