(Gresham, OR)  --  Multnomah County sheriff's deputies are investigating, after a 13-year-old boy drowned in the Sandy River.  Officials say Aaron Peters of Port Angeles, Washington was pulled from the river at Oxbow Regional Park in Gresham yesterday.  Authorities say Peters was swimming with a Native American leadership group around 3:30 yesterday afternoon when he disappeared under the surface.  He was pulled out about 20 minutes later and was taken to Legacy Mount Hood Medical Center, where he was pronounced dead.