Alter Terra, a binational environmental group, is sounding the alarm about the need to dredge the Tijuana River channel just inside U.S. territory to avoid massive flooding near and around the San Ysidro Port of Entry. The group says the floor of the channel has risen by 10 feet over the years, meaning it will take less water for the river to crest over its levees. … The sediment is made up of sludge from raw sewage, dirt from construction sites, soil from Tijuana hillsides and other materials that come in from Mexico. … The other option is to raise the levees, which requires congressional approval and major funding.