To The Victoria Falls
Formation of the Victoria Falls
The Future of the Falls
As we have seen from the process which created them, the Victoria Falls that we see today are only a snap-shot in an on-going process by which the river is successively eroding upstream, in turn forming wide curtains of falling water such, as the present Falls, and then narrow and powerful gorges as it becomes concentrated and erodes back into the rock which forms its bed.
It is believed that the river will erode its next north-south break-through at the western side of the falls, either at the Devil's Cataract, which is already several meters lower than the majority of the Fall-line, or by exploiting the cleft behind Cataract Island, which although it has very little water flow during most of the year, at times of high flow carries a significant volume of water.
However in human terms this process is incredibly slow and in the hundred and fifty years since the Falls were discovered to the world there has been little appreciable change. In 1862, the explorer and artist Thomas Baines visited the region and drew his famous paintings and etchings of the Falls, and those of the Devil's Cataract have been suggested as evidence of visible erosion in this section since this time. However early photographs show very little evidence of change over the last 100 years.
The old wide river bed downstream of the Victoria Falls has, amongst its alluvial (river washed) deposits, revealed the occasional evidence of Stone Age man, in the from of tools, up to 18 kilometres downsteam of the current fall-line. This shows that Stone Age man must has witnessed the Falls at these earlier stages in its formation before the gorges eroded their way down and back through the river bed, perhaps between 250-500,000 years ago. Its has been calculated that it has taken nearly 100,000 years for the river to erode back from the Songwe River to the Third gorge (or sixth fall-line). Using this evidence as a basis to estimate the rate of upstream erosion has lead some to predict a rate of 7 metres every 100 years, or 7 cm each year.
However the temptation to estimate rates of erosion is made complex by the variation in the rates of erosion at different stages in the erosion process (the north-south rate of erosion, when the river flow is concentrated into the narrow gorges, will be much faster than the north-south rate of erosion when the water falls over the wide bed of the river), along with many other factors such as geology, geography and climate.
The main factor affecting the rate of erosion is the volume of water the river carries, and this has varied greatly over the geological timescale. Geological evidence suggests that at times the water flow has been much greater, evidence of this coming from the much rougher alluvial deposits which are carried and deposited by a stronger and faster current, and it is during these periods that the erosive power of the river is at its greatest. The sediments currently carried by the river are much finer sand particles and can be seen in deposits even along the upper gorges at low periods of flow. Even these have a substantial erosive power when carried in fast water and forced against solid rock.
It may even be possible that if water levels fall, rather than increase as they have in the past, perhaps due to global climate change, that the Victoria Falls we see today are the last in this great story. That man's activities could affect the continued evolution of this natural wonder of the world should really give us all pause to think about how we live out lives in this modern age.