I am a biologist, not a fluvial geomorphologist but I like to play one sometimes. As a stream ecologist, you sort have to play fluvial geomorphologist a bit. I will probably talk about this in a way that will make most geologists cringe a bit. I know my Advanced Fluvial Geomorphology professor Steven Kite cringed more than a bit when it came to my first draft of a paper for his class. That said, he was one of the five committee members that granted me a Ph.D.
Streams are complex things that are always changing, that is a large part of what draws many of us to streams. Streams are a balance between hydrology and geology - and ever increasingly, we are recognizing the influences of biology in this relationship. Fluvial geomorphology is the essentially the study of how water moves sediments, and thus is responsible for stream morphology. There are any number of words above that need to be defined and understood. While the angler tends to think of sediments as small particles, for a geologist, sediments are anything moved by water - from the smallest of particles to rock weighing millions of tons. Water, we know, can move them all.
Castro and Thorne (2019) published an open access article in River Research and Applications that describes the relationship between hydrology, geology, and biology in the formation of stream channels. Water (hydrology) is obviously the first factor - channels are shaped by water and what materials they can and cannot move. In particular, channels are often shaped by high flows which rearrange the channel as those of use in the Driftless are quite familiar with after the 2018 floods. Then as the figure above shows, there is a relationship between sediments and water. Too much sediment supply and streams increase in elevation (aggrdation); too little and they degrade. The image above in Lane's sediment balance which has long been used to describe the relationship between sediment supply and streams ability to move that sediment.
What Castro and Thorne (2019) add to Lane's diagram is the biology, the effects of the riparian area and its vegetation. Islands, in-stream woody debris and aquatic vegetation, and other biological controls on stream channels are all important. Biology has generally been overlooked in regards to stream channels and their controls. To me, the role of plants in stream channels and preventing against major stream channel changes was quite evident after the 2018 flood event. While there were some ungrazed or lightly grazed pastures that saw significant damage, from what I saw after the floods, a very large percentage of heavily grazed pastures were greatly altered by the 2018 floods.
Figure 1 from Castro and Thorne's (2019) stream evolution triangle (SET) demonstrates the relationships between the geologic, hydrologic, and biotic controls of stream channel morphology. To orient the reader to the axes, hydrology is a measure of what a stream can move (I wrote about this some time ago). The geology axis is a measure of how easily eroded the banks are - from hard bedrock to soft sands. And the biology axis is how strongly vegetation, large woody debris, and other biota (or formerly alive things) shape stream channels.
The most recent post about ecological succession informs us about biological effects change through time. As Milner and his colleagues (2007) saw, soon after glacial retreat, streams are dominated by water and easily moved sediments. Over time, grasses and eventually shrubs and then trees prevent against bank erosion by the stream. They are also an important source of fertility - something lacking in streams near glaciers. Obviously we are quite removed from glaciation - and in the Driftless Area, our landscape was never glaciated. However, disturbances still play important roles and in much of the Driftless Area, the most important and widespread disturbance is cattle grazing.
The figure above has examples of the geological controls - the further away from the geology corner (lower right corner), the less influence on stream channel moprhology that geology has. Panel a shows that bedrock, because it is less erodible, has the greatest impact on stream channels whereas channels through alluvium (water moved sediments) are more easily altered and less controlled by geology. In panel c, it shows that confined valleys have greater control than do less confined valleys. In many streams, this means there is great geologic control in headwater streams compared to downstream reaches. F
The figure above, also from Castro and Thorne (2019), shows the influences of hydrology on stream channel morphology. Storm-dominated systems with a high amount of impermeable surfaces in free flowing (unregulated) streams are most strongly controlled by hydrology.
Lastly, the effects of biology are shown above. The panels show that (a) obligate wetland vegetation, (b) channel spanning large woody debris, (c) beaver meadows, and (d) mussels have the strongest biological controls on stream channel shapes.
Figure 6 above is also from Castro and Thorne (2019) and shows the stream channels that are associated with different combinations of hydrology, geology, and biology. These are Rosgen Stream Classification System channel types (a post for another day...). Type-A channels are typically formed through strong geologic (bedrock) controls where hydrology and biology have little control on stream morphology. The D-channel types (D and DA) are associated with high amounts of fine sediments. The DA-channel type shows a much greater biological control than does the D-channel type. And the two types of channels we are probably most familiar with are the B and C channel types. Both of which show relatively high sinuosity (a measure of stream length to valley length) and a relatively high depth to width ratio - though not as high as the more sinuous E-type channels.
This was figure 8 from the Castro and Thorne (2019) paper - their caption was, "Whychus Creek, Oregon, restoration project phases over 1 year. Photos courtesy of Paul Powers [Colour figure can be viewed at wileyonlinelibrary.com]". I think this is pretty illustrative of the types of stream channels that are indicative of each corner of the stream evolution triangle (SET). The top channel has little control by biology due to the poorly rooted trees that are high on the bank. By the time the stream had healed (the image in the last panel), the effects of biology are much stronger and a more resilient channel is in place.
Practical Applications of the Stream Evolution Triangle
I think the last set of images from Oregon are pretty illustrative of how stream channels can be modified (improved) to increase flood resilience, improve habitats for fishes and their food sources (both aquatic and terrestrial), and create a sustainable stream channel. Another open access article from Johnson et al. (2020) demonstrates how the SET can be used in river restoration.
Figure 5 from Johnson et al. (2020) is an excellent way to visual how humans impact stream channels and the factors that help shape them. For example, we tend disconnect stream channels from their floodplains and increase stream power (more water in a smaller, more confined channel).
Above is an image from the Driftless Area where biological controls are low to non-existent due to row cropping right up to the stream bank. This channel is unstable and likely to erode into the farmer's field during the next flood event. This stream channel could be stabilized by sloping the bank back and allowing the bank to vegetate with grasses that will better hold soil and prevent erosion. That, of course, would cost the farmer a bit of his/her field - of course, it is looking like they're losing field as it is.
To me, this is the quintessential northern Wisconsin stream that is largely biologically controlled. The substrates are mostly easily moved sand (low geological control) and the watershed is mostly National Forest with intact wetlands so hydrology tends towards a relatively low to moderate influence. However, the large woody debris (LWD) is responsible for much of the channel's morphology. Holes are cut under and around LWD and the stream channel width is largely controlled by riparian vegetation - tag alders in many northern Wisconsin streams.
This summer, our research examined the effects of geology and stream habitats and the results - though Brandon is still working on analyzing and writing them up - are quite interesting. Streams of the sandstone geology were sandy and lower in productivity but the also stay colder and were much more likely to hold Brook Trout than Brown Trout. The streams to the south that are in the dolostone (dolomite) geology, had more gravel, cobble, and other larger substrates. They are more productive (higher conductivities), showed a greater range of temperatures within a day, and they are much more likely to hold Brown Trout than Brook Trout.
Looking at Castro and Thorne's (2019) stream evolution triangle, these difference make perfect sense. The sandbed streams are less hydrologically variable thanks to more high infiltration rate soils (water is more readily absorbed by the sandy soils) and woody debris and streambank vegetation have significant impacts on stream channels. In the dolostone region of the Driftless, the less erodible / transportable sediments have a greater control on stream morphology.
I gave a talk at the Valley Stewardship Network in Viroqua in November of 2022 about climate change, flooding, and stream manipulation. It was a quite enjoyable time - my presentation, in hyper-quick mode is below. Unfortunately, there were some battery issues that kept the presentation from being able to be recorded and shared. I am happy to give it again if any TU Chapters or other conservation groups are interested. The PowerPoint is below - at about 1 slide per quarter second. If you are epileptic, you may want to skip this quickly...
Johnson, M.F., Thorne, C.R., Castro, J.M., Kondolf, G.M., Mazzacano, C.S., Rood, S.B. and Westbrook, C., 2020. Biomic river restoration: A new focus for river management. River Research and Applications, 36(1), pp.3-12.