Children as young as 10 years old have been gathering at a dam on the South Esk river near Lothianburn, Newtongrange, according to reports received by police.
Those gathering have been jumping from the dam into the river. Police said the dam was a “considerable height” above water level.
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The river is shallower than normal in areas following last week’s heatwave and with water in shorter supply across the Lothians, increasing the risk of serious injury.
The children involved range in age up to 15.
Midlothian Police said those visiting the site were also involved in anti-social behaviour and called on parents and guardians to step in.
Community Sergeant Michele Lindsay, based at Dalkeith Police Station, said: “This kind of behaviour is dangerous and puts lives at risk.
“Some of the groups gathering in this area are also involved in anti-social behaviour, which includes shouting abuse at people.
This has a disruptive effect on local communities and will not be tolerated. We will act on any information we receive and, in addition, regularly carry out patrols in areas of concern.
"I am appealing to parents and guardians, in particular during the summer holidays, to assist us in keeping children safe and urge them to make sure that they know where their children are, who they are associating with, and what they are doing.”