The surface of the Red Sea is on a level with the ocean.

A mountain may have fallen and closed the mouth of the Red Sea and
prevented the outlet of the Mediterranean, and the Mediterranean Sea
thus overfilled had for outlet the passage below the mountains of
Gades; for, in our own times a similar thing has been seen [Footnote
6: Compare also No. 1336, ll. 30, 35 and 36.-- Paolo Giovio, the
celebrated historian (born at Como in 1483) reports that in 1513 at
the foot of the Alps, above Bellinzona, on the road to Switzerland,
a mountain fell with a very great noise, in consequence of an
earthquake, and that the mass of rocks, which fell on the left
(Western) side blocked the river Breno (T. I p. 218 and 345 of D.
Sauvage's French edition, quoted in ALEXIS PERCY, _Memoire des
tremblements de terre de la peninsule italique; Academie Royale de
Belgique._ T. XXII).--]; a mountain fell seven miles across a valley
and closed it up and made a lake. And thus most lakes have been made
by mountains, as the lake of Garda, the lakes of Como and Lugano,
and the Lago Maggiore. The Mediterranean fell but little on the
confines of Syria, in consequence of the Gaditanean passage, but a
great deal in this passage, because before this cutting was made the
Mediterranean sea flowed to the South East, and then the fall had to
be made by its run through the Straits of Gades.

At _a_ the water of the Mediterranean fell into the ocean.

All the plains which lie between the sea and mountains were formerly
covered with salt water.

Every valley has been made by its own river; and the proportion
between valleys is the same as that between river and river.

The greatest river in our world is the Mediterranean river, which
moves from the sources of the Nile to the Western ocean.

And its greatest height is in Outer Mauritania and it has a course
of ten thousand miles before it reunites with its ocean, the father
of the waters.

That is 3000 miles for the Mediterranean, 3000 for the Nile, as far
as discovered and 3000 for the Nile which flows to the East, &c.

[Footnote: See Pl. CXI 2, a sketch of the shores of the
Mediterranean Sea, where lines 11 to 16 may be seen. The large
figures 158 are not in Leonardo's writing. The character of the
writing leads us to conclude that this text was written later than
the foregoing. A slight sketch of the Mediterranean is also to be
found in MS. I', 47a.]

The Nile (1093-1098).