上一片里，小水滴冻在了雪线上。太阳一出来，它化开往山下跑，跑着跑着就成了一条河。🌍 星球向导跳上小船，陪它从源头一路漂到大海。Last leaf, Drip froze on the snow line. Once the sun came out, it melted and ran downhill — and the run became a river. 🌍 the planet guide hops aboard to ride with it from source all the way to the sea.
第 1 步：一条河只做三件事Step 1: A river only does three things
从源头到入海口，河水看着一路都在变，其实只换了三次工作。From source to mouth the river looks ever-changing, but it only switches jobs three times.
上游坡陡水急，河水像凿子往下切，切出深窄的 V 形谷，这叫侵蚀。中游坡缓了，水推着石头往前滚，石头越滚越圆，这叫搬运。下游水慢得托不住泥沙，一层层放下，这叫沉积。切、搬、放——记住这三个字就行。Upstream the slope is steep and fast, and the water works like a chisel, cutting a deep narrow V-shaped valley — erosion. Midstream the slope eases, water rolls stones along and grinds them rounder — transport. Downstream the water slows and can no longer hold its load, laying it down layer by layer — deposition. Cut, carry, drop — those three words are enough.
中国人最熟的两条河，正好讲清“流域”这个词。The two rivers every Chinese child knows happen to explain the word “basin”.
第 2 步：长江黄河，母亲河Step 2: Yangtze and Yellow, the mother rivers
长江最长，黄河最黄，都发源于高原，养活了两岸的农田和城市，所以叫母亲河。一条干流加上所有支流，收集雨水的整片地叫流域。两条流域中间那道山脊，雨水各流一边，叫分水岭。The Yangtze runs longest, the Yellow runs yellowest. Both rise on the plateau and feed the farms and cities along their banks — hence mother rivers. One main channel plus all its tributaries drains a whole patch of land called a basin. The ridge between two basins sends rain each way — the divide.
切下来的石头去哪儿了？答案堆在河的两头。Where did all the cut-away rock go? The answer piles up at both ends of the river.
上游切得深，就留下又深又窄的峡谷。泥沙被搬到入海口，水一慢就卸货，日积月累堆出扇形的新陆地，叫三角洲。下一片的平原，很多就是这样一层层堆出来的。Cut deep upstream and a steep narrow canyon stays behind. Carried to the mouth, the load drops as the water slows, piling day after day into a fan of new land — the delta. Much of the plain in the next leaf was laid down exactly this way, layer upon layer.
🎮 你来当领航员（30 秒）🎮 Your turn: river pilot (30 seconds)
道理讲完了。先按“放一滴水”，看小水滴从雪线漂到大海；再看 4 张现场照片，判断它是上游、中游还是下游。Theory done. First hit “Release a drop” and watch Drip float from the snow line to the sea; then read 4 field photos and say whether each shows the upper, middle or lower course.
一句话记住它：河流是会流动的血管：上游切、中游搬、下游放；长江黄河是母亲河；峡谷是切出来的，三角洲和平原是堆出来的。Remember it in one line: a river is a flowing vein — cut up top, carry in the middle, drop at the bottom; Yangtze and Yellow are the mother rivers; canyons are carved, deltas and plains are piled up.
三段：上游侵蚀切 V 谷，中游搬运磨圆石头，下游沉积放下泥沙Three stretches: upper erodes V valleys, middle transports and rounds stones, lower deposits its load流域与分水岭：一条河收集雨水的整片地叫流域，中间分水的山脊叫分水岭Basin and divide: all the land one river drains is its basin; the ridge splitting two basins is the divide塑造地貌：切出峡谷，堆出三角洲和平原（下一片就是平原）Land-shaping: carve canyons, pile up deltas and plains (the next leaf is the plain)
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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