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A water cannon is a device that shoots a high-velocity stream of water. Typically, a water cannon can deliver a large volume of water, often over dozens of meters. They are used in firefighting, large vehicle washing, riot control and mining. Most water cannons fall under the category of a fire monitor.
Water cannons were first devised for use on fireboats. Extinguishing fires on boats and buildings near the water was much more difficult and dangerous before fireboats were invented. The first fireboat deployed in Los Angeles was commissioned on 1 August 1919. The first fireboat in New York City was Marine 1, deployed 1 February 1891. There may have been other fireboats elsewhere even earlier.
Fire trucks deliver water with much the same force and volume as water cannons, and have even been used in riot control situations, but are rarely referred to as water cannons outside this context.
The first truck-mounted water cannon was used for riot control in Germany in the beginning of the 1930s.
The most modern versions do not expose the operator to the riot, and are controlled remotely from within the vehicle by a joystick. The German-built WaWe 10.000 can carry 10,000 litres (2,200 imp gal) of water, which can deploy water in all directions via three cannons, all of which are remotely controlled from inside the vehicle by a joystick. The vehicle has two forward cannons with a delivery rate of 20 litres per second (260 imp gal/min), and one rear cannon with a delivery rate of 15 litres per second (200 imp gal/min)
Water cannons designed for riot control are still made in the United States and the United Kingdom, but most products are exported, particularly to Africa and parts of Asia such as South Korea.
Use of water cannon in riot control contexts can lead to injury or death, with fatalities recorded in Indonesia (in 1996, when the cannon's payload contained ammonia), Zimbabwe (in 2007, when the use of cannons on a peaceful crowd caused panic), Turkey (in 2013, when the payload was laced with "liquid teargas"), Ukraine (in 2014, with the death of activist and businessman Bogdan Kalynyak, reportedly catching pneumonia after being sprayed by water cannon in freezing temperatures) and South Korea (in 2016, when a 68 year old farmer died after injuries sustained by a water cannon the previous year). Water cannons in use during the 1960s, which were generally adapted fire trucks, would knock protesters down and on occasion, tear their clothes.
On 30 September 2010, during a protest demonstration against the Stuttgart 21 project in Germany, a demonstrator was hit in the face by a water cannon. Dietrich Wagner, a retired engineer, suffered from the damage to his eyelids, and damage to the retinas. The eye injuries thus inflicted on the man resulted in near-complete loss of eyesight. Graphic imagery was recorded of the event, sparking a national debate about police brutality and proportionality in the use of state force.
According to a report issued in the United Kingdom, using plastic bullets instead of water cannons was justified because the latter "are inflexible and indiscriminate", although several people had previously been killed or seriously injured by plastic bullets.
The presence of the media at riots has had a significant impact on water cannon use. There is much pressure on police departments to avoid bad publicity, and water cannon often play badly in the press. It is considered that this is a likely reason that they are not used more often in certain countries.
Confrontations that took place in the era of the American Civil Rights Movement where water cannons were used by authorities to disperse crowds of protesting African Americans, has led to the demise of water cannon in the United States.
In 1997, pink dye was reportedly added to the water used by South Korean and Indonesian police to disperse a riot. The implication is that they might use this mark to make it easier to arrest rioters later. The United Kingdom, which had sold the water cannon to Indonesia, condemned this practice (although the Royal Ulster Constabulary had used a water cannon with purple dye during The Troubles in Northern Ireland) but later approved the sale of further water cannons to them. Most modern water cannons are also capable of adding tear gas to the stream.
Electrified water cannon
In 2004, Jaycor Tactical Systems was experimenting with additives (salt and additives to reduce the breakup of the stream into droplets) that would allow electricity to be conducted through water. They have demonstrated delivery from a distance of up to twenty feet (6 m), but have not yet tested the device on people.
Although referred to as an electrified water cannon, this experiment involved a water jet much less powerful than a water cannon.
Water cannon differ from other similar devices in the volume of water delivered in a given time, the nozzle speed, the pressure that it is delivered at, and to a lesser extent the total volume that can be delivered. They are also generally portable. The method of employment is also important in labeling a device a water cannon. Nevertheless, the distinction between a water cannon and other similar devices is fuzzy. For example:-
- Pressure washers generally produce an extremely high pressure stream where the power of the stream drops off significantly over a very short distance.
- Water pistols and other toys deliver much less water at a much lower pressure with a much lower volume of water.
- Ultra high pressure water jet cutters are used to cut a wide variety of materials including granite, concrete (see hydrodemolition), ceramics, fabric and even Kevlar. One such cutter delivers 55,000 psi (380 MPa) through a nozzle 0.003 inch (8 micrometres) in diameter at 1 kilometre per second. This can cut a person in half at close range. There are reports of accidental deaths involving the industrial use of high-pressure water.
Water cannon are still in use on a large scale in Chile, Belgium and other parts of Europe.
The State of New South Wales in Australia purchased a water cannon in 2007, with a view to using it during an APEC meeting in Sydney that year. It was not used. It was the first purchase of a water cannon in Australia.
The annual riots on 1 May in Berlin, the Schanzenfest fair in Hamburg, which regularly ends in riots, or other demonstrations, are usually accompanied by water cannon, which support riot police. German communities use their water cannon in hot summers to water public parks. The most commonly used water cannon in Germany, which has more than 60 water cannons in total, is the Wasserwerfer 10000.
Until 2014, although manufactured there, there were only six water cannons in the United Kingdom, all held by the Police Service of Northern Ireland. Water cannon use outside Northern Ireland is not approved, and would require the statutory authorisation of Parliament in England, or of the devolved assemblies in Scotland and Wales. In June 2014, London's Deputy Mayor for Policing and Crime Stephen Greenhalgh authorised the Metropolitan Police to buy three second-hand water cannons from the German Federal Police. Mayor of London Boris Johnson said that the purchase had been authorised before Parliamentary approval, as the three cannons cost £218,000 to purchase and would require a further £125,000 of work before being deemed suitable for service, as opposed to £870,000 for a single new machine. But after a study of their safety and effectiveness, Home Secretary Theresa May said in Parliament in July 2015 that she had decided not to license them for use.
Truck-based water cannon, and fire hoses used as improvised water cannons, were used widely in the United States during the 1960s for both riot control and suppressing peaceful civil rights marches, including the infamous use ordered by Eugene "Bull" Connor in Birmingham, Alabama in 1963.  The newsreal footage of police turning water cannons and police dogs on both nonviolent student protestors and bystanders, including children as young as six, were widely viewed as shocking and inappropriate and helped turn public sympathies towards civil rights. 
Water cannons are used in hydraulic mining to dislodge rock material or move sediment. In the placer mining of gold or tin, the resulting water-sediment slurry is directed through sluice boxes to remove the gold. It is also used in mining kaolin and coal.
Croatian Police water cannon CVT-6000
German police water cannon WaWe 9000 content 9000 liter tank
French National Police water cannon
Water cannon of the French National Police deployed to prevent rioting following Nicolas Sarkozy's election, 6 May 2007
Demonstration against G8 meeting, 5/9/2007
A Dutch police water cannon.
Moscow OMON riot control water cannon police vehicle "Lavina-Uragan" on Ural-532362.
Internal troops ABS-40 "Lavina" riot control water cannon on BAZ-6953 chassis
Water cannon vehicle of Police of Panamá, friendly named "Pitufo" (Smurf).
The term "water cannon" could also refer to:-
- Similar land vehicles used for firefighting
- Numerous large toys, for example images 
- Waterjet in hydraulic mining
- A type of railway wagon used to remove fallen leaves off the track: e.g. seen at Alexandra Palace on 25 October 2003
- Tool for powerwashing large construction equipment. See riveer.com for images, details and video
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