Lake Grabensee is one of the three "Trumer Seen" in the Salzburger Seenland area (Salzburg Lake District). In fact, the Grabensee is the smallest of these lakes, which are all the "descendants" of one large lake that formed after the last Ice Age. Like the other two Trumer Seen lakes, the Mattsee and the Obertrumersee, the Grabensee is also rather swampy and surrounded by marshes. It is only about two kilometres long and 800 metres wide.
The creek Mattig enters and leaves the Grabensee, it then runs northwards to merge with the Inn River near the town of Braunau. The swamps and marshes around the Grabensee are part of a nature reserve, which results in rather limited opportunities for swimming and other water sports. Motor vehicles are not permitted on the lake, which is common on Salzburg′s lakes, but in the case of the Grabensee the authorities went one step further and outlawed windsurfing and any form of boating as well.
The Grabensee is among the warmest lakes of Austria. This is due to the small volume of the water body; the lake is only up to 14 metres deep and therefore, heats up quickly. Water temperatures of up to 30 degrees can occur on the peak of the summer heat in extreme summers. All of the Grabensee lies within the municipality of Berndorf.
That does not keep the Upper Austrian community of Perwang to call itself "Perwang am Grabensee". This is because a tiny section of the shoreline belongs to Perwang, which operates a public lido. The lido (Strandbad) is the only place where you can enter the Grabensee for swimming. "Lido" is a bit of an exaggeration, since the facilities comprise of little more than an area where the grass is cut, a small wooden wharf and a raft, fixed on the lake.
German Wikipedia on the Grabensee
Information on sailing, gear, boats and vacations
More on the Salzburger Seenland Lake District
Information on the Salzkammergut