Staffordshire Police Sergeant Fay Mills described the incident at the reservoir within the country park as “tragic”.
She spoke as teenagers were again spotted swimming at the site, despite clear warnings of the dangers.
Officers now believe that the young girl had got into “extreme” difficulty around 30 to 40 metres off the reservoir’s beach area before the 21-year-old went into the water.
The young girl made it safely out of the water, after she was pulled out by another passer-by.
But the man, who is yet to be named by Staffordshire Police, was pronounced dead at the scene on Sunday evening.
His body was only found after a search involving a number of boats, police, firefighters and members of the public.
His family have been informed and are being supported by specialist officers. He has not been named but is believed to come from the Tamworth area.
Staffordshire Police detectives have launched an investigation and are now urging anyone who was at Chasewater at the time, to come forward.
WATCH: Police reveal latest on investigation
Speaking at the scene, Sergeant Mills said: “We were called to reports of two people in trouble at Chasewater Reservoir on Sunday at 6pm.
“What we believe so far is that a nine-year-old girl, who we believe was about 30 to 40 metres out in the water got into trouble in the reservoir and a 21-year-old male has gone in to assist her.
“The young girl made it out safely.
“Tragically, the man who had gone into support her and save her life, has died.
“It is tragic. He has gone there and seen a nine-year-old child in difficulty and he has gone in there to save her and sadly lost his life.”
Sergeant Mills added: “We are still investigating if there was anyone else in the water with the girl at the moment, so we are trying to establish exactly what went on at the time.
“Our investigation is still ongoing at the moment and we are trying to determine the circumstances.
“Next of kin are aware. We do know the people involved and the families have been told.
“We are now appealing for anyone in the area at that time or any other witnesses to contact us.”
Signs stating 'no swimming' line Chasewater reservoir, but only a day after the tragedy youngsters were seen in the water. They were swimming close to the area where the man died.
Chasewater is a popular family attraction with walking trails and play areas for children, and the reservoir is often used by boats and water skiers.
A water sports centre and sailing club are based there. Floral tributes were yesterday left near to the scene of the tragedy.
Tributes have been paid to the 21-year-old man on social media, from residents, local councillors and Cannock Chase MP Amanda Milling.
Walkers out in Chasewater Country Park on Monday morning described the man’s death as a tragedy, with one adding: “It is just devastating for this to happen here and he has lost his life at such a young age. My thoughts are with his family.”
Anyone with information about the tragic incident should call detectives at Staffordshire Police on 101. They should quote the incident reference number 654 of June 30.