Another of the small southern tributaries of the Niemen which offered excellent opportunities for resistance of which the Russians promptly availed themselves, was the Zelvianka River, which joins the Niemen just west of Mosty. The fighting which went on there for a few days was almost exclusively in the form of rear-guard actions, and was typical of a great deal of the fighting during the Russian retreat. Whenever the Germans advanced far enough and in large enough numbers to endanger the retreating armies, the latter would speed up as much as possible until they reached one of the many small rivers with which that entire region abounds. There sufficiently large forces to delay the advance, at least for a day or two, would be left behind to use the natural possibilities of defense offered by the waterway to the best possible advantage, while the main body of the army would move on, to repeat this operation at the next opportunity. In most instances these practices held up the German and Austrian advance just exactly in the manner in which it had been designed that it should. Furthermore, the Russians would not give way until they had inflicted the greatest possible losses on their enemies, and in that respect they were frequently quite successful. For first of all many of these rivers have either densely wooded or very swampy banks which lend themselves admirably for defense to as brave a fighting body as the Russian army, and which proved exceedingly treacherous to the attacker; and in the second place the Russians, of course, had the advantage that they were fighting on their own soil, while the Germans were in a strange and often hostile country. In spite of this, however, the German advance, taken all in all, could not be denied, and in practically every one of the cases just described, the final outcome was in a very short time defeat for the Russians and a successful crossing of the watery obstacle by the Germans. This was true also at the banks of the Zelvianka, where the Germans on September 9, 1915, stormed successfully the heights near Pieski, capturing 1,400 Russians. This success was followed up by further gains on the next day, September 10, 1915, that again yielded a few thousand prisoners. A few days later the crossing was forced and the Germans began to attack the Russians behind the next Niemen tributary, the Shara.