Modeling fluvial systems with a stream table - fans/deltas.

This site focuses on depositional fan forms created on a stream table in the Department of Geography and Geology at the University of Nebraska at Omaha. There is an overview site discussing stream table modeling of river (fluvial) systems in general, and a site that focuses on the channel forms created. Aspects of channel evolution are also further discussed in passing in this document.

In that a mostly exposed fan shaped body builds out (progrades) from the foot of a significant break in slope, this feature depicted here is somewhat like an alluvial fan at the foot of a mountain. In that small lobes of sediment form in the 1 inch or so of standing water at the margin of the fan it behaves somewhat like a birds foot delta. So it might be considered a fan/delta compex. In the adjacent photo, taken after several hours of flow, you can see the small fan shaped body of sediment that has formed. Note the oblique view of a six inch orange protractor on the right side of the image. At the time the photo was taken the right portion of the fan is where a channel is actively depositing sediment. As you scroll down through this document note how the position of deposition changes in the successive photos. You can also see 2 channels actively feeding (shown with arrows) the fan. Note their evolution with time also.


This is a similar view after an hour or so has passed. Note that the middle portion of the fan is now active. More photos would show the channels on the fan switching back and forth depositing a strip of material here, then there, and through this history forming the fan. If you think about it the geometry of the fan is somewhat conical (funnel shaped), but a cone with a very wide apical angle. A channel does not sweep gradually back and forth (like a wind shield wiper). The behavior is more complex. Usually there are several channels operating at once, with one or more of the population carrying most of the flow. Gradually a side channel captures more of the flow and then it becomes dominant, and as it does so new channels form and old channels are abandoned, and the process continues. What are the dynamics that drive this constant rearrangement and that result in such a regular geometry? Note also the channel entrenchment since the above photo, and how the left channel of above has been abandoned.


This photo is after some 5 hours of flow. Using the fish for scale you can note the growth of the fan. It has a lobate form. Each lobe represents deposition of a channel as it enters the standing water. An old (no longer presently active) lobe is marked, as is the active part. Multiple channels exist at this point on the fan, but the one in the center is dominant. Note also the meander bend migration evident, although entrenchment is not as evident.


This oblique view upstream is after a days worth of flow and the fan now spans the width of the table. Note that the left side is the active side.


This shows a close-up of the front where individual lobes of sediment are evident. Much longer lobes are evident at deltas of rivers such as the Mississippi. With time the above water portion of the lobes of the Mississippi delta disappear due to subsidence from compaction of underlying sediments and from wave action and erosion due to Gulf storms. Those interested in reading more about river channel switching and lobe abandonment may want to read Controls on Nature by John McPhee .


One more interesting feature that can be modeled is the patterns of sedimentation caused by a sea or lake level increase or decrease. This is simply done by increasing the depth of water that the fan/delta complex builds into. In this photo you can see two lobate depositional fronts. The higher one was formed when the water level was higher, and can now be considered a paleo-shoreline. The water level was decreased in order to take a good photo. The active channel seen in the left, associated with the restored lower water level, has locally destroyed evidence of the shoreline deposits formed during the previous high water stand. This is typical, that a decrease in water level (often termed a regression) erodes through deposits left during pregious higher level stands.


If you have any questions or comments about this web page please contact Harmon D. Maher Jr. at University of Nebraska at Omaha (harmon@cwis.unomaha.edu). Thank you!