Over the centuries, this river has borne witness to epic struggles between East and West, Allies and Axis, and most recently, capitalism and communism. It may be a mere tributary of the mighty Danube but the Sava is high on the list of Europe's major rivers. On its 1,000km journey from west to east it cuts a swathe through the lands of the former Yugoslavia, connecting four countries and three capitals, and dividing old city centres from their newer suburbs. My own travels beside its greenish waters brought encounters with adventure tourism, endangered wildlife and unusual wines, not to mention castles, fortresses, monuments and museums.
The Sava is born high in the Julian Alps, near Slovenia's border with Italy, and – as befits a serious river – has several tributaries of its own, two of which can claim to be the source. The Sava Bohinjka hurtles down the mountainside and has become the place to enjoy, if that's the right word, the thrills of white-water kayaking and canyoning. These can be booked in the picturesque resort of Bled, the most attractive of Slovenia's many lovely towns. It is a base for walkers, cross-country skiers, climbers ascending lofty Mount Triglav (2,864m) and those who just want to chill out beside Bled's pristine lake, whose tiny teardrop island is decorated with a fairy-tale castle.
Bled lies in the fork between the Sava Bohinjka and its fellow tributary, the Sava Dolinka. They join to form the Sava proper near the little town of Radovljica. Apart from its views, this is worth visiting for its charming museum of bee-keeping, which celebrates a traditional Slovenian enterprise.
I joined up with the river again at Slovenia's compact capital, Ljubljana, where it provides a shimmering focus for frittering away lazy summer evenings at outdoor cafés. From an over-the-top railway station – about the size of Clapham Junction, for one-hundredth of the trains – I took the international train which follows the course of the Sava into Croatia. We clattered through a high narrow valley with the occasional glimpse of a red-roofed farm clinging to a steep meadow before the valley broadened out, beyond the railway junction of Zidane Most. This is the region of Posavje ("Land of the Sava"), one of Slovenia's largest wine-making regions.
"You mustn't leave here without trying the Cvicek," said one of my fellow passengers. It's gorgeous: ruby-red with a dry, slightly acidic taste. A good place to stop off for a tasting is Krsko, the town where much of Slovenia's energy is generated and whose prettier neighbour, Brezice, 12km down-river, boasts a castle with a fine ethnographic museum and a spectacular room called the Knight's Hall, which drips with Baroque painted scenes.
After entering Croatia, the Sava winds its way round the southern edge of Zagreb, separating the main city from the new suburb of Novi Grad where the massive residential tower blocks built during the socialist era of Yugoslavia have been joined, this year, by the Museum of Contemporary Art, the largest cultural institution in Croatia.
The river is navigable, in theory, between here and distant Belgrade but it's not until you reach Sisak that boats of any size can make it upstream from the Serbian capital. Sisak is an unremarkable town in most respects but it is one of the main gateways to the Lonjsko Polje Nature Park. Access to the park is via a narrow, winding, often pot-holed road which follows the Sava's lazy meanders. Lined with woodlands, it is dotted with tiny villages of wooden houses. The first that you come to, Cigoc, gained the accolade of the first "European stork village" for reasons which are immediately apparent if you visit between April and August, when the roofs of all the houses in the village are decorated with sprawling untidy nests and the air is filled with the click-clacking sound which the white storks make with their beaks.
Paths and cycling trails have been constructed to help visitors discover the park, though it's as well to be prepared for watery walks since the level of the apparently tranquil and torpid river can rise up to 10m in 48 hours, inundating the flood plains and the so-called retention areas. Apart from the wetlands and the riparian forest, the park is also rich in small fields and oak woods where you can find three unusual and endangered breeds of livestock roaming free: the Posavina horse, the long-horned Podolian cattle and the rather attractive, shaggy-coated, spotted Turopolje pig, which lives mainly on acorns.
The oaks have also provided wood to build the local houses, some of which are more than 200 years old, and which have (or lack) some surprising features. "Why do you think that the houses have no chimneys?" asked the Park's chief guide, Edoard Bogovic, as he showed me round a house at the educational centre in the "architectural heritage" village of Krapje. And the answer? So that that the space inside the lofty roofs can be used for smoking sausages, hams, garlic and paprika.
Visitors to the Park can find out what it's like to stay in one of these houses, as well as enjoy the local cuisine, thanks to the spread of agroturizam. Pioneers in this agricultural tourism business are the Ravlic family, who have converted their traditional farm in Muzilovcica into a guesthouse and restaurant. I enjoyed their hearty al fresco lunch of cured sausage while ducks and chickens scrabbled around and the sun glinted on the nearby Sava.
Despite the natural beauty and images of rural tranquillity, there are constant reminders of the wars that have periodically ravaged this area. Along the "borderers'" walking trail from Krapje, wooden observation towers have been built, replicas of the chardaks that marked the conflict-strewn border between the Austro-Hungarian and Ottoman empires.
At the eastern end of the park, near Jasenovac, it is very unwise to leave the marked paths and dykes due to the danger of undiscovered land mines, laid during the Balkan wars of the 1990s, while Jasenovac is the site of a Second World War extermination camp, which was run by a puppet Nazi regime, the Ustase. A museum and a striking sculpture of a lotus-flower are monuments to a time when the Sava frequently ran red with blood.
At Kutina on the northern edge of the Lonjsko Polje Park, I caught a train to Slavonski Brod, whose main tourist attraction is the massive 18th-century fortress that was built to defend the Austro-Hungarian border against the invading Ottomans. From the cafés on the airy pedestrian square, the Trg Mazuranica, you can look across the river, now much broader and a frontier to boot, to the matching town of Bosanski Brod in Bosnia and Herzegovina. Since the break-up of Yugoslavia and the creation of new states the river has seen virtually no traffic, so a French cruise company recently made waves when its boat, the Victor Hugo, brought 80 foreign tourists to Slavonski Brod en route for Sisak. But this is as yet an isolated occurrence, leaving most travellers to choose between the motorway or the railway for the onward journey to Belgrade. There is one more point where road, rail and river meet again: at the Serbian town of Sremska Mitrovica, once one of Rome's regional capitals. No fewer than 10 Roman emperors were born in or around Sirmium, as it was then called, and although most of the Roman city is still to be uncovered, the current excavations can be seen at Nikola Pasic square.
If you dropped a stick into the Sava at Sremska Mitrovica, it is hard to believe that it would take longer to reach Belgrade than the train, which creaks across the Sava for the first time, just before it reaches Belgrade station, its terminus. The Sava is also in its final throes, about to mix its greenish waters with the unexpectedly brown-coloured Danube. The confluence is a great natural spectacle which you can gaze upon from a terrace high up in the grounds of Belgrade's fortress, Kalemegdan.
An appropriate final reminder of the Sava's contrasting legacies is to be found in the island at the very centre of the confluence. Great War Island, Veliko Ratno Ostrvo, was once the centre of continual conflict between Turkish and Austrian armies, but it now contains a bird reserve and – in summer – a popular beach. For now, it's all quiet on the Balkan front.
Travel essentials: The Sava
* You can also fly to Ljubljana on Adria Airways (00 386 1 369 10 10; adria.si ) from Gatwick and Manchester; to Zagreb on Wizzair from Luton or Croatian Airlines (00 385 1 66 76 555; croatiaairlines.com ) from Gatwick or Heathrow; and to Belgrade from Heathrow on BA (0844 493 0787; ba.com ) or JAT Airways (00 381 11 3114 222; jat.com ).
* From Ljubljana to Belgrade, an open second-class ticket costs £65 from Trainseurope (0871 700 7722; trainseurope.co.uk ).
* Lonysko Polje Nature Park accommodation: Pension Domagoj, Krivaj 83, Lipovljani (near Kutina; 00 385 44 680 761). B&B costs €18 per person.
* Restaurant and Agroturizam Ravlic: Mužilovcica 72 (00 385 44 710 151). Lunch is €14, B&B costs €27 per person.
* Croatian National Tourist Office: 020-8563 7979; croatia.hr .
* Park website: pp-lonjsko-polje.hr .