The Yangtze flows from the Tibetan Plateau to the East China Sea (6300 km); total drainage area 1.808 10⁶ km² . Here we describe the main hydrological characteristics of the Yangtze (precipitation, runoff, variability, seasonality, floods, low flows) for the whole catchment and the upper, middle and lower sections, since 1955. Briefly, the Yangtze has mean annual precipitation (runoff) of 1037(517) mm, both with a marked summer dominance. Interannual variability of precipitation (runoff), as measured by the coefficient of variation, is low by world standards at 0.066 (0.126). The variability of floods is similarly low: flash flood index 0.18.

            The upper catchment has the most marked summer precipitation dominance. It is both the driest and least variable section and with no statistically significant trends in either precipitation or runoff. Both precipitation and runoff increase downstream through the middle and lower basins, variability increases, as does winter precipitation. There is a small but statistically significant increase in runoff and the runoff ratio in the middle basin, possibly caused by changes in the nature of the surface due to accelerated urbanization post 1980 and increased area of water storage.

The Yangtze has a long (~8,000 yr) history of human occupation and human impacts have increased dramatically since the economic reforms began in China in the late 1970s. The changes since 1980 that would be expected to have a significant hydrological impact include population growth, agricultural intensification, increasing urbanisation, dam construction and industrial development. We find little direct evidence in the overall hydrology of the Yangtze that reflects these changes.

Shanghai has recently increased its dependence on the Yangtze for its water supply by constructing intakes and storages in the estuary where salinity can be a problem during the low flow months of the year (November to March).  Water diversions, such as the south-north water transfers, and sea level rise are increasing the risk that the estuary water will be too saline to consume in these low flow months, even though the total amount of water entering the estuary is sufficient to meet the needs of Shanghai.  We conclude that risks to water security in Shanghai do not arise from water availability in the Yangtze but from managerial decisions about how Shanghai sources its water from the river