The worst drought in more than five decades along the Yangtze River has seen salt water seep into a key reservoir in Shanghai, potentially threatening the city's drinking water. Seawater usually intrudes into rivers at their estuaries in winter, the driest season, but it is rare in May, experts said. Low rainfall along the Yangtze's lower reaches since the start of the year is causing rivers and lakes to dry out, affecting crops and aquatic animals in a region known as the mainland's environmental lifeline and 'the land of fish and rice'. Chenhang Reservoir, which provides raw water for watering plants in northern Shanghai, has suffered a week-long saltwater intrusion and is expected to continue receiving seawater for the next two days, the Dongfang Daily reported, citing sources at the municipal water supply administration. The news also hit the headlines of other local media. 'Normally the Yangtze River estuary sees about 12 days of salt water intrusion in a whole year, mostly in winter,' said Xia Qing, former deputy director of the Chinese Research Academy of Environmental Sciences. 'Water in the reservoir cannot be transferred to water plants directly if it's mixed with salt water. You have to desalt it first, but it's a costly project and even if they can afford it, they don't have the equipment to hand.' Although the reservoir was full before the salt water came and it has been transferring water from the nearby Bao Steel Reservoir, it has still seen water levels drop significantly and fall short of demand in the region, the Dongfang Daily said. In Shishou, Hubei, low water levels in the Yangtze River have threatened the survival of finless porpoises, a highly endangered freshwater dolphin. They live only in the middle and lower reaches of the river and number just 1,000, Xinhua said. Water is three metres lower than a year ago - and the lowest in decades, state media reported. When the level drops, the dolphins have less room to move and could become stranded on the banks, Tao Le, of the Swan Island National Nature Reserve in Shishou, was quoted as saying.