Longitudinal transport timescales in a large dammed river - The Changjiang River.

As a key component of the global water cycle, river flow transports both freshwater and biotic/abiotic matters from land to sea, while in recent decades its rhythm has been strongly disturbed by human activities, especially damming. Yet little is known about the long-distance transport processes along the world's major fluvial systems and the impact of large dams on their timescales. Here, taking the Changjiang River (Yangtze River) as an example, we built a hydrodynamics-based model to investigate the water age and residence time in the mainstream from the upper reach ~700 km upstream of the Three Gorges Dam (TGD) to the estuary ~1900 km downstream of the TGD. We find that since the mainstream was dammed by the TGD, the water age increases significantly by approximately 2 to 5 times from the estuary to the dam. Downstream of the dam the longitudinal ageing rate of water becomes discordant in an annual cycle, and the replenished discharge in dry season accelerates the water transport. Due to the stationary assumption, the widely applied hydraulic residence time of water is substantially larger and smaller than the age-based dynamic residence time in the large reservoir during the impounding and releasing periods, respectively.