[2] We investigated the effect of the Three Gorges Project and other dams on the load of phosphorus (P) to the middle and lower Yangtze River (MLY) and discussed the alteration of P on the ecosystem of the MLY. We collected data for continuous flow and sediment over the past 60 years and observed the concentrations of total P (TP) and particulate P (PP) in the pool reaches of the Three Gorges Reservoir (TGR), both before and after the impoundment in 2003. As a result, we obtained highly positive correlations between P and sediment and revealed two changes that were caused by the impoundments: (1) the sediment load to the MLY decreases by 91% and the river becomes almost clear; and (2) the loads of TP and PP to the MLY are sequestered by 77% and 83.5% annually and 75% and 92% in dry seasons, respectively. Because P was the limiting nutrient for bioactivity in the MLY before 2003, such significant reductions, along with the many other consequences of the dams, will not only further reduce the bioavailability of P but also increase the existing high ratio of nitrogen (N) to P. Therefore, it is quite possible to alter the nutrient regime and reduce the aquatic primary productivity of the MLY. Given that many large dams with huge reservoirs are under construction or planned upstream and elsewhere, studies focused on the long-term effects of sediment and P reduction deserve a high priority for the protection of lowland rivers and aquatic ecosystems.Citation: Zhou, J., M. Zhang, and P. Lu (2013), The effect of dams on phosphorus in the middle and lower Yangtze river, Water Resour. Res., 49,[3659][3660][3661][3662][3663][3664][3665][3666][3667][3668][3669]

A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment-induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross-section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment-induced density effects and hindered settling are included.The high concentrations typical for the Yellow River influence both fluid and sediment dynamics significantly. Therefore a brief summary of sediment-fluid interactions is given first (see Winterwerp and van Kesteren, 2004, for more details), followed by a description of hyperconcentrated flow, and then an overview of hyperconcentrated floods in the Yellow River.

Dams affect ecosystems, but their physical link to the variations in fluvial fluxes and downstream ecological consequences are inadequately understood. After estimating the current effects of the Three Gorges project and other reservoirs upstream on the Yangtze River on the fluvial phosphorus (P) in the middle and lower Yangtze River, we further investigated the long-term effects of dams on the fluvial regimes of P and P-enriched sediment (PES). Simultaneously measured P distributions with sediment size (PDSS) from the Three Gorges Reservoir (TGR) proved that the areal density of particulate P (PP) bound on graded sediment can be measured using the surface area concentration of the total sediment. A PDSS relationship is obtained and the selective transport and long-term sedimentation of P are simulated using a nonuniform suspended sediment model, which incorporates the PDSS formula. The computations revealed that a reservoir would significantly lower the downstream availability of P in the dry season and promote high pulses of P in summer when the reservoir is flushed as sedimentation accumulates. As a result, the P buffering and replenishing mechanism in the pristine ecosystem from upstream supplies and local resuspension are permanently eliminated when a regulating reservoir is built upstream. This change is irreversible if reservoir regulation continues. Changes could potentially aggravate the existing P-limitation, decrease the water's ability to adjust nutrient/pollutant fluctuations, accumulate a greater surplus of carbon and nitrogen, and even exacerbate blooms in favorable conditions.