Turdetania itself is marvellously blessed by nature; and while it produces all things, and likewise great quantities of them, these blessings are doubled by the facilities of exportation; for its surplus products are bartered off with ease because of the large number of the merchant vessels. This is made possible by the rivers, and by the estuaries as well, which, as I have said, resemble rivers, and, like rivers, are navigable inland from the sea, not only for small boats but also for large ones, to the cities of the interior. For the whole country beyond the seaboard that lies between the Sacred Cape and the Pillars is a plain for a considerable distance inland. And here, at a large number of places, are inlets which run up from the sea into the interior, resembling moderate-sized ravines or simply river-beds, and extending for many stadia; and these inlets are filled by the overflows of the sea at the flood-tides, so that one can sail inland thereon as readily as on the rivers — in fact, better, for it is like sailing down the rivers, not only because there is no opposing current, but because, on account of the flood-tide, the sea wafts you onwards just as the river-current does. And the overflows are greater on this coast than in the other regions, because the sea, coming from the great ocean, is compressed into the narrow strait which Maurusia forms with Iberia, there meets resistance, and then easily rushes to those parts of the land that yield to it.
The excessive tides are a bit tricky but commercially advantageous it seems.
Now, while a number of the inlets of this kind are emptied at the ebb-tides (though some of them do not become wholly dry), yet a number of them enclose islands within themselves. Such, then, are the estuaries between the Sacred Cape and the Pillars, for they have an excessive rise of tide as compared with those in the other regions. A rise of tide like this affords a certain advantage to be utilised by sailors, namely, the estuaries are made more numerous and larger, oftentimes being navigable even for a distance of eight stadia; so that, after a fashion, it renders the whole country navigable and convenient both for exporting and importing merchandise. And yet it also affords a certain annoyance; for, on account of the vehemence of the flood-tides, which press with superior force against the current of the rivers, navigation on the rivers is attended by no small danger to the vessels, alike in their descent and ascent. But in the case of the estuaries the ebb-tides too are harmful; for the ebb-tides too grow violent in proportion to the strength of the flood-tides, and on account of their swiftness have oftentimes even left the ship stranded on dry land. Again, the cattle which cross over to the islands that lie off the rivers or the estuaries have at times actually been engulfed; at other times they have merely been cut off, and in their struggle to get back to the land lacked the strength to do so, and perished. But the cows, they say, are by observation actually aware of what happens, wait for the retirement of the sea, and then make off for the mainland.
He also mentions isthmuses which sound like tidal causeways
... canals that have been dug in a number of places are an additional aid, since many are the points thereon from which and to which the people carry on their traffic, not only with one another but also with the outside world. And further, the meetings of the waters when the flood-tides reach far inland are likewise helpful, for the waters pour across over the isthmuses that separate the waterways, thus rendering the isthmuses navigable also; so that one can cross over by boat from the rivers into the estuaries and from the estuaries into the rivers.