Where the eastern and western currents of American life merge as smoothly as one river flows into another is a place called Nebraska. There we find the Platte, a river that gave sustenance to the countless migrants who once trudged westward along the Mormon and Oregon trails. We find the Sandhills, a vast region of sandy grassland that represents the largest area of dunes and the grandest and least disturbed region of mixed-grass prairies in all the...