Consequences of Concomitant Reduction


Consequences of Concomitant Reduction

The concomitant reduction in sediment delivery to downstream areas also has several consequences (Figures 7-9). Ultimately the decreased sediment supply to the river mouth translates to net erosion of beaches and loss of land in coastal regions, as natural coastal erosion by currents and subsidence caused by compaction of delta sediments is not offset by delivery of sediment. For example, prior to construction of the Aswan High Dam began in 1960, the annual sediment flux to the Nile Delta was ~100 million tons. This sediment supply was enough to offset erosion and natural subsidence.

See caption. Sediment increases by deforestation, mining, agriculture road construction. Decreased by dams, water diversion & bank hardening
Figure 7. Sediment loads at the mouth of several of the world’s major rivers, as a function of time (from Syvistky & Kettner, 2011). A ratio of 1.0 indicates that the sediment load is unchanged from pre-Anthropocene conditions. For many rivers, including the Nile, Colorado, and Indus, modern sediment loads are far smaller (<10%) of those in the past. However, a few rivers like the Amazon and Po sediment loads have slightly increased.
Source: Syvistky and Kettner

Without continued sediment delivery, subsidence and coastal erosion lead to significant losses of land area (Three Gorges: A “Mega-Dam” and its Impacts), much of which is prized for agriculture or development. Currently, parts of the Nile Delta are subsiding at up to 1 cm/yr. This phenomenon is common to most of the world’s major river systems, including the Mississippi, Colorado, Yangtze, and Indus Rivers (Figure 7). For example, much of New Orleans is subsiding at over a half-centimeter per year, with some areas sinking more than 2.5 cm/yr (Figure 9). The combination of subsidence, coastal erosion, and sea-level rise has led to land loss from the Mississippi Delta of almost 1100 acres per year since the mid-1970s.

The same processes also place these areas at especially high risk for flooding in major storm events. Much of the inundation of the Gulf Coast caused by Hurricane Katrina in 2005 occurred in areas that lie below sea level due to subsidence. The effects of subsidence are compounded by the loss of barrier islands as their sediment supply is not replenished. Further, extraction of groundwater from the subsurface – and in some cases oil and gas - exacerbates land subsidence, and can also lead to saltwater intrusion in coastal aquifers.

Image described in caption below
Figure 8. Satellite image of the Colorado River Delta (imagery from NASA Earth Observatory). Light blue and gray areas are all below water, including a large portion of the delta that was formerly subaerial prior to subsidence and coastal erosion. From the center of the image to the Southeast, the dark blue channel that appears to be a river is actually a flooded offshore extension of the former river.
Source: NASA Earth Observatory
Satellite image of modern subsidence rates in New Orleans, zooming in on an area by Lake Borgne
Figure 9. Modern subsidence rates in New Orleans, as determined by satellite radar interferometry (after Dixon et al., 2006).