Lake Constance

A lake with no line

Lake Constance (the Bodensee in German) lies on the Rhine at the northern foot of the Alps.^[5] Its shores belong to the German states of Baden-Württemberg and Bavaria, the Swiss cantons of St. Gallen and Thurgau, and the Austrian state of Vorarlberg.^[1] The big basin is the Obersee (Upper Lake), which includes the long north-western arm called the Überlinger See; a short stretch of the Rhine at Konstanz links it to the smaller Untersee (Lower Lake).^[5][4]

Across most of the Obersee, no treaty says where Germany, Switzerland and Austria meet.^[5][1] The IGKB, the commission the shore states set up to protect the lake, calls it the only area in Europe where the borders between the neighbouring states have never been exactly fixed.^[1]

There was no need for one at first. Before the Thirty Years’ War (1618–48) the lake lay inside the Holy Roman Empire, so it was an inland lake with no borders to draw.^[1]

The parts that are settled

The Untersee was divided in 1854, when the Grand Duchy of Baden and the Swiss canton of Thurgau agreed on the middle of the Rhine and of the lake as their border.^[1] The survey offices’ map still labels that line simply “in the middle of the lake”, and notes that a new treaty defining it by modern survey is being prepared.^[6]

Between Konstanz in Germany and Kreuzlingen in Switzerland, the border runs from the shore out into a funnel-shaped bay, the Konstanzer Trichter.^[5][1] On 28 April 1878 Switzerland and Baden signed a treaty to settle, “in a neighbourly way”, the disputes over the border at and on the lake by Konstanz.^[7] Part of its line through the bay follows a sightline from the middle of the tower of Konstanz railway station.^[7] Germany and Switzerland confirmed the line in a new treaty in 1938.^[1]

Nobody disputes the Überlinger See: it belongs to Baden-Württemberg alone.^[1] The survey offices’ map closes it off with a line from Meersburg across to Eichhorn on the Konstanz shore.^[6]

Three theories for the open water

For the rest of the Obersee, lawyers describe three theories.^[1][3] The first is the condominium theory: the whole Obersee, up to the shoreline, is under the joint rule of all the shore states.^[1] (A condominium is territory that two or more states govern together.)

The second is the Halde theory. The Halde is the shallow shelf along the shore, down to a depth of 25 m.^[1] Each state owns the Halde in front of its own shore, and only the deep water beyond it, the Hohe See (“high lake”), is shared.^[1][6]

The third is real division: a border down the middle of the lake.^[1] Switzerland claims that line, Austria holds that the lake is shared, and Germany’s position has been called ambivalent.^[5][3] The survey offices’ map shows the Halde theory as Austria’s official view and the open water as a condominium in the view of Germany and Austria; it draws the Swiss claim after the 1973 navigation treaty.^[6]

Vorarlberg wrote its view into the constitution it adopted in 1984. The Halde off its shore and the Hohe See both count as its territory, but on the Hohe See its sovereign rights are limited by the same rights of the other shore states.^[8] Kramsch reports that the neighbouring states met this with silence.^[5]

Running a lake without a border

In practice, the IGKB says, the open water of the Obersee is treated as the common property of the three states and run jointly.^[1] At Steckborn on 27 October 1960, Baden-Württemberg, Bavaria, Austria and Switzerland agreed to protect the lake from pollution and set up a standing commission to do it.^[9] Two German states signed, not Germany itself; the federal government could send observers.^[9]

Navigation came next. A treaty signed “on Lake Constance” on 1 June 1973 by Germany, Austria and Switzerland replaced rules going back to 1867, and says outright that it does not touch the course of state borders.^[4] It splits the Obersee into three zones, each policed by the state whose shore it lies in front of.^[4] Police may cross into another state’s zone to deal with an accident or chase a boat suspected of breaking the rules, but not into the strip 2–3 km off that state’s shore, where only its own police act.^[4]

In 2026 Switzerland and Germany were preparing a treaty to set out their whole border, from Basel to Konstanz, in one place instead of a patchwork of old agreements.^[3] It leaves out the Obersee. Swisstopo, the Swiss mapping office, said there is no defined national border there and that the shore states want to keep the status quo.^[3] The canton of St. Gallen objected that a treaty meant to make the border clearer misses the point if the Obersee is left out.^[3]

“Other matters of sovereignty on Lake Constance, in particular the course of state borders, are not affected by this convention.”^[4]