A cross-sectional representation of the duct from the Nymphaeum to the Fosso di Fiora.

The distinguished archaeologist Thomas Ashby wrote in his book "The Aqueducts of Ancient Rome" (1935):

The only remains detected by the author are about 125m. due east of the ruined chapel of S. Fiore, and east of the railway to Viterbo, between the stations of Manziana and Oriolo, but nearer the latter, about 300m. south-east of castello 51,144. Here, just where a track branches eastwards from the north and south track marking the communal boundary between Bracciano and Oriolo, the Aqueduct is crossed by the Fosso di Fiora. About 10 m. downstream from the crossing, the stream has broken into the specus, and falls some 15 m. into a deep pool here. The outer specus supporting-wall is exposed for a height of 10-15 m. and a length of 10 m. It is built in the usual brick and opus reticulatum with an admixture of ashlar below, but is liable to be covered with thick vegetation. The specus is 0.91 m. wide, running east and west: the thickness of its walls was not measured, but is probably 0.65 m. or so, as at Vigna Orsini. The tufa reticulatum cubes are 0.07 m. square, and stand upon three brick courses. The outer wall has a band of opus retuculatum below the three courses, followed by five brick courses standing on unfaced concrete. At another point two blocks of cut stone are visible, both well below the specus level, which must be something like 320 m. above the sea.

Here's what he missed:

Click on the image to view a high resolution file (390Kb download).

This schematic representation shows the main sections of the Aqueduct Tunnel at Santa Fiora with the Church as it currently appears, and the well, chiusino, and settling tank as they were in the late sixteenth and early seventeenth centuries.

A The Spring Water Collection Tunnel. Here the water was filtered from the water table between large stone blocks which line the tunnel.
B The Well of Running Water / Mother Well.The bottom of this well measures eight metres from the top. Water was collected using a mechanism called a Shaduf - a lever pivoted in a tree - one of the oldest machines invented by mankind.
C Tunnel lined with Opus Reticolatum. This is the Aqueduct Proper - displaying very traditional Roman Building Techniques.
D "Chiusino" or water lock. Beneath a stone hut is a system of sluice gates and a cross-ways of two aqueducts: The Aqua Traiana, and either the Bracciano Aqueduct of Duke Paolo Giordano Orsini from 1578, or perhaps the 'new' Bracciano Aqueduct of Duke Livio Odescalchi from 1718.
E We have been told that this large circular tank, perhaps a settling tank lies beneath a dwelling next to the Stone Hut above D. It once contained ancient foundations and mechanisms pertaining to its use as a Bottino.
F This now dried up lake of water from the Fiora sources used to spill into the Valley of the Fosso di Fiora below it.