12 September 2012

Landslides and the red Yangtze River

Posted by Dave Petley

An interesting story coming out of China today concerns the Yangtze River at Chongqing, upstream of the Three Gorges Dam.  To the understandable consternation of local people, on Thursday the river developed an intense red colouration.  Various news agencies have images of this phenomenon, but the Daily Mail has the best photo compilation that I’ve seen so far (and, as an aside, it is accompanied by the most amusing comments at the bottom of the page):


Inevitably, there is a great deal of speculation as to the cause of this change in colour, ranging from a “red tide” algal bloom (unlikely as this is marine phenomenon), pollution such as a toxic chemical spill (also unlikely given the scale – the Yangtze is a huge river at this point, such that this would need to be a huge input of chemicals), the Yunnan earthquake (again unlikely given the location of the affected area), or sediment generated by soil erosion and landslides.  Incidentally, it is also being interpreted as an indicator of momentous events in China, with links being made to the forthcoming change in leadership of the country.  It is slightly dispiriting that such interpretations are still made.

By far the mostly likely cause of the change of colour is a large-scale input of iron-rich sediment to the river.  It is notable that a large flood wave passed down the river last week, caused by heavy rainfall in Sichuan Province.  This was reportedly the largest flood wave for two decades.  Thus, I would hypothesise that the intense rainfall caused extensive sediment mobilisation both in terms of landslides on slopes and debris already in the river, and that this has caused the change in water colour.

Of great interest to me is the question of where this sediment mobilisation occurred.  One leading candidate might be the Wenchuan Earthquake area, where we know that very large quantities of sediment were released from the slopes.  Could it be that this is the main part of that sediment plume passing downstream?  If so, I am surprised that it has taken so long, but it could be that a portion of the sediment has been stored in the channel and only mobilised during this extreme rainfall event.  An alternative hypothesis that has been proposed is that this was caused by landslides on the banks of the Three Gorges reservoir, where we know that extensive mass movements are occurring.  However, this appears to have occurred upstream of the Three Gorges reservoir, so that is highly unlikely.  Alternatively, it could be that there is some other area in the Yangtze catchment that is suffering extensive landsliding, although it is not clear where and why this might be occurring.

None of the newspaper reports indicate that this change in river colour has a precedent in Chongqing, indicating that something out of the ordinary has occurred.  It will be interesting to discover the nature of the causal event. But in the meantime, it is interesting to note that at time of writing this change in water-colour does not seem to feature on Xinhua News, the official Chinese news agency website.