Over time the course that a river follows changes. Evidence of this process can be found by looking at an aerial photo of a meandering river or a delta: silted up loops or oxbow lakes mark the sites of previous channels next to the river’s current course.
Conditions favourable to channel switching across a delta or floodplain build up slowly. The existing channel becomes more winding or silted up, whilst an alternative channel path develops, perhaps where flooding commonly breaches the river bank. The switch itself is thought to happen suddenly, triggered by a large event such as a flood. This process is called avulsion. When initiated by dinosaurs it is given the name ‘dinovulsion’.
The initial proponents of dinovulsion suggested that dinosaurs could be responsible for river avulsions across the alluvial plain that once covered Jurassic Utah. Large dinosaurs dying in rivers would be a suitable size and shape to almost completely block the channels. Ice jams and logjams in modern rivers cause avulsion – so ‘dinojams’ would have a similar effect!
Unfortunately for supporters of dinovulsion little evidence exists to support this theory. Although some fossil dinosaurs have been found that are thought to have died in river beds, there is no indication that this happened often enough to be a significant cause of river cloggage. It seems much more likely, given the abundance of large fossilised trees across the ancient floodplain, that fallen vegetation was the main cause of channel blockage.
However, although it seems unlikely that dinosaurs could have directly caused avulsion, they may well have acted to enable it. In the present-day Okavango Delta in southern Africa, hippos trample down trails along the sides of rivers in order to feed. As they are large, live in groups and use the same paths repeatedly, they are able to create deeply incised trails. Crucially, their feeding behaviour also means that they form trails near to the rivers and aligned with the prevailing slope, ideal for water to flow down if the old river channel becomes blocked by the accumulation of plants floating and growing in the channel.
Perhaps herds of sauropods could have interacted with their environment in a similar way. They tick many of the same boxes as hippos – they could reach enormous sizes and studies of their tracks has indicated that they too may have lived in groups. Trampled down, or ‘dinoturbated’, trackways have been found through floodplain deposits through the Cretaceous of Utah, often near to palaeostream edges. These pathways contain evidence of repeated use; they may have been used regularly as paths to reach the water.
It seems then that some dinosaurs may have exhibited the required behaviours to enable channel avulsion, creating new channels for rivers to follow when their old ones were blocked. In fact, dinoturbated trackways are often found on top of floodplain muds but at the base of channel deposits within a succession, which may represent a record of this process occurring. In answer to our question: dinosaurs probably didn’t cause rivers to shift their channels, but in some cases may have made it easier for them to do so.
Jones, L.S., Gustason, E.R. (2006). “Dinosaurs as Possible Avulsion Enablers in the Upper Jurassic Morrison Formation, East-Central Utah.” Ichnos, 13, p. 31–41.
McCarthy, T.S., Ellery, W.N., Bloem, A. (1998). “Some observations on the geomorphological impact of hippopotamus (Hippopotamus amphibius L.) in the Okavango Delta, Botswana.” African Journal of Ecology, 36, p. 44–56.
Difley, R.L., and Ekdale, A.A. (2002). “Footprints of Utah’s Last Dinosaurs: Track Beds in the Upper Cretaceous (Maastrichtian) North Horn Formation of the Wasatch Plateau, Central Utah.” Palaios, 17, p. 327-346.
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