The Boy with the U. S. Weather Men

by Francis Rolt-Wheeler

"Of course," agreed Ross, "but what has that got to do with it?"

"A great deal," the Forecaster replied succinctly. "The faster a river flows, the more sediment it can carry without allowing it to drop to the bottom; the slower it flows, the more readily is the sediment dropped. If you put some mud in a glass of water and keep stirring it with a spoon, the mud will never sink to the bottom. Even if you let it stand perfectly still, it will take several days before the finest particles sink to the bottom of the glass and the water becomes clear."

"Yes," agreed Anton, "I've often wondered why."

"Well," the Weather Man continued, "if you look closely at the mud in the bottom of the glass, you'll see that the bigger particles are at the bottom and then those a little smaller and so on up, until your top layer is made of a mud composed of particles so fine that you'd have to get a microscope to see them."

"I don't quite see why," said Ross. "I know bigger things are heavier, but why should a big bit of earth sink more quickly than a small bit, when they're both made of just exactly the same stuff?"

The Weather Man looked at him.

"Some of these days," he said, "remind me to talk to you about sunlight and dust, and I'll tell you a heap of things you don't know. Right now, get this idea in your head. The larger a piece of matter is, the smaller is the surface in proportion to the bulk. A feather of swan's down will float in a high wind, but if you roll that feather into a ball, it will fall. Why? You haven't made it any heavier. You've only reduced the amount of surface which was borne up by the air. It's the same way with mud, the bigger pieces sink first because they have less surface in proportion to their weight."

"Yes," answered Ross, "I can see that now."

"Very good, then," the Forecaster continued, "when the Mississippi overflowed its banks and the water got out of the current of the main stream, so that it flowed more gently, the sediment began to fall, the larger pieces first and those that were finer until it was only at the most distant point from the river that the finest mud settled. This has gone on, year after year, for thousands of years.

"Therefore, you see, the lands nearest the river are higher than those farther away. In two big basins, the St. Francis and the Yazoo basins, the slope and the drainage is away from the river, instead of towards it."