There is no 'barrier' and there is plenty of 'transgression', you just have a low level of mixing.
Saltwater is heavier than freshwater so it stays at the bottom of the lake, and as the lake is still there is little mixing (which requires motion).
Salt does pass to the freshwater (freshwater contains salt anyway, just much less of it), it just happens at a slow rate.
This is something that is common in all estuaries, you have a freshwater layer and a saltwater layer. How thick these are and how 'separate' depends on flow rates, currents, topography, etc.
Salt wedge[edit]
In this type of estuary, river output greatly exceeds marine input and tidal effects have a minor importance. Fresh water floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land. As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example of a salt wedge estuary is the
Mississippi River.
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