The Venice Lagoon is brackish, which means sea water thinned by fresh water off the land. Measurements across the lagoon through 2019 put salinity at roughly 12 to 36 PSU, against about 34.7 PSU for water carrying no freshwater signal at all, so the lagoon runs from about a third as salty as the Adriatic up to as salty as the Adriatic (Sankara Narayanan et al., 2026, p. 10).
Where you are decides what you are floating on. The two monitoring stations under marine influence held 27 to 35 PSU through the year, while the two under stronger local freshwater influence were both lower and far more variable (Sankara Narayanan et al., 2026, p. 12). The gradient runs the same way every year: saltiest along the main tidal channels and at the three inlets at Lido, Malamocco and Chioggia, freshest in the northern and inner western sectors where the land water arrives (Sankara Narayanan et al., 2026, pp. 3, 13).
This page gives the measured figures: how salty the lagoon is and why, how much the answer changes between the inlets and the mainland edge, which month is freshest, what the tide actually does to the salt, why the water looks green-brown, and how any of it is measured.
Is the Venice Lagoon salt water or fresh water?
Neither, in the strict sense. The lagoon is classed as a polyeuhaline transitional environment, which places it at the salty end of the brackish scale (Cibic et al., 2025, p. 3). Salinity here is reported in PSU, practical salinity units, close enough to grams of salt per kilogram of water to read as the same thing. Water in this system with no freshwater signal left in it measures about 34.7 PSU, and the lagoon as a whole was measured at about 12 to 36 PSU through 2019 (Sankara Narayanan et al., 2026, p. 10).
Four things keep that range so wide. The lagoon is shallow, averaging about 1.2 metres across roughly 550 square kilometres and some 50 kilometres from north to south. The tide draws Adriatic water in and out through three inlets. Fresh water arrives through a network of minor rivers, regulated channels and drainage canals rather than from one large river. And the shape of the basin reflects centuries of engineering, including the deliberate diversion of several rivers away from the lagoon to stop it silting up and keep it navigable (Sankara Narayanan et al., 2026, pp. 3, 4).
So the water at your feet in Venice is sea water, diluted by a variable amount depending on where you stand and what month it is.
Why the sea wins: the lagoon's daily water budget
The reason the lagoon is salty rather than fresh comes down to volume. Every day the lagoon exchanges about 400 million cubic metres of water with the Adriatic, while the inflow from rivers and the subsoil together comes to about 3.7 million cubic metres (Cibic et al., 2025, p. 3). The sea therefore delivers well over a hundred times as much water per day as the land does. Across a single 12 hour tidal cycle, roughly 60 per cent of the volume held in the lagoon is exchanged with north-western Adriatic water through the inlets at Lido, Malamocco and Chioggia (Cibic et al., 2025, p. 3).
The land side matters, it is simply much smaller. Twelve main tributaries drain a catchment of about 1,850 square kilometres holding around a million people, and they deliver a mean freshwater discharge of about 35.5 cubic metres per second (Cibic et al., 2025, p. 3).
Venice also has the largest tides in the Mediterranean, which is why that exchange is so effective, and why it shapes the morphology of the lagoon along with its oxygen, salinity, nutrient and sediment distribution (Cibic et al., 2025, p. 3). The same mechanism that brings acqua alta to St Mark's Square is the one that keeps the lagoon salty.
How salty the water is where you are
One figure for the whole lagoon hides most of what is interesting. Through 2019 the two monitoring stations under marine influence held 27 to 35 PSU, while the two under stronger local freshwater influence ran both lower and far more variable (Sankara Narayanan et al., 2026, p. 12). One of those sits at the mouth of the Dese, the only river the salinity study names, in the northern lagoon (Sankara Narayanan et al., 2026, p. 4).
Mapped across the basin the pattern is consistent: lower salinity in the northern and inner western sectors, higher values along the main tidal channels, at the three inlets and in the Adriatic coastal water just outside them (Sankara Narayanan et al., 2026, p. 13). A separate study working in the Palude di Cona, a shallow area in the northern lagoon ringed by salt marshes, measured salinity of about 22 in summer and about 25 in autumn (Cibic et al., 2025, pp. 3, 8).
For a visitor the practical reading is this. Water at the inlets and out in the main shipping and vaporetto channels is effectively sea water. Water in the northern lagoon and along the mainland edge is measurably diluted, and most diluted in spring. The dominant gradient, as the 2026 study puts it, runs from marine conditions at the inlets to seasonally enhanced freshwater influence further in (Sankara Narayanan et al., 2026, p. 12).
The freshest month and the steadiest season
Salinity in the lagoon has a clear annual cycle. Winter is the stable part of it, with much of the lagoon between 25 and 30 PSU and lower values persisting in the northern and inner sectors (Sankara Narayanan et al., 2026, p. 13).
The strongest freshening comes in May, when inflow from the northern tributaries is high. Satellite retrievals for 2019 show salinity dipping to about 10 to 15 PSU near the tributaries that month, with values of around 20 to 25 PSU reaching well into the inner lagoon (Sankara Narayanan et al., 2026, p. 13). At one inner station the spring drop was steeper still, falling from about 28 to 30 PSU down to about 14 to 22 PSU through the period of elevated river discharge (Sankara Narayanan et al., 2026, p. 12). That is the largest single swing the study recorded anywhere in the lagoon.
Salinity then recovers through summer and stays relatively high into early autumn, before localised reductions reappear towards the end of the year (Sankara Narayanan et al., 2026, p. 13). One study year could be a freak, so the authors checked: the recurrence of the spring freshening and of the landward to inlet gradient indicates these are persistent features of how the lagoon works rather than an isolated annual anomaly (Sankara Narayanan et al., 2026, p. 16).
What the tide does to the salt
Two cloud-free satellite scenes eight days apart in February 2019 isolate the tidal effect, because conditions were otherwise comparable. On 5 February the scene was acquired on a flood tide, with the Punta della Salute gauge in central Venice reading 0.65 metres, and salinity across the lagoon ran approximately 25 to 33 PSU as saline Adriatic water pushed in through the inlets. On 13 February the scene caught an ebb tide at minus 0.15 metres, and salinity was roughly 2 to 3 PSU lower across the northern and inner sectors, while the channels, the inlets and the coastal water outside stayed higher (Sankara Narayanan et al., 2026, p. 13).
The mechanism is worth stating precisely, because it is easy to get backwards. A rising tide does not make the water in the lagoon saltier. It slides saltier water landward and pushes diluted water back, so the boundary between the two travels with the tide. A substantial part of the short-term salinity variation measured at any fixed point comes from that landward and seaward displacement of water masses, rather than from the saltiness of the water itself changing (Sankara Narayanan et al., 2026, p. 15).
Over longer periods three influences combine: tidal exchange, fresh water delivered from the drainage basin, and the weather, with rainfall deciding how much the rivers carry (Sankara Narayanan et al., 2026, p. 15).
Why the water looks green-brown rather than blue
The colour follows directly from what is dissolved and suspended in the water. Reflectance from lagoon water peaks around 560 to 580 nanometres, the green to yellow part of the spectrum, and the landward and river-influenced stations show the highest green-red reflectance of all (Sankara Narayanan et al., 2026, p. 8). That colour carries a land signature: most lagoon observations sat outside or near the upper edge of the reference range for marine water, which points to terrestrial and local organic inputs rather than a purely marine source (Sankara Narayanan et al., 2026, p. 8).
Transparency varies for four reasons the same study names: suspended sediment, freshwater inputs, tidal exchange and boat traffic (Sankara Narayanan et al., 2026, p. 15). Wind adds a fifth. A scene from 20 March 2019, taken under a south-westerly of 10.84 metres per second, shows bottom sediment stirred up into a localised optical feature in the central lagoon (Sankara Narayanan et al., 2026, p. 16).
The water is also shallow enough that the seabed contributes to what a satellite sees, particularly at low water or when the water is clear (Sankara Narayanan et al., 2026, p. 16). Brown water after a windy day is resuspended lagoon floor, and it tells you nothing about how salty the water is.
How the lagoon's salinity is measured
Two systems watch this water and they work differently. SAMANET runs multi-parameter water quality stations distributed across the lagoon, recording at 30 minute intervals at one metre depth. The network has ten stations, six of which had salinity records available for 2019 (Sankara Narayanan et al., 2026, p. 4). That gives high precision at a handful of points and nothing in between.
The satellite method fills the gaps. Coloured dissolved organic matter can be measured from space by Sentinel-3, and because that organic matter arrives with the fresh water it serves as an optical stand-in for salinity. The 2026 study is the first to develop and evaluate satellite-based salinity retrieval for the Venice Lagoon (Sankara Narayanan et al., 2026, p. 3). The relationship is a straight line, salinity = -15.7 x aCDOM(440) + 34.7, whose intercept of about 34.7 PSU amounts to the fully marine endmember (Sankara Narayanan et al., 2026, p. 10).
Accuracy matters when quoting any of these numbers. Against the station records the retrieval returned an R2 of 0.68, a bias of -0.01 PSU and a root mean square error of 2.22 PSU, so treat the mapped figures as good to roughly 2 PSU (Sankara Narayanan et al., 2026, p. 12). Field campaigns in May and June 2018, July 2019 and June 2024 supplied the in-water measurements behind the method (Sankara Narayanan et al., 2026, p. 4).
