oss. "I hadn't thought much about that."

"Indeed you hadn't," his friend replied. "You've got to remember, Ross,
that the Mississippi doesn't run in a straight line; it bends and twists
like a snake. In the bends the current strikes on the outwardly curving
bank, and, as you know, the water is always deep there. This causes a
rapid caving and erosion of the bank. At the foot of each bend, the main
flow crosses to the other side, where it strikes the bank which has
become concave there, and eats into that bank just as, a few hundred
yards higher, it has been eating into the opposite one."

"I know you've always got to pilot a boat first on one side of the river
and then on the other," said Ross thoughtfully.

"You have. And, if you remember, you'll see that it is generally on the
side nearest to the concave shore that the boats pass."

"Yes," agreed Ross thoughtfully, "I guess it is."

"Now, you can easily see," the Forecaster continued, "that the river
might keep its own channel clean if it flowed straight down with a
current of equal strength. But, as the current crosses from side to
side, it slackens speed at each of these crossings. Therefore, as the
current becomes slower, it drops some of the heavier particles of sand
or mud, forming a bar at every bend, sometimes so high as to prevent
navigation."

"That's what the dredges are for, isn't it?" asked Ross.

"Yes. The Government has twelve large dredges at work all the time,
keeping the navigation channel open."

"I don't see, yet, why the stone wall idea wouldn't work," protested
Ross.

"I'm just showing you," was the reply. "If you built your heavy wall on
the bank, the water would strike the concave bank at one of these
crossings, eat away the earth under the wall and your wall would topple
in. Then the current would cross the stream, undermine the bank on the
other side and your masonry would crumble there, too. So much for the
wall."

"Suppose you sunk that wall, away down deep, below the level of the
bottom of the river?" suggested Ross.

"That might work," the expert replied, "but it would cost more money
than the United States could afford to spend. Besides, Ross, where would
you build this wall? Right on the bank?"

"Of course."

"But the Mississippi is half a mile wide at some places and three miles
wide at others. If the river were absolutely walled in, you'd have swift
currents at one place and slow in another. Then your channel would fil