THE MAGDALENA is Colombia’s biggest and most prized river — a crooked, rambling basin that cuts through the western half of the country for about 1,000 miles. The basin, which runs from the southwest corner near Ecuador through two subranges of the Andes mountains, is the aquatic lifeblood of the country: supplying water to 80 percent of the country’s population, it is also home to 2,735 species of animals, 20 percent of the world’s bird species, and soon up to 15 new hydroelectric dams. As Colombia emerges from 50 years of internal conflict, foreign development is rushing in, and a race to harness and manipulate the power of the Magdalena is on.