The use of water for irrigation and hydropower results in a reduction of river discharge peaks. This leads to silting in of estuaries downstream, but the interactions between river discharge, tides, sea-level rise and the extraction of water are very complicated. Thus it is hard to predict the consequences of irrigation and hydropower in rivers. An international team of researchers, led by Jaap Nienhuis of Wageningen University, now found a way to predict river delta change due to irrigation and hydropower. With the help of Google Earth.
Thus far the morphological changes (the shape of the delta) resulting from discharge reductions remained elusive, not only because of complicated interactions between river discharge, tides, and sea-level rise, but also because of the slow rate in which deltas adjust. This made it difficult to test models of delta change. To circumvent these problems Nienhuis and co-authors from Wageningen University, Florida State University, and Tulane University made a simple model relating river discharge and tides to channel width. “With a calculation at the back of an envelope, we can now estimate the consequences of river discharge change,” Jaap Nienhuis says. The researchers applied their model to 74 river deltas globally with channels that ranged from 38 m to 50 km wide, including the Mississippi, the Amazon, and the Yangtze, and found that it fits the data really well. By designing a model that can be applied on a diverse set of rivers, the researchers were able to effectively capture how discharge and tides are reflected in deltaic channel patterns.
By applying the model, the authors found that discharge decreases resulted in a short-term increase in tidal dominance and tidal intrusion, followed by gradual silting in by tides that would lead to narrowing of the channels. The study cites a case of the Ord River in Australia where a new dam upstream resulted in estuary infilling in less than 30 years. “River dams and water use globally decrease the mean discharge as well as the discharge peaks,” Nienhuis says. “This makes large-scale infilling of deltaic channels really likely over the next decades. Estuary infilling may have significant consequences for channel navigability and dredging costs. So policy should really focus on these morphological changes to river deltas, because they ultimately drive significant flooding and erosion hazards that affect coastal populations.”