The threat of destructive flooding from a monstrous Sierra Nevada snowpack that supplies Los Angeles with its water led Mayor Eric Garcetti to declare an emergency Monday to protect the city’s aqueduct system and the people who live nearby. Flooding is not a threat in the nation’s second-largest city. But it could swamp the rural Owens Valley hundreds of miles north, which has long had a fraught relationship with the metropolis that surreptitiously bought rights to its water and channeled it south more than a century ago.