The total amount of sediment which moves within the lagoon in a year is about 2,200,000 cubic metres. There is, however, a considerable sediment deficit. Of the total amount, 30,000 cubic metres of new sediment enter through the rivers, whereas 70,000 cubic metres originate from the erosion of lagoon salt marshes and 2,100,000 cubic metres derive from the re-suspension of lagoon bed sediment. 2,100,000 cubic metres are re-deposited in the lagoon, causing the silting up of channels, while 700,000 cubic metres go out of the lagoon through the inlets.

To combat erosion, in recent years, about 400,000 cubic metres of sediment a year, deriving from the dredging of lagoon channels, were kept within the lagoon when found compatible for the reconstruction of mud flats and salt marshes.

The lagoon was not always like this. It is more precise to say it has been substantially changed over the centuries: in respect to the past, the lagoon is now bigger, deeper, flatter, saltier and dirtier and human activities, together with natural processes, have been a crucial factor in the process which has transformed the lagoon from an environment at risk of silting up and becoming land, to one that is at risk of becoming part of the sea.
In fact, in terms of morphological evolution, the dominant evolutionary trend in the lagoon has for centuries been one of gradual silting up and in the past Venetians fought a constant battle against this process.
To defend the so called "live lagoon" from silting up it was necessary to distance all solid materials, both those transported by the rivers emptying directly into the lagoon and those which the sea deposited at the inlets thereby blocking them.

On the one hand, the problem of the inlet beds was never solved by the Republic of Venice; all the works built at the inlets did not prove very effective, until the construction of the jetties which began in the second half of the 19th century.
On the other hand, defence against material coming from rivers was very successful: the diversion of the rivers Brenta and Sile, which emptied out into the lagoon, and the distancing of the mouths of Piave and Po rivers, which were too close to the inlets, stopped the lagoon from becoming part of the mainland. But it also resulted in an opposite process which leads to the lagoon losing sediment to the sea, thus becoming more vulnerable to eroding forces.

But besides the drastic reduction of the quantities of sediment entering the lagoon, other causes contribute to the erosion processes that is transforming the lagoon into a marine environment: natural phenomena, exploiting water supplies, sand removal and sand loss, wave motion, the disappearance of eelgrass and certain fishing techniques.

Natural phenomena, like eustasy and subsidence, contributed to the erosion process. Subsidence occurs naturally, but humans can contribute by exploiting the underground water supplies. In the historic centre of Venice, this activity, carried out for industrial needs and halted in the 70s, caused the ground level to lower by 9 cm. The combined phenomena of eustasy and natural and man induced subsidence have caused, since the beginning of the century, a 23 cm loss in ground level in relation to sea level.

Wave motion caused by winds, currents and motor boats increase the erosion of salt marshes and mud flats. At the same time, the disappearance of the morphological features of the lagoon, together with the increase in lagoon bed depth, have meant a decrease in the natural capacity of the lagoon to withstand the force of waves.

Sand removal is a consequence of the diversion of the rivers. But the flow of sediments into the lagoon from the sea has also been drastically reduced for two reasons: first because the entrance of solid materials is blocked by the outer jetties, second, because the supply of sediment to the coast has diminished greatly due to human interventions to river banks. Thus, today silt, sand and other solid materials no longer reach the lagoon and deposit on the lagoon beds.

Human interventions for industrial needs greatly influenced sand loss in the lagoon environment. In particular, the digging out of big artificial canals has accelerated the erosion process which have caused sediment in shallows and salt marshes to be lost to the sea.

The disappearance of eelgrass, essentially caused by water pollution, deprived the lagoon of an essential acquatic plant. With its long roots, eelgrass consolidates the lagoon bed, thereby contrasting erosion. In addition, the deterioration of the lagoon environment endangers typical salt marsh vegetation that helps capture and deposit sediments and increase the amount of organic material in the soil.

Finally, some fishing techniques have also a devastating effect in lagoon beds, turning them into barren areas. For example, the illegal practice of turbosuction fishing for the gathering of clams, which occurs at a depth of up to 40-50 cm into the lagoon bed, devastating all living organism.

The sediment dynamics within the lagoon is such that it is provoking the gradual disappearance of all the morphological features which are indigenous to the lagoon environment.

The evolutionary tendency towards erosion can be seen in the progressive disappearance of salt marshes and mud flats, the gradual and constant increase in water depth, the disappearance of tidal creeks (natural channels cutting into shallows and salt marshes) and a reduction in the cross section of channels. In short, the lagoon is losing its shape and its morphological structures and becoming flatter, with the sea current expanding in all directions at random as in a bay.

Once the process of erosion is underway, it is self-perpetuating, with the general deepening of the shallows causing a considerable increase in wave motion which in turns leads to greater erosion. This is accompanied by a deterioration in water quality as a result of pollution and the lagoon sediment loss to the sea.
Losing its lagoon characteristics and taking on those of a marine environment has very serious consequences on both the ecosystem and human activities.

The principal elements at risk, besides the lagoon morphology itself, are the lagoon biodiversity, due to the disappearance of animal and plant species and the destruction of urban centres and the structures protecting them, due to the increase in wave motion.