题目解析：
【题目翻译】根据2段，关于水流中的砂坝下列哪个选项是正确的？
A：它们开始在河道和在银行扩大成形。
B：它们很少形成河流，用于商业导航。
C：它们每年继续生长。
D：它们通常不持续超过一年。
【判定题型】：题目问的是文章中的具体细节信息，故根据题目问法可以判断本题为事实信息题。
【关键词定位】：根据关键词“ bars in streams”,定位到Passage 2第二句，原句为“Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks.”意思是沙坝是细长的沙丘，是河道和河岸上形成的瞬态特征。
【逻辑分析】沙坝是瞬态特征，所以它们不会一直存在。文章中还说到砂坝通常在一年内形成，下一年就会被消磨掉。用于通商航行的河道必须频繁地修整，因为砂坝每年都会变。
【选项分析】
A选项：砂坝一开始在河道中形成，然后延伸到岸上。错误，因为文中说“Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks.” 砂坝是河流内部和河流两岸所形成的沉淀物的瞬态特征。所以，在河流内和岸上，都可以形成砂坝，这两者是并列关系，不分先后。故A选项与原文不符，排除。
B选项：砂坝很少在用于通上航行的河道中形成。错误，因为文中说“Rivers used for commercial navigation must be recharted frequently because bars shift from year to year.” 用于通商航行的河道必须频繁地修整，因为砂坝每年都会变。这说明通上航行的河道中每年都会形成砂坝，故B选项与原文矛盾，排除。
C选项：它们每年都会增长。错误，因为文中说“They commonly form in one year and erode the next.” 砂坝通常在一年内形成，下一年就会被消磨掉。它们的位置可能会变换，但是不会增长，因为第二年就被消磨掉了。故C选项与原文不符，排除。
D选项：它们通常存在不超过一年。正确，出处同C选项“They commonly form in one year and erode the next.”这句话恰恰能说明砂坝存在不会超过1年的时间，故D选项符合原文，为正确答案。
A stream deposits its sediment in three environments: Alluvial fans and deltas form where stream gradient (angle of incline) suddenly decreases as a stream enters a flat plain, a lake, or the sea; floodplain deposits accumulate on a floodplain adjacent to the stream channel; and channel deposits form in the stream channel itself. Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks. They commonly form in one year and erode the next. Rivers used for commercial navigation must be recharged frequently because bars shift from year to year. Imagine a winding stream. The water on the outside of the curve moves faster than the water on the inside. The stream erodes its outside bank because the current's inertia drives it into the outside bank. At the same time, the slower water on the inside point of the bend deposits sediment, forming a point bar. A mid-channel bar is a sandy and gravelly deposit that forms in the middle of a stream channel.
