GREIN WHIRLPOOL From Passau to Linz the Danube is hemmed in by mountains, but its valley becomes wider below the latter town where the river subdivides into several arms which unite again at the once famous whirlpool near Grein. Below Grein, the river flows through an other narrow defile as far as Krems but once more subdivides as its passes toward Vienna. The district between Linz and Vienna is renowned for its beauty and for the numerous places of histori cal and archaeological interest along the river's banks. At Vienna the river is 316 yd. wide, and 429 ft. above sea level and below the town is the district of Marchfeld which is a low-lying country across which the Danube frequently subdivides forming numerous islands. An important left bank tributary, the March, which drains Moravia, joins the main stream here. Before reaching Bratislava (Pozsony, Pressburg), the Danube passes through the narrow gap between the lower spurs of the Alps and the Car pathians and enters upon the middle section of the river. At this gap the river also passes out of Austria and for a few miles is entirely in Czechoslovakia but it very soon becomes the boundary between that country and Hungary, until it reaches Esztergom (Gran), below which town it enters Hungary.
The Danube flows for the first too m. of its middle course upon alluvial and Quarternary deposits of the Little Hungarian Plain. This latter is separated from the Great Hungarian Plain by the Bakony Wald ridge, the innermost arc of the Carpathian mountain system, and the Danube breaks through this ridge at Esztergom. After leaving Bratislava, the river divides into three channels forming several islands, but these join together again at Komarom where the river is also joined on its left bank by the river Waag which rises in the Carpathians. Higher up the stream at Gyor (Raab), the southern branch of the divided Danube is joined by the river Raab which rises in the Styrian Alps. Between Esztergom and Waitzen (Vacz) the valley becomes narrow until at the latter town the river turns southward to flow in that direc tion for 23o m. across the great Hungarian plain. In this long stretch, the Danube meanders about in a wide alluvium-filled val ley, frequently dividing into two or more streams and passing Budapest, Baja (where it leaves Hungary and enters Yugo slavia) and Mohacs. At Almas, 14 m. east of Osijek, the Danube is joined by the important right bank tributary, the Drave (height 81 ft.), which rises in Tirol and drains a large portion of the Eastern Alps. The Danube is again diverted eastward at Borovo by the Fruska Gora, and it flows along the northern edge of this range, passing Novi Sad (Ujvidek), until it reaches Belgrade. Be tween these two towns, the Danube receives the important left bank affluent, the Tisa (Theiss), which, rising in the Carpathians, drains the greater part of the western slopes of those mountains, as well as the great Hungarian plain. At Belgrade, the Danube is joined by the important right bank affluent, the Save, which, rising in the Julian Alps, flows eastward and drains the greater part of western Yugoslavia, whilst the eastern part of that coun try is drained by the Morava, which joins the Danube between Belgrade and Bazias, also on its right bank.
The whole character of the Danube valley changes suddenly at Bazias, and between that town and Turnu Severin, the river has worn out for itself a channel through the mountain ridge which joins the Carpathian arc with the Balkan mountains. A large part of the great Hungarian plain, which covers an area of about 30,00o sq.m., is remarkably flat and low-lying, and the altitude rarely exceeds 30o ft. In consequence, natural drainage by the Tisa and the Danube is very poor, and where artificial drainage has not been carried out, the banks of the rivers are in many places lined by wide swamps and marshes, which in winter form large ice-fields. Until comparatively recent geological times, this plain formed an extensive inland sea, whose final effluent followed the present course of the Danube through the Kazan defile and the Iron Gates. By the lowering of its channel through the gap (the level of the Danube at Orsova is now 42 ft. above sea level), this epicontinental sea was drained, leaving the great Hungarian plain covered with a thick deposit of alluvial sands and gravels. Hemmed in by precipitous rocks, the river passes through the stupendous Kazan defile (162 yd. wide), then widens out to nearly a mile at Orsova, but becomes narrower again at the Iron Gates. The river has been cleared of numerous obstructions to make possible navigation along this stretch.