From the air, Lewis and Clark Lake along the Nebraska-South Dakota border appears immense as it stretches some 25 miles behind Gavins Point Dam.It would be logical to think of the sprawling lake — formed by damming the Missouri River — as a big deal in flood protection.It is formed by one of six massive dams on the Missouri River, all operated by the U.S. Army Corps of Engineers.Together they constitute the largest reservoir system in the United States.And together, they deny the Missouri River its nearly annual habit of flooding twice a year because of snowmelt — first from the plains and then from the mountains.When an epic storm hit in mid-March, the gargantuan system had 22 percent of its space set aside for flood storage, and a full 96 percent of that stood empty and ready to take in water.The dam, as it turns out, is not designed for significant flood control. So when the Niobrara dumped its floodwater into Lewis and Clark Lake — on the peak day of flooding, 31 times the river’s average March flow was pouring into the lake — the Corps of Engineers had few options but to open the floodgates.The timing couldn’t have been worse: Downstream, the Platte River was pouring even more water into the Missouri River, which was swelling to record levels. South of the Platte, levees were overtopped, broken or weakened. Low-lying neighborhoods, businesses and cropland flooded.