C) THE WORKS ON THE LEFT BRANCH
A threshold was built also in the left branch, with the purpose to balance the water flows, upstream Ponte Fabricio near Ponte Garibaldi. (pict.C1; see also B1 and B2)
However the need to optimise the functionality  of the thresholds for the different flow conditions of the Tiber that, in spite of its length, has a rather variable and torrential flow, brought to an overall design consisting in moving the right branch threshold downstream Ponte Cestio, as above mentioned, and replacing the fixed threshold on the left branch with a variable height one.
The works have been carried on in various steps during many years; in 2003 it has been undertaken the final phase with the realization of the mobile threshold and the conclusion of the project.
The thresholds are also essential to limit the erosion of the riverbed, but due to them the Tiber will never be navigable in the island area: this is the price to pay for the solving of the river hydraulic problems around the Tiber Island.