A search has begun for an 11-year-old boy who went missing after being seen entering a river in South Yorkshire.Police were called at 7.59pm on Saturday after the schoolboy was seen in the River Don near Ferry Boat Lane in Mexborough but was not spotted getting out.Doncaster Response Team Inspector Neil Pryce said officers knew where the boy got into the water because of witness accounts and items left on the riverbank. Officers are searching for the boy with fire crews, medics, the National Police Air Service and an underwater search team.Inspector Pryce said: 'We know, from witnesses and items left on the riverbank, where the boy entered the water.'Utilising specialist knowledge and skills, a coordinated multi-agency operation is under way and we are doing everything we can to find this young man.'We are invaluably assisted by an underwater search team, supported from above by the police helicopter and drones. This is a tragic incident and we understand that members of the public want to do all they can to help but we must ask that, for everyone's safety, only those who are involved in our operation enter the River Don.'South Yorkshire Police said: 'Officers, alongside emergency crews from the National Police Air Service, South Yorkshire Fire and Rescue Service, and the Yorkshire Ambulance Service, responded and a specialist search operation remains under way.' The boy was seen entering the River Don in Mexborough, near Doncaster, on Saturday eveningThe boy's family have been told and are being supported by specialist officers.His disappearance comes after at least 13 young people from swimming in open water during the May heatwave, which saw the record for the month's hottest day broken twice in two days.A 15-year-old girl died on Saturday after getting into difficulty on a Merseyside beach on Monday.Chiedza Nyanjowa's family said she was a 'bubbly' person who 'loved Christ and going to church'. Chiedza Nyanjowa was believed to be the 13th person to die during the May heatwaveOn Friday Police Scotland named a 16-year-old who died after getting into difficulty in the water in Stirlingshire as Charlie Noble.And on Wednesday the body of 14-year-old Baltazar L'Qui was found in the River Thames while another teenage boy was found in a pond in Swanscombe, Kent.The same day, a missing teenage boy's body was found in a lake near Blackwater in Hampshire after he went missing while swimming.On Tuesday, two boys were found dead, including 17-year-old David Junior-Tita from Crewe, who died at Pickmere Lake in Cheshire.Junior Slater, 12, also died after getting into difficulty in the River Ribble in Lancashire.Junior Slater, 12, and Declan Sawyer, 15, both died while swimming in open water during this month's heatwaveThat day police also recovered the body of a drowned 16-year-old boy from Sheffield who died in a Rotherham lake after going missing while swimming.Muhammad Secka, who had drowned the previous day, was described as a 'lovely, kind boy' who dreamed of becoming a doctor.There were four more deaths on Monday as temperatures reached 34.8C at Kew Gardens and broke the record for the hottest-ever May day.Among them were 13-year-old Reco Puttock, who was pronounced dead after being found in Leadbeater Dam in Halifax, West Yorkshire.The same day 16-year-old Lillianna Tomlinson was found dead at Warwickshire's Kingsbury Water Park.Heroic grandfather Phil Crow, 68, also died after suffering a cardiac arrest when he ran out to the sea trying to rescue his wife and granddaughter when they got into difficulty at Tregirls Beach in Padstow, Cornwall.On Sunday, an unnamed 72-year-old woman died off West Angle Beach in Pembrokeshire. In Lincoln that day 15-year-old Declan Sawyer also died after getting into difficulty at Swanholme Lakes.The UK Health Security Agency (UKHSA) issued heat health alerts for parts of England this week and warned of a greater risk of water-related deaths.It said there was a higher likelihood of cold-water shock and drowning as more people took to lakes and ponds in the warm weather.