Images from a local resident, Clive Williams, show a significant drop in the volume of water at Lake Vyrnwy reservoir in Powys.
The water level is approximately 15 feet lower than normal, according to Mr Williams, who is a regular visitor to the dam.
The healthcare assistant from Welshpool said: “I have seen it this low once before in 1996, but nothing like this since then. It must be about 15 feet lower than it usually is”
“They are taking water out of the reservoir to the top up the river levels so the hot weather does not affect the wildlife.
"The water has been warm to swim in, it is like a warm bath, which is nice.
“We usually head to Lake Vyrnwy to walk and take photos of the wildlife. But it has been too hot recently so we have been paddleboarding.”
The images show the water being well in from the sides of the reservoir, and people can walk on areas usually completely covered.
Other pictures show marks on the dam walls where the initial water line was, with the current water level significantly lower.
It comes after the UK experienced severe weather last week, with temperatures of more than 40 degrees registered in some parts of the UK.
In Shropshire, where a near 32-year-old temperature record was broken during last week's heatwave, water companies said even though they were monitoring water levels, they did not think the situation had become severe enough yet to bring in a hosepipe ban.
A spokesperson for Severn Trent said: “There hasn’t been a hosepipe ban in our region for more than 20 years. We continue to monitor reservoir levels closely.”
“It is essential that people stay safe and hydrated and we’d like to thank all of our customers for their support in being mindful about non-essential water use as the hot weather continues.”