Wefer stared at the map and slapped his leg in delight. "You're right!" he exploded. "Ensign, we're headed for that confluence Colonel Grahame just pointed out. Button up for noise, and get the weapons turret uncovered topside."
"Aye, sir," answered the ensign.
They reached the juncture point between the tributary point and the mainstream, which Cletus had pointed out. The Mark V crept out of the channel into the relatively shallow water near the river-bank opposite the mouth of the tributary and stopped there, its turrets less than five feet below the river surface. The sensor float was released from the upper hull of the vehicle and popped to the surface - a small, buoyant square of material with the thin metal whisker of a sensor rod rising one meter from it into the air, the two connected by a fine wire to the communications equipment of the Mark V. The sensor rod had to view the scene around it by available light only; but its resolving power was remarkable. The image of the scene it sent down to the hemispherical screen in the command room of the Mark V below was very nearly as clear as if broad daylight, rather than a fingernail paring of a moon, was illuminating the conjunction of the two streams.
"Not a hull in sight," muttered Wefer, rotating the view in the hemispherical screen to take in the full 180 degrees scanned by the sensor rod. "I suppose we'll just have to sit here and wait for them."
"You could be taking a few precautions, meanwhile," suggested Cletus.
Wefer glanced aside at him. "What precautions?"
"Against their getting away downstream if by some chance they manage to slip by you," said Cletus. "Is there anything to stop you now from moving enough material into the channel downriver so that, if they do come by, they'll run aground just below us?"
Wefer stared at him in astonishment, which slowly changed to delight. "Of course!" he exploded. "Ensign! Take her downstream!"
The Mark V moved roughly a hundred yards downstream; and, extending its massive dozer blade crosswise in front of it, began to shovel sand and silt from beneath the water near the river's edges into the main channel. Fifteen minutes work filled the channel for some fifty yards to a level even with the rest of the river bottom. Wefer was inclined to stop at that point, but Cletus suggested he further refine it into a barrier consisting of a wide, sloping ramp rising gradually to within half a dozen feet of the surface. Then, also at Cletus' suggestion, the Mark V returned, not merely upstream, but up into the tributary some fifty yards behind the point where it met with the waters of the main river.
Here the water was so shallow that the Mark V sat with its turret out in the air. But a few moments work with the dozer blade sufficed to dig a shallow depression so that they could lie in wait completely underwater.
Then the wait began. It was three hours - nearly midnight - before the sensor rod on its float, invisible against the shadow of the foliage lining the tributary's bank, picked up the image of a motor launch sliding down the main channel of the tributary, its motor turning at a speed barely sufficient to keep the drogue pod, towed behind it, underwater.
They waited, holding their breaths, until ship and drogue had passed. Then Wefer jumped for the command phone, from which he had, some hours since, displaced the ensign.
"Wait," said Cletus.
Wefer hesitated, staring at Cletus. "Wait?" he said. "What for?"
"You know that launch isn't going to be able to get past the barrier you built downstream," answered Cletus. "So why not sit here a little longer and see if another boat comes along?"
Wefer hesitated. Then he stepped back from the command phone. "You really think another one might come along?" he asked, thoughtfully.
"I wouldn't be surprised," said Cletus, cheerfully.
The answer was hardly out of his mouth before the sensor picked up another approaching motor launch with pod in tow. By the time this was well passed and out into the main river, still another launch had appeared. As Wefer stood staring with incredulous delight into the hemispherical screen, twenty boats towing pods passed within thirty yards of the submerged Mark V.
When a couple of minutes had gone by following the passage of the twenty boats and pods, Cletus suggested that probably it was time they were checking up on what had happened downstream. Wefer put the Mark V in motion. It surged up out of its shallow hole and plunged under the surface again down into the main channel of the tributary.
They reached the central channel of the main river, and turned downstream. Their infrared searchlights underwater, as well as the sensor rod being towed on its float above them, gave them a picture of wild confusion just ahead of them. Of the twenty launches that had passed them, fully half were firmly aground in the sloping ramp of river bottom that the Mark V had built. The rest, still afloat but with their drogue pods bobbing helplessly on the surface behind them, were valiantly trying to tow the stranded vessels free. Wefer commanded the Mark V to a halt. He stared into the screen with mingled elation and dismay.
"Now what?" he muttered to Cletus. "If I charge on down there, the ones that aren't stuck are just going to turn around and beat it upriver and get away. Of course I've got the weapons in the turret. But still, a lot of them are going to get past me."
"As a suggestion," said Cletus, "how's this Mark V of yours at making a wave?"
Wefer stared at him. "A wave?" he said - and then repeated, joyously. "A wave!"
He barked orders into the command phone. The Mark V backed up a hundred yards along the channel of the main river and stopped. The two wings of its dozer blade, which had been folded back against its body to reduce drag while traveling, folded forward again and extended themselves to right and. left until the blades' full area of twenty yards of width and ten feet of height were exposed. Delicately, Wefer tilted the front of the Mark V upward until the top half of the blade poked through the surface of the river and the treads were swimming freely in the water. Then he threw the engines into full speed, forward.
The Mark V rushed down the river in a roar of water, checked itself and sank itself to the bottom of the channel just fifty yards short of the still-floating launches. For a moment a wall of water hid the scene ahead; and then this passed, speeding like an ever-diminishing ripple farther downstream.
Left behind was a scene of wreckage and confusion.
Those launches that had already been aground had had their decks swept by the wave that the Mark V had created. In some cases they had been flipped on their sides by the wave or even turned completely upside down. But the greatest effect was to be seen upon those launches that had still had water under their keels and had been trying to tow the grounded ones loose.
Without exception these free-floating boats had been driven aground as well. In many cases they had been literally hammered into the soft soil of the piled-up river bed. One launch was standing on its nose, its prow driven half a dozen feet into the sand and silt below.
"I think they're ready for you now," Cletus said to Wefer.
If anything more was needed to complete the demoralization of the guerrillas aboard the launches it was the sight of the black shape of the Mark V roaring up into view out of the river depths, the two heavy energy rifles in its turret sweeping ominously back and forth. Almost without exception those who had managed to cling to their battered crafts dove overboard at the sight and began to swim frantically for the banks of the river.