Polymath by John Brunner

to let the offshore current disperse theirsewage, which was at least an advance on the crude latrines of the first month here; but pipes made of hollow tree-trunks sealed with plastic film were being readied—one and a half miles of them—to take their drains well away from the town.
    And now… this!
    He wasn’t the only person Cheffy had summoned to witness the horrifying phenomenon. Jerode was here already. Work had stopped on all the riverside projects, and members of the subteams in charge of them were clustered around, speculating in low, worried voices. On glancing upstream Lex saw that two or three of the team who had been constructing the sedimentation tanks were making their way down the middle of the drying riverbed.
    Jerode, acknowledging his arrival with a nod, called to Lex. “What do you make of all this?”
    “When did it happen?” Lex countered grimly.
    “The level began to drop about an hour ago, maybe an hour and a half,” Cheffy said. “It was slow to start with, so I guess we didn’t notice for a while. Then it dropped really sharply. You could watch it going down, like water running out of a tub. Do you think it’s a seasonal stop-page? Because if so we can just tear up our plans.”
    “No, not a seasonal stoppage,” Lex said. “Remember we got here at the end of the summer, and this river was still deep and wide. Besides, if it were only a question of the headwaters not being fed—with snow or whatever—the stream would dry up slowly over days or weeks. This is due to a blockage higher up. A solid obstacle.”
    Jerode, who had come closer, nodded. “That makes sense. What could have caused it—a landslide? Or…” He scratched his bald pate thoughtfully. “I seem to recall that back on Earth they have an animal which dams rivers. A—a beeger?”
    “Beaver!” Cheffy exclaimed. “Yes, of course. Lex, do you reckon there could be a dam-building animal here?”
    “There not only could be,” Lex said after a pause. “There is.”
    Cheffy and Jerode stared at him, puzzled.
    “Man,” Lex said. “If you remember, we found that this river leads directly to where the other party set down.”
    “Lex, that’s absurd!” Jerode snapped. “We’ve tried and tried to make contact with them—we’re still trying now and then—and there’s total radio silence. They must bedead!” And added in a lower tone, “Much as it galls me to find I’m agreeing with Ornelle.”
    Lex shrugged. “Didn’t mean you to take me so seriously, Doc. All I was saying was: it’s possible for all we know.”
    “Look—whatever it is that stopped the water,” Cheffy said, “we’ve simply got to get it back! With explosives if we have to.”
    Jerode hesitated. “Yes, of course…. Well, I was intending to put a proposal to the next assembly, since things are—I mean were—going so well. Another expedition to the plateau, to find out if there are any survivors up there, see if their ship’s worth cannibalizing if there aren’t. As a matter of fact”—he rounded on Lex—“I was going to ask if you’d lead it.”

IX

    The news about the river spread like wildfire. Within half an hour all work had stopped except Fritch’s, and he was keeping his gang on the job only because they were halfway through raising a main roof-beam, thirty feet long, thick, and very heavy. Everyone who had been on the beach drifted back, even Lex’s salvage team, and the rest of Cheffy’s upstream workers came shouting to know what they should do now. It was inevitable that someone should suggest an emergency assembly.
    As usual, rumors that an assembly was in the offing brought the self-important—Nanseltine, Rothers, and their kind—clustering around Jerode like flies on honey, to pester him with ill-considered ideas and nonnegotiable demands. It took the better part of an hour to drive people back to work with assurances that the assembly would meet at dusk.
    Then Jerode sent for the members of the steering