上一篇里，河流搬着泥沙一路向东，跑到跑不动的地方停了下来。泥沙停下来铺开的地方，就是平原——🌍 蓝色星球向导已经在那里摆好了餐桌。Last leaf, the river carried mud and sand east until it could run no more. Where the load settles and spreads out, you get a plain — and the 🌍 Blue Planet Guide has already set the table there.
第 1 步：大地的餐桌Step 1: The land's table
先回答一个问题：什么样的地才算平原——又低又平。First, one question: what counts as a plain — low and flat.
平原有两个硬指标：低（海拔大多在 200 米以下），平（一眼望不到头）。华北平原的麦浪、恒河平原的稻田，都是平原端出来的“餐桌”——全世界一半以上的粮食，长在平原上。A plain passes two tests: low (mostly below 200 metres) and flat (no end in sight). The wheat waves of the North China Plain and the rice paddies of the Ganges Plain are dishes from the land's table — over half the world's grain grows on plains.
粮食多了，人就聚过来了——人一多，城市就长出来了。Where grain piles up, people gather — and where people gather, cities grow.
第 2 步：城市爱长在平原上Step 2: Cities love growing on plains
盖楼先找平地：平原上修路直、通水电容易，火车汽车都跑得顺。北京、上海这样的大城市，全坐在平原上——人多、楼多、路直，就是平原城市的模样。Builders look for flat ground first: straight roads, easy water and power, smooth trains and cars. Great cities like Beijing and Shanghai all sit on plains — crowds, towers and straight roads, the classic plain-city look.
平原这么好，到底是谁把它铺出来的？Plains are wonderful — but who paved them?
第 3 步：河流铺出平原Step 3: Rivers pave the plains
上游水急，把石头泥沙一路往下冲；到了下游水变慢，搬不动了，就一年一层铺下来。冲积平原，意思就是河水冲出来、积出来的平原。华北平原是黄河铺的，恒河平原是恒河铺的。Upstream the water runs fast and drags stones and sand downhill; downstream it slows, cannot carry the load, and lays it down layer by layer, year by year. An alluvial plain is a plain rushed out and piled up by a river. The Yellow River paved the North China Plain; the Ganges paved the Ganges Plain.
🎮 你来当土地局长（30 秒）🎮 Your turn: land-use chief (30 seconds)
道理讲完了。🌍 向导接了 4 块地：下面轮到你，给每块地派个最合适的活。Theory done. The Guide has 4 plots waiting — your turn to give each plot its best job.
一句话记住它：平原 = 又低又平的“大餐桌”；粮食多、人多、城大，文明爱在这里诞生。Remember it in one line: a plain is a low, flat giant table — grain, people and cities pile up, and civilizations love to be born here.
又低又平：海拔多在 200 米以下，一眼望不到头Low and flat: mostly below 200 m, no end in sight大粮仓：华北平原、恒河平原养活上亿人Breadbasket: the North China and Ganges plains feed hundreds of millions城市摇篮、河流铺就：平地好建城；上游冲、下游铺Cradle of cities, paved by rivers: flat fits cities; eroded upstream, spread downstream
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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