题目解析：
【题目翻译】根据第1段，流水的能力增加是因为什么？
A：下游分选减少
B：洪水减退
C：河流速度增加
D：颗粒的尺寸增大
【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“competence”,定位到Passage1。其中一句是原句为“When the current slows down, its competence (how much it can carry) decreases……”，意思是当水流放缓的时候，其携带能力（能够携带的颗粒量）会减弱。
还有一句是第一段第2句话，第二句话是“Competence increases with velocity, so a river should be able to transport larger particles than its tributaries carry.”，意思是“水流的携带能力随着流速的加快而提升，因此一条河流可以输送的颗粒应该比它的支流所携带的颗粒更大。”
【逻辑分析】这两句话，都能说明河流的携带能力与速度有着密切联系。随着河流速度的增加，携带能力会增加。
【选项分析】
A选项：顺流而下沉淀物分类的减少。错误，因为文中只提到“This downstream sorting is curious because stream velocity generally increases in the downstream direction.”，说明水流的速度沿着下游方向越来越快。但是下游方向沉淀物分类的减少，与水流的携带能力无关。故A选项排除。
B选项：洪水消退。我们根据关键词“flood wanes”，定位第一段第3句“If current velocity continues to decrease—as a flood wanes, for example—finer particles settle out on top of the large ones.”这句话说，洪水消退时水流速度会减慢。但是洪水消退和水流的携带能力并无直接关系。其次，根据后文提到的“Competence increases with velocity”，我们可以判断，洪水消退时，水流速度减慢，所以其携带能力也会减弱，所以与题干矛盾。综上，B选项错误。
C选项：当河流速度变大时，它的携带能力也增强了，携带物就多了。C正确。
D选项：颗粒大小的增加。错误，因为本段中没有提到颗粒大小与水流携带能力之间的关系，属于未提及的内容。故D选项排除。
A large, swift stream or river can carry all sizes of particles, from clay to boulders. When the current slows down, its competence (how much it can carry) decreases and the stream deposits the largest particles in the streambed. If current velocity continues to decrease - as a flood wanes, for example - finer particles settle out on top of the large ones. Thus, a stream sorts its sediment according to size. A waning flood might deposit a layer of gravel, overlain by sand and finally topped by silt and clay. Streams also sort sediment in the downstream direction. Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of fine silt and clay. This downstream sorting is curious because stream velocity generally increases in the downstream direction. Competence increases with velocity, so a river should be able to transport larger particles than its tributaries carry. One explanation for downstream sorting is that abrasion wears away the boulders and cobbles to sand and silt as the sediment moves downstream over the years. Thus, only the fine sediment reaches the lower parts of most rivers.
