WUHAN, June 26 (Xinhua) -- Authorities promised swift steps Thursday to clean up the algae-plagued tributaries of the upper Yangtze River as water was affected in central China's Hubei Province.

    The blue algae bloom started last week in Yichang City along the Xiangxi River and had expanded about 23 km, stretching from Xingshan County to Zigui County where it converged with the Yangtze.

    The outbreak also occurred along the Shennong Stream, a major tourist site. It converged with the Yangtze River in Badong County. The river water had since turned emerald green and opaque.

    The slow water flow, deep humus and rich nutrients from farm and domestic run-off have provided ripe conditions for the algae bloom, Badong environmental protection officials said.

    But heavy rains in the areas have curbed the outbreak these days.

    Blue algae, which exists widely in water bodies and is not harmful itself, grows easily in polluted water with high concentrations of nitrogen and phosphorous and a temperature of about 18 degrees Celsius.

    The provincial officials earlier vowed to rule out agricultural and industrial pollution along the rivers.

    Net-pen fish culture in the river would be banned within a month and polluting factories closed until the emissions standards were reached, said Hubei provincial governor Li Hongzhong.

    A timetable had also been made to ban planting vegetables that required large use of fertilizer and pesticide in areas along the rivers.

    More sewage treatment plants would be built to ensure first-grade water emission standards, Li said.

    Local governments are closely monitoring the water quality and the spread of the algae and have advised residents against eating fish or drinking water from the river.

    Blue algae choked the eastern Taihu Lake last summer, triggering a drinking water crisis for residents in Wuxi, Jiangsu Province.