Several genetic studies have found that eastern freshwater cod, in the southernmost of these four rivers, and Mary River cod, in the northernmost of these four rivers, are more closely related to each other than to Murray cod. This suggests that Murray cod only managed to cross into east coast river systems once. It is not clear which of the four river systems was the original entry point, but geomorphological evidence suggests the headwaters of the Clarence River. The mechanism by which Murray cod crossed the Great Dividing Range are considered to be either river capture, where the headwaters of an easterly-flowing coastal river cuts back (in the erosional sense) and intercepts and "captures" the headwaters (and fish fauna) of a previously westerly flowing stream, or a "wet divide", where extreme flooding or a very wet climatic phase created swamps in a topographic low point that effectively connected the headwaters of western flowing and eastern flowing streams.