Activity at Loch Vaa, near Aviemore in the Cairngorms, has puzzled experts given the fluctuating highs and lows of the water there.
Fed by a spring, it was reported in May last year that the water level had inexplicably dropped by 1.4metres.
Now, those who care for the loch have reported that the water levels have now rised by an extra 2.5metres – or 8ft.
Brian O’Donnell, who manages the loch, told BBC: "The boats in the boathouse were hitting off the roof.”
"It is the highest I have seen in 20 years. This has happened during what is likely to have been one of the driest Aprils, and after a poor winter with little snow-melt."
"It is pretty amazing and we might never know what is going on with the Loch Vaa."
After losing tens of millions of gallons of water over the course of about nine months to May last year, Loch Vaa returned to normal in September 2019 - but only after almost 40 days of rain, the BBC reported.
The Scottish Environment Protection Agency then said the loch had suffered due to a "relatively dry" winter.