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记录 英语
TPO阅读52 溪流沉淀
Stream Deposit
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)
decreas and the stream deposits the largest particles in the streambed. If
current velocity continues to decrea - as a flood wanes, for example - finer
particles ttle out on top of the large ones. Thus, a stream sorts its
dimentaccording to size. A waning flood might deposit a layer of gravel,
overlain by sand and finally topped by silt and clay. Streams also sort diment
in the downstream direction. Many mountain streams are choked with boulders and
hbu cobbles, but far downstream, their deltas are compod mainly of fine silt and
clay. This downstream sorting is curious becau stream velocity generally
赤壁赋原文及翻译 increas in the downstream direction. Competence increas 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 diment moves downstream over the years.
卡特里娜飓风 Thus, only the fine diment reaches the lower parts of most rivers.
A stream deposits its diment in three environments: Alluvial fans and
deltas form where stream gradient (angle of incline) suddenly decreas as a
stream enters a flat plain, a lake, or the a; floodplain deposits accumulate
on a floodplain adjacent to the stream channel; and channel deposits form in the
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stream channel itlf. Bars, which are elongated mounds of diment, are
transient features that form in the stream channel and on the banks. They
commonly form in one year and erode the next. Rivers ud for commercial
navigation must be recharged frequently becau 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 becau
the current's inertia drives it into the outside bank. At the same time, the
除此之外 slower water on the inside point of the bend deposits diment, forming a point
bar. A mid-channel bar is a sandy and gravelly deposit that forms in the middle
of a stream channel.
Most streams flow in a single channel. In contrast, a braided stream flows
in many shallow, interconnecting channels. A braided stream forms where more计算机专业考研数学
diment is supplied to a stream than it can carry. The stream dumps the
excessdiment, 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 veral channels and shifts back and
forth across its floodplain. Braided streams are common in both derts and
glacial environments becau both produce abundant diment. A dert yields
large amounts of diment becau it has little or no vegetation to prevent
erosion. Glaciers grind bedrock into fine diment, which is carried by streams
flowing from the melting ice. If a steep mountain stream flows onto a flat
plain, its gradient and velocity decrea sharply. As a result, it deposits most
of its diment in a fan-shaped mound called an alluvial fan. Alluvial fans are
common in many arid and miarid mountainous regions.
A stream also slows abruptly where it enters the still water of a lake or
t out ocean.The diment ttles 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, rembling 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
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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 ven different delta channels during the
