Uruguay River level in Salto, today. At the port, every 15 minutes.

It rained from 24 March to 23 April 1959. In Salto, those who lived through it say it rained all day, every day, for a week, and the Uruguay River rose 18 metres in front of Salto and Paysandú: the official record at the port of Salto reads 18.17 m, and witnesses spoke of 19. There was no dam: Salto Grande opened twenty years later.

The water reached calle Brasil in front of the Customs house and came up Julio Delgado to a block from the city hall; it reached calle Uruguay. It covered the old OSE waterworks and threatened the engines of the power plant on calle Colón: a week without water or electricity, with the city cut off. More than 2,000 people were evacuated, most of them housed at the racecourse. The river took more than a month to go down. For Salto it was, as the local paper put it, the year of the greatest drama.

Since 1979 the Salto Grande reservoir has been cutting the peaks. In June 2017 more water went through the dam than in 1959, more than twenty times the volume of the reservoir, and the port read 15.10 m: three metres less than in 1959, with more water. In January 2015 up to 23,000 m³/s went through, five times the normal flow, and the lake had been lowered beforehand to receive it: the river stayed at 12.15 m in Salto. In December that year, with 30,000 m³/s, the dam kept the flood two metres lower than it would have been, and it still reached 16.37 m, the second highest since 1959. According to the Comisión Técnica Mixta, in 1983, 1992, 1997 and 1998 regulation lowered the level in the downstream cities by up to three metres.

What the dam cannot do is stop a large flood: when the reservoir is full, what comes in goes out. To gain room, the CTM lowers the lake before the water arrives and lets it rise above normal while it passes: in December 2015 the reservoir reached 37.14 m, when normal is 35, and in January 2019 it went above the 36 m expropriation level to keep Salto at 13 m. That is why the port gauge is read today with the dam in mind: what it shows is what the power plant let through.

Salto Grande (1979) sits 15 km upstream of the port. Its reservoir regulates the river: it can turbine about 8,400 m³/s and discharge up to about 60,000 through the spillway. The mean flow of the Uruguay River is 4,622 m³/s; the highest measured since 1898 was 37,714 m³/s, on 9 June 1992. According to the Comisión Técnica Mixta, in the great floods regulation lowers the level in the downstream cities by up to three metres: the 2015 flood would have been two metres higher without the dam, and the August 2026 one would have reached 12.20 or 12.30 m instead of 11.56.

What it cannot do is hold back a large flood: when the reservoir is full, what comes in has to go out. That is why the number that matters is how much water is coming from the upper basin, in Brazil: heavy rain there takes about ten days to reach Salto Grande, and two days if it falls in the lower basin. Every day, around 1 pm, the CTM publishes a bulletin with the maximum level expected at the ports of Concordia and Salto for the following day; that is the figure the Intendencia, the CECOED and the press repeat.

In normal conditions the river at the port of Salto sits at around 5 metres.

The height of the river at the port of Salto is read on the port gauge, the local zero used by the Prefectura (coast guard), the CECOED emergency committee and the Salto press. The Comisión Técnica Mixta de Salto Grande runs a telemetric station there, Salto (31.39° S, 57.98° W), which measures the level every 15 minutes and publishes it through an open web service. The official height above sea level is 1.1 m higher than the gauge: when a source says a safety level of 13.1 m, on the port gauge that is 12.0 m.

A robot run by this platform reads that station every half hour and keeps the latest reading, the previous 90 days and, day by day, the lows, highs and means since the historical file ends. The historical file was built once from the same service with every reading since 1 January 2019: 272,073 readings, of which 17 were discarded as out of range or as impossible jumps of the station. A day counts in the history with 20 readings or more. The station has measured since August 2016, but in 2017 it read 1.7 m below the port (13.4 m on 12 June, when the port stood at 15.10), so nothing before 2019 is used; since 2019 it matches the official readings to within a few centimetres.

The station sits downstream of the dam, and it shows: the level at the port rises and falls every day with the operation of the power plant. That is why each day has a low, a high and a mean, and why a single reading can differ from one taken an hour earlier without the river having risen. Real floods show in the daily mean, not in one reading.

Each threshold and landmark has its own source: the port closure, the Prefectura del Puerto de Salto; the alert level, the Comisión Técnica Mixta; the safety level, SINAE; the low streets and evacuations, CECOED; the bridges and the historic floods, the flood-risk map of the Intendencia de Salto and DINAGUA (September 2017) and the Salto press.