Why does a wave travel upstream on the Qiantang River?
钱塘江上的浪，为什么朝上游跑？
江水通常朝海边流，钱塘江上却会出现一道向上游推进的潮头。
River water usually flows toward the sea, yet a tidal front can advance upstream on the Qiantang River.
如果只看浪前进的方向，好像整条江突然换了主意。
Watching the wave’s direction can make it seem as though the whole river has changed its mind.
这种现象叫涌潮，它和海潮进入河口的方式有关。
This is a tidal bore, a phenomenon related to how a rising tide enters an estuary.
许多受潮汐影响的河流，水流换方向是慢慢发生的。
In many tidal rivers, the change of current direction happens gradually.
向下游的水流先减慢，经过一段流动很弱的时间，再转成向上的涨潮水流。
The downstream current first slows, passes through a period of weak flow, and then becomes an upstream flood-tide current.
涌潮出现时，变化却集中在一个比较明显的波前，也就是向前移动的水面前沿。
With a tidal bore, the change is concentrated at a distinct wavefront—the advancing edge of the water surface.
要形成这样的潮头，河口的形状和潮水的涨落幅度都很重要。
Both the estuary’s shape and the tidal range matter in forming such a front.
比较平浅、向内收窄的河口，加上较大的潮差，才可能提供合适的条件。
A relatively flat, shallow estuary narrowing inland, together with a large tidal range, can provide suitable conditions.
潮差指高潮和低潮之间的水位差，不是每一道浪本身的高度。
Tidal range means the difference between high- and low-tide levels, not the height of each individual wave.
所以，有涨潮的河流很多，能形成清楚涌潮的却只是一部分。
Many rivers experience rising tides, but only some develop a distinct tidal bore.
潮头向内陆前进时，经过区域后面的水流可以转为朝上游流。
As the front moves inland, the current behind it can reverse and flow upstream.
这不是整条河永久倒流，而是涨潮过程中，一段水流被迅速改变。
It is not the whole river permanently reversing, but a rapid change in a section of current during the rising tide.
钱塘江的特别景象，让本来常常缓慢的水位与水流变化，变成了可以看见向前跑的潮头。
The Qiantang’s spectacle turns a change in water level and flow that is often gradual into a visibly advancing front.
