(Los Angeles Times) — On the edge of the Nebraska sand hills is Lake McConaughy, a 22-mile-long reservoir that in summer becomes a magnet for Winnebagos, fishermen and kite sailors. But officials here in Keith County, population 8,370, imagined this scene: an Al Qaeda sleeper cell hitching explosives onto a ski boat and plowing into the dam at the head of the lake.  The federal Department of Homeland Security gave the county $42,000 to buy state-of-the-art dive gear, including full-face masks, underwater lights and radios, and a Zodiac boat with side-scan sonar capable of mapping wide areas of the lake floor.  Up on the lonely prairie, Cherry County, population 6,148, got thousands of federal dollars for cattle nose leads, halters and electric prods — in case terrorists decided to mount biological warfare against cows.