Misconception – Waterfalls can get taller, but they don’t retreat

There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to one of those misconceptions. There will be others. Find them here.

Misconception: Waterfalls can increase in height over time, but do not retreat

Streams as they flow from their headwaters (the beginning) to their base level (their end, which is usually a standing body of water like a lake or ocean). The overall profile of a ‘perfect’ stream is steep at the top and nearly horizontal at the bottom.

If the base level changes, especially if it drops, then the river tries to erode itself back down to the ideal profile. Erosion increases at the base level end and works its way upstream. In the process, sharp drop form along the profile and work their way upstream. These drops are called knickpoints, and are often associated with waterfalls.

Perhaps one of the best known knickpoints is Niagara Falls. Both American Falls and Horseshoe Falls are a knickpoint that formed when the baselevel between Lake Erie and Lake Ontario shifted.

Niagara Falls are retreating. That is, they are eroding back, at a rate of about 1 foot per year, though it may have been much faster in years past. The retreat began after the ending of the last major ice age about 12500 years ago. The melting and removal of the ice changed the dynamics of base level on the entire Great Lakes region, causing the formation of the knickpoint that is now the Niagara Falls as we recognize them.

(Here are some other interesting facts about the Falls).

In case you’re interested, back in 1969 water flow over the falls was cut off to install a hydroelectric plant. Here’s a page that features photos of what the falls looked like with no water.