LOS ANGELES — About 5 miles of Southern California beaches remained closed Wednesday due to a massive sewage spill that flowed into the Los Angeles River. A buried pipe near downtown Los Angeles collapsed Monday, causing a blockage and a spill of 2.4 million gallons of raw sewage onto streets and into storm drains that feed into the river, which empties into Long Beach harbor 20 miles downstream. Crews managed to contain, divert or vacuum up at least 750,000 gallons, but the rest flowed into the river. About 4 miles of sand in Long Beach and a mile of coast in neighboring Seal Beach were closed to swimmers and waders while health officials tested the waters for bacteria.