Today in my flvuial processes class, we’re going to discuss a great paper by Gran et al. on “Landscape evolution, valley excavation, and terrace development following abrupt postglacial baselevel fall.” The paper is set in a landscape I know well – southern Minnesota’s Minnesota River Basin. For my students in northeastern Ohio, this landscape would feel somewhat similar to the area around here, but not quite. Similarly, the glacial history has some degree of overlap (glaciers covering the region and retreating about ~14,000 years ago), but there are some unique elements of each place. Fortunately, I found a series of short video clips that do a nice job of setting the Minnesota River story in its context.
The tributaries to the Minnesota River are still relaxing after all of this geologic excitement. As a result, many of the tributaries exhibit knickpoints or knick zones – steepened areas on the channel profile.
You don’t have to go to the primary literature to learn about the knickpoints and what’s going on in the Minnesota tributaries, thanks to a nice short discussion by Karen Gran. There’s even a huge knickpoint left on the Mississippi River in Minneapolis as a result of the Glacial River Warren and draining of Lake Agassiz. You can read about that one here.
For a great discussion of fixed versus mobile knickpoints, check out this essay by Ben Crosby, who did his PhD work on a particularly knickpoint rich landscape in New Zealand. More good discussion of knickpoint mobility can be found in this blog post by Chuck Bailey. Finally, Steven Schimmrich has a nice series of blog posts on knickpoint retreat at Niagara Falls.
If that’s not enough, knickpoint retreat is often associated with the formation of terraces. There are two types of terraces to come to terms with. Cut-and-fill terraces, where the unconsolidated sediments are actually alluvium, show that the river has had periods of both aggradation and incision (and lateral planation of the valley bottom). In rivers with strath terraces, aggradational periods are either relatively minor or non-existent and the terraces represent alternating periods of incision and lateral planation. Callan Bentley has illustrated each type of terrace below, and you should see his post and comments for good discussion of the intricacies of naming and describing these features.