Thurs-Demo: The one with Terraces

Part of my research deals with the history of rivers. Well, a few rivers in particular, but they are an example of how the landforms associated with rivers and streams can tell us about past changes to the river system. Terraces are one of my favorite. Terraces reflect periods of transition for the river system. Periods when something happened - whether it be tectonics, climate, or changes in the base level (lowest point) terraces mark a change from a stream that is relatively "stable" to a stream that is able to erode down into its floodplain.

Callan, over at Mountain Beltway, had a post about terraces last year. The picture below shows some of the work I'm doing along the Red Cedar River - a tributary of the Chippewa River - which itself is a tributary of the Mississippi River.



Depending on how picky I feel like being, I count over six terrace levels - possibly as many as ten. One of the questions I'd really like to answer is "how did these terraces form?" Did they form as a result of changes in sediment supply? Cutting off sediment supply (or increasing water discharge) allows the stream to pick up sediment along the floodplain and erode downwards. Or base level drop? A drop in the main stream would lower the mouth of the tributary. This would increase the slope of the stream locally. Increased stream flow would increase erosion. I've sketched out the two most likely scenarios for the Red Cedar below (I'm ruling out tectonics for now, but isostatic readjustment as a result of glacial retreat may play a role).



It's important to note that these terraces may not be synchronous - depending on the rate of erosion, one part of the terrace may form much later than another. Also, the direction that the terrace develops is different. With sediment supply, the erosion starts upstream and progresses downstream. By contrast, a drop in base level occurs at the mouth first and erosion progresses upstream.

This is rather an abstract concept. It's hard to intuitively understand these processes. That's where the Emriver model comes in. By manipulating the system, it becomes easier to see what's going on. But how well does the Emriver replicate these terrace forming processes? Pretty good, actually. Here's a video where I managed to alter sediment supply (by limiting the sediment mobilized upstream) and dropping the standpipe to simulate a fall in base level. Pay close attention to the direction that the incision propagates.

Connecting the dots.

I've been working on my presentation for GSA - which is part of my developing research into the fluvial geomorphology of the Red Cedar River in western Wisconsin. The picture below represents some of what's occupying my mind right now. The dots represent the elevation of stream terrace surfaces along the river valley. The y-axis is elevation in feet above sea level and the x-axis is horizontal distance (in meters) measured from the mouth of the Red Cedar River. I obtained the measurements in GIS, getting the elevations from a LiDar dataset and using the "measure" tool to get horizontal distance. The orange and green lines represent some possible interpretations for correlating the different terrace surfaces.



You can see that the two interpretations are quite different in some aspects. The orange lines are my primary interpretation - I focused on emphasizing terrace elevations that were parallel with each other. But, while it's tempting to correlate terraces so that they are parallel with the modern stream profile (blue dots at the bottom), this may not be the case. So, as an exercise in "changing my perspective," I drew the green lines by "squishing" the plot horizontally (extreme vertical exaggeration). This emphasized the possible surfaces that were not parallel.

Those of you with some training in fluvial geomorphology may have noticed another interesting feature. The sheer number of terraces along a 20 km reach of a relatively small river. I have probably over-interpreted some of this data; fourteen terraces, some separated by only six feet is probably the high-end estimate. Even with a more "conservative" correlation, there are at least eight terraces along this stretch of river. I'll let further exploration of that point (as well as my take-home message about the terrace profiles) wait until GSA - if you want to get the full story, come to my talk on Tuesday afternoon.