本篇文章共分 4 段，將溪流沉積物的形成過程、三種主要的河川沉積環境以及各自的特色與樣態一一加以說明陳述。

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Stream Deposit 溪流沉積物

1. 溪流沉積物的形成

   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.

   一條大的、湍急的溪流或河流可以攜帶各種大小的顆粒，從粘土到巨石。當水流變慢時，它的能力（它能攜帶多少）就會減少，水流就會把最大的顆粒沉積在河床上。如果水流速度繼續下降—例如，隨著洪水的減弱—較小的顆粒就會沉澱在大顆粒的上面。因此，溪流根據大小對其沉積物進行分類。一個流速減弱的洪水可能會沉積一層礫石，上面是沙子，最後是淤泥和粘土。溪流也會在下遊方向分類沉積物。許多山區的溪流被巨石和卵石堵塞，但在下游，它們的三角洲主要由細小的淤泥和粘土組成。這種下游分類很奇怪，因為溪流的速度通常在下遊方向增加。溪流能力隨著速度的增加而增加，所以一條河流應該能夠運輸比其支流所攜帶的更大的顆粒。對下游分選的一個解釋是，隨著沉積物長年累月地向下游移動，磨蝕將巨石和卵石磨成沙子和淤泥。因此，只有細小的沉積物到達大多數河流的下游部分。

2. 溪流沉積的環境

   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.

   一條溪流在三種環境中沉積其泥沙： 沖積扇和三角洲是在溪流進入平坦的平原、湖泊或海洋時，溪流坡度（傾斜角度）突然減小的地方形成的；洪泛區沉積物在與溪流通道相鄰的洪泛區積累；而河道沉積物則在溪流通道本身形成。砂壩沉積是細長的沉積物堆，是在河道和河岸上形成的短暫特徵。它們通常在一年中形成，並在第二年被侵蝕。用於商業航行的河流必須經常補水，因為河床每年都在變化。想像一下一條蜿蜒的河流。曲線外側的水比內側的水移動得快。溪流侵蝕其外側的河岸，因為水流的慣性促使它進入外側的河岸。同時，彎道內側較慢的水沉積了泥沙，形成了一個點砂壩沉積。河道中段砂壩沉積是在河道中間形成的沙質和礫質沉積物。

3. 溪流沉積物與河道變化

   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.

   大多數溪流在一個單一的渠道中流動。相反，辮狀溪流在許多淺的、相互連接的渠道中流動。當提供給溪流的沉積物多於它的承載能力時，就會形成辮狀溪流。溪流傾倒多餘的沉積物，形成河道中間的砂礫沉積。這些沉積物逐漸填滿河道，迫使河水溢出河岸，侵蝕新的河道。因此，辮狀河道同時在幾個河道中流動，並在其洪泛區來回移動。辮狀溪流在沙漠和冰川環境中都很常見，因為兩者都產生大量的沉積物。沙漠產生大量的沉積物，因為它很少或沒有植被來防止侵蝕。冰川將基巖磨成細小的沉積物，被從融化的冰層中流出來的溪流帶走。如果一條陡峭的山澗流到一個平坦的平原上，其坡度和速度會急劇下降。因此，它的大部分沉積物沉積在一個稱為沖積扇的扇形土丘上。沖積扇在許多乾旱和半乾旱的山區很常見。

4. 三角洲的特徵

   A stream also slows abruptly where it enters the still water of a lake or ocean. The sediment settles out to form a nearly flat landform called a delta. Part of the delta lies above water level, and the remainder lies slightly below water level. Deltas are commonly fan-shaped, resembling the Greek letter “delta” (∆). Both deltas and alluvial fans change rapidly. Sediment fills channels (waterways), which are then abandoned while new channels develop as in a braided stream. As a result, a stream feeding a delta or fan splits into many channels called distributaries. A large delta may spread out in this manner until it covers thousands of square kilometers. Most fans, however, are much smaller, covering a fraction of a square kilometer to a few square kilometers. The Mississippi River has flowed through seven different delta channels during the past 5,000 to 6,000 years. But in recent years, engineers have built great systems of levees (retaining walls) in attempts to stabilize the channels.

   溪流在進入湖泊或海洋的靜水時也會突然變慢。沉積物沉澱下來，形成一個幾乎平坦的地貌，稱為三角洲。三角洲的一部分位於水位以上，其餘部分則略低於水位。三角洲通常呈扇形，類似於希臘字母 “delta”（∆）。三角洲和沖積扇都變化迅速。沉積物填滿了渠道（水道），然後被拋棄，而新的渠道則像辮狀溪流一樣發展。因此，為三角洲或沖積扇提供水源的河流會分裂成許多渠道，稱為支流。一個大的三角洲可能以這種方式擴散，直到覆蓋數千平方公里。然而，大多數扇形區要小得多，只覆蓋幾分之一到幾平方公里。密西西比河在過去的 5,000 至 6,000 年裡，流經了七個不同的三角洲河道。但近年來，工程師們建造了巨大的堤壩系統（擋土牆），試圖穩定渠道。