A three-mile stretch of the Santa Margarita River and the land that surrounds it, property that could have been at the bottom of a reservoir if plans in the 1950s had been realized, will soon be sold to a wildlife conservancy for $10 million. The acquisition of the 1,384-acre property will be known as the Santa Margarita River Preserve and will insure that the extensive and popular trail system along the river will remain forever. The Fallbrook Public Utility District agreed earlier this year to sell the land it has owned for about 60 years to The Wildlands Conservancy.