The Mouth of the Tiber 1816-1862
The reason that all the planning maps show the Tiber Creek replaced by a canal and the Mall filled with dry land past the White House is that L'Enfant had planned a monument dedicated to GW to be located due south of the President's House and due west of the Capitol (marked A below). At least that's what they tell us.
I repeat, L'Enfant's map does not call for a monument on the edge of the river as some have suggested, but rather shows it in the center of an area that has been landscaped (filled in). As we saw earlier, the mouth of the creek remained unchanged until at least 1840. Construction on the monument began in 1848, but not in the location indicated above.
This is Latrobe's 1816 vision of the mouth of the creek, including sea walls and a lock. The building that you see is a proposed university. His idea appears to be just landscape enough area around the monument location for an equestrian statue as was planned.
What Latrobe's map shows us is that the position where the two axes cross was on the edge of the river in 1816, and given that the Potomac rises and falls with the tides, it was probably often as not under water.
By 1846 the map looked like this renewed version of the Morrison map including sea wall and island. Note that the mouth of the creek was never closed off as Latrobe envisioned it, and L'Enfant never drew a map looking like this.
By 1875 the area that L'Enfant had showed filled in on his map was completed on the ground. This map was produced by S. Augustus Mitchell in 1875 for his New General Atlas.
Looking closer at the above image, we see the current location of the monument, and we notice that the proposed position for that lies right on the sea wall. The fact is that no effort was ever made to landscape far enough north and west to permit a monument to be built on that spot until the Washington Monument was positioned where it is now, 371 feet east and 123 feet south of the proposed location.
Note that extending the line south from the White House and Maryland Avenue from the Capitol Building gives us the current location of the Jefferson Memorial, and the line through the Wash Mmt from the CB extends to the Lincoln Memorial (not on the square axis). The 1888 street car map below shows that the land for the Jeff Mml had finally been positioned.