While Napoleon and the Grand Army in force advanced along the south, or right, bank of the Danube, Mortier had been detached across the river to hold in check any attempt to interfere with the main operations from the Bohemian side. A body of Austrian cavalry, under the Archduke Ferdinand, had managed to cut their way through from Ulm at one point just before the closing of the net round General Mack. With the aid of the local militia levies these might prove troublesome on the line of communications. To deal with them, three divisions, drawn from as many corps, were amalgamated as Mortier’s special corps, which numbered in all between twenty and twenty-five thousand men: Gazan’s division, lent by Marshal Lannes; Dupont’s, lent by Ney; Dumonceau’s, lent by Marmont. To keep Mortier in touch with the main body of the army, and that he might be reinforced in emergency, a flotilla of Danube craft was at the same time improvised, and placed in charge of the Seamen of the Guard, a battalion of whom had accompanied Napoleon for the campaign. The flotilla was to keep pace with Mortier and link him with Napoleon. Mortier crossed at Linz and moved forward; his three divisions each a day’s march apart, for convenience of provisioning. He marched so fast, however, that he outstripped the connecting boats. [Sidenote: THE DANUBE FLOTILLA STOPPED] At the moment the fighting opened, the flotilla was miles in rear. It had been stopped and its progress blocked near Moelkt, unable in the swollen state of the Danube to pass the dangerous Strudel, or whirlpool, there, raging just then, after the heavy autumn rains, with the force of a swirling maelstrom. The flooded river had made it extremely difficult work all the way, even for the picked Seamen of the Guard, to navigate with safety the assortment of boats and timber rafts, clumsy structures of logs and spars lashed together, 160 feet long each, and planked over, with cabins on the planks, which composed the flotilla. On them, together with a quantity of spare stores and ammunition for the army, convalescents and footsore men of various regiments were being carried, who, it was intended, would thus be on the spot to reinforce Mortier first of all in case of danger. Immediately after passing Dürrenstein, the leading division, General Gazan’s, numbering some 6,000 men, unexpectedly stumbled across part of the Russian rearguard. All unknown to Mortier, the Russian army corps which had been entrenched in front of Vienna had abandoned its position and had hastily withdrawn north of the river, crossing a short distance from Dürrenstein.