The inverse estuary (the backwards sea)

Every estuary you have ever stood in works the same way. A river runs fresh water in at the top, the open ocean lies salt at the mouth, and somewhere in between the two blend along a gradient you could taste your way down. Gulf St Vincent — the broad, shallow arm of sea that Adelaide sits beside — does the exact opposite, and it is uncommon enough that oceanographers keep it as a textbook case.

The trick is that the gulf has no real river to speak of, and a long, killing-dry summer that lifts water off its surface faster than the trickle of creeks and winter rain can put back. So instead of being freshened from within, it is slowly boiled down. Out at the mouth, off Cape Jervis, the water is ordinary ocean at a salinity of about 36.5. Up at the northern head, past the top of the city, it climbs to around 39 by the end of winter and past 42 by the end of summer — saltier than the open sea, a shallow bath concentrating under the sun (Lennon 1987).

That inversion is not a curiosity bolted onto the coast; it is the bottom rung of the whole gulf-to-range gradient the book follows. The same Mediterranean summer-drought that will dictate the soils, the sclerophyll leaves and the fire regime higher up the slope is here writing itself into the sea, turning an estuary inside out. And because the gulf flushes slowly and from the bottom up rather than with the tide (Bye 2014), whatever the city adds at the salty, sluggish head — nutrients, stormwater, desalination brine — is slow to leave. The backwards sea is productive and strange and, as later guides show, more fragile than it looks.