题目解析：
【题目翻译】为什么作者要提到“冰川把基岩磨成细小的沉积物，而细小的沉积物是由融冰流出的小溪携带的”的信息？
A：为了解释为什么重度沉积的辫状河在冰川环境中很常见。
B：解释为什么有些山溪的大部分沉积物沉积在扇形的土丘中。
C：确定干旱和半干旱山区最常见的泥沙来源
D：为了帮助解释为什么冰川沉积物降低了陡峭山溪的坡度和速度。
【判定题型】：根据题干问法“why …… include”，题目询问“the information”(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“xx”,定位到Passage 3，定位到2句话：前一句话“A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion.”
再往前看一句“Braided streams are common in both deserts and glacial environments because both produce abundant sediment.”
【引用内容分析】前一句话“A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion.”这句话是在解释为什么沙漠地带能产生大量沉积物。
再往前看一句“Braided streams are common in both deserts and glacial environments because both produce abundant sediment.”这句话是说辫状河在沙漠和冰川环境中都很常见，因为沙漠和冰川都能产生大量的沉淀物。至此，这段话的结构已经非常明显：即先说明沙漠和冰川都能产生大量沉积物，然后再各用一句话解释它们为什么能产生沉积物。
【选项分析】
A选项：解释为什么含有大量泥沙的辫状河在冰川环境很常见，正确。
B选项：解释为什么一些山溪会将泥沙沉淀在冲积扇地区。错误，因为冲积扇和题干这句话的关系不大，是下文新展开的一个层次，故B选项排除。
C选项：解释干旱和半干旱山区最常见的沉淀物来源。错误，因为题干这句话的目的，不是为了解释干旱和半干旱山区沉淀物来源，而是为了说明冰川和前文所提到的辫状河之间的联系，解释为什么辫状河在冰川环境下很常见。故C选项错误。
D选项：解释为什么冰川沉淀物会降低陡峭的山溪的坡度和水流速度。错误，因为文中并未提到该信息，故排除。
Most streams flow in a single channel. In contrast, a braided stream flows in many shallow, interconnecting channels. A braided stream forms where more sediment is supplied to a stream than it can carry. The stream dumps the excess sediment, forming mid-channel bars. The bars gradually fill a channel, forcing the stream to overflow its banks and erode new channels. As a result, a braided stream flows simultaneously in several channels and shifts back and forth across its floodplain. Braided streams are common in both deserts and glacial environments because both produce abundant sediment. A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion. Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice . If a steep mountain stream flows onto a flat plain, its gradient and velocity decrease sharply. As a result, it deposits most of its sediment in a fan-shaped mound called an alluvial fan. Alluvial fans are common in many arid and semiarid mountainous regions.
