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向下侵蝕又稱下切河流、下切侵蝕、下切，是一種河流的侵蝕作用。由於河水受重力作用，產生由高往低的動能，對河床底部進行挖蝕、磨蝕，以增加河道深度。由於河床坡度越大，流速越大，使得向下侵蝕的力道增加，故通常在河的上游進行。常見的地型有河階、V型谷。
, Downcutting, also called erosional downcut … Downcutting, also called erosional downcutting, downward erosion or vertical erosion is a geological process by hydraulic action that deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor. The speed of downcutting depends on the stream's base level, the lowest point to which the stream can erode. Sea level is the ultimate base level, but many streams have a higher "temporary" base level because they empty into another body of water that is above sea level or encounter bedrock that resists erosion. A concurrent process called lateral erosion refers to the widening of a stream channel or valley. When a stream is high above its base level, downcutting will take place faster than lateral erosion; but as the level of the stream approaches its base level, the rate of lateral erosion increases. This is why streams in mountainous areas tend to be narrow and swift, forming V-shaped valleys, while streams in lowland areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed. The term gradient refers to the elevation of a stream relative to its base level. The steeper the gradient, the faster the stream flows. Sometimes geological uplift will increase the gradient of a stream even while the stream downcuts toward its base level, a process called "rejuvenation." This happened in the case of the Colorado River in the western United States, resulting in the process that created the Grand Canyon.the process that created the Grand Canyon.
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向下侵蝕又稱下切河流、下切侵蝕、下切，是一種河流的侵蝕作用。由於河水受重力作用，產生由高往低的動能，對河床底部進行挖蝕、磨蝕，以增加河道深度。由於河床坡度越大，流速越大，使得向下侵蝕的力道增加，故通常在河的上游進行。常見的地型有河階、V型谷。
, Downcutting, also called erosional downcut … Downcutting, also called erosional downcutting, downward erosion or vertical erosion is a geological process by hydraulic action that deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor. The speed of downcutting depends on the stream's base level, the lowest point to which the stream can erode. Sea level is the ultimate base level, but many streams have a higher "temporary" base level because they empty into another body of water that is above sea level or encounter bedrock that resists erosion. A concurrent process called lateral erosion refers to the widening of a stream channel or valley. When a stream is high above its base level, downcutting will take place faster than lateral erosion; but as the level of the stream approaches its as the level of the stream approaches its
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向下侵蝕
, Downcutting
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