题目解析：
【题目翻译】为什么作者要求读者“想象一条蜿蜒的小溪”？
A：为了解释水柱的存在如何改变水流的速度和方向。
B：解释为什么沙洲比冲积扇或其他类型的沉积物更常见。
C：说明商业航行在许多河流上面临的特殊困难。
D：帮助解释砂坝是如何形成的。
【判定题型】：根据题干问法“why …… ask”，题目询问“Imagine a winding stream”(文章引用内容)的修辞目的，可以判断本题为修辞目的题。
【关键词定位】：根据关键词“Imagine a winding stream”,定位到Passage x 第x句，原句为“Imagine a winding stream. The water on the outside of the curve moves faster than……the slower water on the inside point of the bend deposits sediment, forming a point bar.”
【引用内容分析】河流外缘的水流速度要比河流中心的水流速度快，以及河流外缘的水流和内部的水流分别造成的影响。从中我们得知，河流内部的水流因为较为缓慢，所以会沉淀砂石，形成点坝。所以从这段话中，我们可以知道点坝是如何形成的。
【选项分析】
A选项：解释砂坝如何改变水流的速度和水流的方向。错误，因为文章没有说砂坝可以改变水流的速度的方向。故A选项未提及，排除。
B选项：解释为什么砂坝要比冲积扇或者其他形式的沉积物更普遍。错误，因为同样，这个比较关系在文中并没有提及，故B选项排除。
C选项：解释说明在很多河流中通商航行所遇到的困难。错误，因为文中只说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。并没有提到通上航行会遇到困难。故C选项未提及，排除。
D选项：正是解释说明了点坝是如何形成的，符合原意，正确。
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 recharted 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.
