"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 fill up in the wide places and you'd be as badly off as before."
"Make it all the same width, then," said Ross.
"Build two-thirds of the whole two thousand miles by some underwater system, constructing the wall under water? If you had ever read of the difficulty of building one lighthouse foundation, my boy, you wouldn't talk so glibly about building huge retaining masses of masonry under water."