Navigable by barges from Ulm (by larger craft from Regensburg), the Danube is an important artery; in volume, however, Danubian commerce is far below that of the Rhine. The Danube is linked to the Main and Rhine rivers by the Rhine-Main-Danube Canal; other canals link it with the Oder and Tisza rivers. Navigation is impeded by ice in winter and by seasonally varying water levels. Hungary and Czechoslovakia (succeeded by Slovakia) agreed in 1977 to develop hydroelectric projects, which have been pursued amid controversy. Pollution of the Danube has diminished once-rich fishing grounds and rendered the water unfit for drinking and most irrigation; cleanup has proceeded slowly. Romanian efforts in the 1980s to drain land in the delta for agriculture damaged Europe's largest wetlands, which are now being rehabilitated. During the Kosovo crisis in 1999, NATO air strikes destroyed bridges across the river in Serbia, obstructing river commerce; the debris was completely cleared in 2003.