Knee-deep in the water, the fisherman casts his line. He pays no attention either to the small sheep outlined on the opposite bank or to their shepherd's distant shouts. The two men can see each other quite clearly but do nothing to acknowledge it. They act as if they were on different planets.

And in a way they are. The fisherman is wading in Serbian waters, the shepherd and his flock are in Romania, and we are observing the scene from Bulgaria. We are all divided by the Danube and its tributary, the Timok.

The Timok, as geography textbooks tell us, is the river which runs alongside the final kilometres of the Bulgarian-Serbian border. Its mouth is Bulgaria's northernmost point.

The statistics cannot prepare you, however, for what awaits you at this piece of land on the border. Changeable and beaten by the two rivers, the spot is covered with sticky mud. It comes in curious shapes, at times in colours and outlines mimicking rocks.

The surrounding panorama is idyllic, and tolerable in the colder months due to the absence of mosquitoes. Thick reeds overgrow the bank, where trees lower their branches towards the water, and only the occasional fishermen's boat or a barge disturbs the Danube's peaceful surface.

A track which Bulgarian fishermen and shepherds have trodden among the vegetation leads to the mouth of the Timok. To find it, however, you need to have asked at least three people in Kudelin, the nearest village. Even following their directions, however, may not prevent you from getting lost in the labyrinth of pathways that criss-cross the low, arable land of the Bulgarian border. Instead of arriving at the river, you may just as well end up at the local mental hospital.

The Timok is not the only river whose course and outflow form a Bulgarian border. At the village of Rezovo, in the southeast of the country, the Rezovska river flows into the Black Sea.

The difference between the two rivers is immense. The border with Turkey is guarded by wire fences and warning signs, and on either bank you can see national flags fluttering.

Nothing like this is on view at the mouth of the Timok. You get the feeling that if you were to splash across the river, you'd be able to exchange a few words with the Serbian fisherman.

It goes without saying that the Timok used to be heavily guarded during the years of Socialism, as the relatively liberal Yugoslavia began on its western bank. The isolation has preserved the natural landscape along the course of the river, but has driven away the people who once lived there. The settlements in the region are now almost deserted. The short-lived "boom" that resulted from smuggling during the twice-imposed embargo on Yugoslavia in the 1990s is now a thing of the past. Today many young people from the area either live and work in Bulgaria's big cities or have left the country.

Only monuments – the one dedicated to Soviet pilots, in the village of Baley, for example – bear witness that this quiet region used to be at the centre of historic events. Kudelin, for instance, is named after the semiautonomous Bulgarian ruler from the 1270s. The land around the mouth of the Timok was among the last Bulgarian territories to fall under Ottoman rule at the end of the 14th Century. The Bulgarian rebel Haidut Velko used to roam through the region at the turn of the 18th and 19th centuries. In November 1885 some of the battles of the Serbo- Bulgarian War were waged at the nearby town of Bregovo.