Chinese Premier Wen Jiabao made an unannounced visit to this beleaguered city Saturday to pledge his support as a long slick of toxic river water continued moving through town. But he offered no explanation for the government’s attempt to cover up a chemical spill that has left millions here without running water for four days.The visit, the first by a senior Chinese leader to Harbin since Beijing admitted on Wednesday that a “major water pollution incident” had occurred in the region, was the latest move by the ruling Communist Party to repair its image and blunt public outrage caused by initial efforts to conceal the environmental disaster on the Songhua River.