Police were called to Ardsley Reservoir in Tingley on the evening of Friday, May 29.
A group of young people were found gathering near and swimming in the reservoir.
The young girl had got into difficulty while swimming and needed help.
Emergency services were called to get her out of the water.
She was taken to Leeds General Infirmary for treatment.
Officers stayed on scene and dispersed the group.
Following the incident, police patrols will be increased in the local area.
West Yorkshire Police have now spoken out to warn people about the life threatening dangers of swimming in open water.
Chief Inspector Allan Raw of Leeds District Police said: "This incident in Tingley serves as a stark reminder of the very real dangers of swimming or playing in open water, no matter how tempting it may appear in warm weather.
"Fortunately, the young person involved in this incident is expected to recover but the consequences could have been much worse.
"Water such as reservoirs or lagoons can be very cold, even in the summer months, and people suddenly entering cold water are susceptible to hyperventilation, which could result in the body going into seizure.
"There can also be hidden currents and unseen dangers such as mud banks or items under the water which a swimmer can become caught on.
"As inviting as the water can look, it simply isn’t worth the risk."
A WYP Leeds South spokesman said: "Ardsley Reservoir, Tingley. !!NO SWIMMING!!
"We understand how tempting swimming in water can be, especially whilst we are having some hot weather.
"But we are telling you, not to!
"Last night there was an incident at Ardsley Reservoir which involved a number of people swimming in the water.
"This resulted in someone needing rescuing from the water, mbulance attention and assistance from a number of officers and emergency services.
"They were taken to hospital due to low blood pressure and they had inhaled a lot of water.
"Luckily, on this occasion they survived and are doing ok.
"That is not always the case, people have died from swimming in reservoirs.
"We cannot stress to you enough how dangerous it is!
"It may look beautiful but it can be deadly!
"Cold water shock can kill you in 60 seconds!
"!!DO NOT SWIM IN THE RESERVIOR!!"
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