Police were called to Ardsley Reservoir in Tingley on the evening of Friday, May 29 when a group of young people were found gathering and swimming in the reservoir.
A young girl had got into difficulty while swimming and needed help to get out of the water.
She was taken to hospital with minor injuries.
Officers from the Leeds South neighbourhood team said they were "appalled" to find a huge amount of litter had been left across the reservoir.
A spokesman for the team said: "Following on from an incident at Ardsley reservoir on Friday officers carried out patrols today to observe people’s actions and behaviours.
"We were appalled to see the amount of litter that has been left by people visiting the area!
"This is a lovely area that we are lucky to have on our door step, which people like to enjoy walking or running round it for exercise.
"Yet there is a minority who simply are spoiling it for others. This is a disgrace!
He added: "Littering is an offence, and anyone who is caught littering can be dealt with by way of a fixed penalty notice.
"This is permitted by section 88 of the Environmental Protection Act 1990.
"A fixed penalty notice sets a penalty to be paid within a set amount of time. The maximum fine for littering is £2,500.
"Please take your litter home with you!"
Following Friday's incident, West Yorkshire Police issued a warning 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."
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