A rescue operation saved 150 fish after a section of a Buxton river ran dry.
The Environment Agency investigated after the discovery of 100 dead trout in the River Wye at Burbage recently.
Officers determined the cause to be a fissure appearing in the limestone rock, draining away the water to a one kilometre section of the river off Macclesfield Old Road.
Environment Agency spokesman Tim Carter said: “On arrival it looked like a one kilometre section part of the headwaters of the River Wye had literally disappeared. The waterfall had worn down and a fissure had appeared in the limestone rock, draining the water away.
“Luckily another tributary one kilometre downstream provided enough water to ensure there were not major difficulties further downstream.”
Mr Carter said a similar incident had happened 35-years-ago, but at a different location.
He added: “We have enlisted the support of a limestone specialist and will be using a water dye technique to locate where the water has gone. Some water is now avoiding the fissure and running downstream. We are keeping a close eye on the situation and need to assess all the associated risks with possible actions we could take such as filling in the fissure.”