Again the call of ""Mark Twain!"" rings out from a stately riverboat, this time as it paddies the toxic Mississippi in a wry post-apocalyptic novel. Some years after ""The Flash"" and a subsequent round of devastating plagues, six orphans convert an old floating museum into a working steamer and set up a regular route to deliver news, mail, and cargo to the few isolated settlements left along the river. Their troubles begin when they pick up King, an aging fugitive; suddenly they can't seem to shake a gang of relentless pursuers. Eventually, King reluctantly admits that he knows where some guns are hidden--a ticket to power in this time of bows and arrows. The intriguing premise and well-built cast deserve more work on details. There's little physical description here, and no sense of how the different communities have learned to cope; characters' pasts remain unexplored, and the wonderful fact that the River Rats have gathered storage batteries and made themselves into the world's last rock band gets disappointingly little play. Not up to David Brin's Postman (1986); still, a sturdy, not overearnest sf adventure.