- "The Ancients don't put Stargates on a flood plain, and I certainly don't think the tide just came in."
- ―Dr. Rodney McKay[src]
A Flood is a situation in which a body of water rises and overflows an area, especially at a typically dry location.
History
In 1998, when Rogue NID stole the Touchstone from Madronans they brought it to Earth and experimented with it which caused unusual weather conditions, such as flooding in Palm Springs, California and Florida. (SG1: "Touchstone")
In 2004, during the Lantean storm, Atlantis experienced flooding on the North, West and East Piers. When the Genii invasion of the city ended, Dr. Rodney McKay reported that they had begun to pump out the lower levels of the East Pier which were flooded. (SGA: "The Eye")
Anything salvageable from the flooded sections were moved to the main tower. Detailed records were kept of what was found and from where. (SGA: "First Contact")
Atlantis' outbreak system may have been affected by the flooding. When the Atlantis expedition explored the city after the storm, they encountered a nanovirus which the City-ship's automated detection systems didn't originally initiate a quarantine lockdown for. It only did after Peterson transported out of the east side and entered the mess hall. (SGA: "Hot Zone")
In 2006, Dr. McKay and Captain Hugh Griffin ran a test flight on Jumper 6 after it had previous been shot down on Olesia. During the test, the ship crashed into Lantea's ocean and the forward section ended up flooded after the windshield gave into the ocean's pressure. Griffin sacrificed himself in the incident, saving Rodney in the jumper's other section.
After the jumper impacted against the ocean floor, the rear compartment also began to flood due to micro-fissures the impact caused. While the leaking couldn't be stopped, Rodney slowed it by creating a positive pressure environment after his hallucination of Samantha Carter suggested it. (SGA: "Condemned", "Grace Under Pressure")
In 2008, on the planet M44-5Y9, a research team led by Dr. Nichols were studying the effects of global warming. When the team was late for a check-in, AR-1 went to the planet and found the whole valley the Stargate was located in flooded. When they redialed Atlantis, they couldn't lower the Stargate's shield else the entire control tower would also become flooded. The constant pressure of the water kept the Stargate active until the Atlantis expedition could send them a Puddle Jumper to wait out in until the flood receded.
M44-5Y9's flood was speculated to have been caused by an ice dam breaking, as the Ancients don't place Stargates on flood plains. Also, Dr. Nichols team had set up camp at the foot of a massive glacier and the flood waters were freezing.
Having been suffering from a viral infection before he even went on the mission and then having been left in the freezing water for a while, Dr. McKay's resistance was low, and he succumbed to the Second Childhood parasite. (SGA: "The Shrine")
Alternate timelines
In an alternate timeline, when the Atlantis expedition traveled to Atlantis, the city's shield rapidly began to collapse, flooding the city as its power ran out. Colonel Marshall Sumner and others drowned in the flooding. The city's bulkheads shut which trapped people with the rising waters. Dr. Rodney McKay worked to get the bulkheads open as well as the roof hatch to the gate ship bay as the control room flooded. However, he had no time, as the room flooded within seconds, killing him as well. Lt. Aiden Ford and Dr. Carson Beckett drowned while attempting to get people into Puddle Jumpers. (SGA: "Before I Sleep")