1. Fluvial geomorphology
ESS 400a
Summer 2014
Lecture for Saturday mid day, 8/2/2014
Now that we’ve seen some examples of fluvial landforms, lets have a crash course in fluvial geomorphology and hopefully some of the observations you’ve made will click!

2. The fluvial system Headwaters Straight channel Tributary
Meandering channel
Delta
Let’s start with a drawn out look at the whole of the fluvial system – from headwaters to mouth.
Channels start in steeper highlands and water flows down the system. As it does so, the morphology of the channel changes. We start in the headwaters with straight channels, then progress to meandering (maybe some braided), and then to a delta. What is driving all the water flow?
Water flow

3. The fluvial system Gravity! The engine for geomorphic work Headwaters
Straight channel
Tributary
Meandering channel
Delta
Gravity! The fluvial system and its geomorphic components are all driven by gravity, which determines how much energy is available to do work.
Water flow

4. The fluvial system
Gravity drives changes….but the amount of water and sediment is controlled by basin size.
Drainage basin (watershed): the area contributing water and sediment to the channel
While gravity has a large influence on driving changes, the amount of water available to make those changes is also important. One way of thinking of how much water a system has is by considering its drainage basin or watershed size. A watershed is the area contributing water and sediment into a channel, and its divides are often high ridges.

5. The fluvial system Quiz time! 1 2
Where is the river flowing fastest – point 1 or 2? 1
Now, where do you think water is flowing the fastest?
Raise of hands, count on the board. 2

6. The fluvial system Quiz time! 1 Point 2 – why is this? 2
Where is the river flowing fastest – point 1 or 2? 1
Point 2 – why is this?
The answer is point 2 – down in the meandering, wide channel. Those who answered correctly, why did you think so? 2

7. The fluvial system
The volume of water increases downstream as more tributaries and run-off join
This volume of water is termed discharge, and is the volume of water flowing through a channel per time unit. 1
Therefore, Q = v * A
So as Q increases, velocity and area increase!
So in fluvial geomorph terms, why is the velocity higher at point 2? As you go downstream, more water enters the channel from tributaries joining and groundwater and run-off. We call the flow of water through the channel the discharge, and is simply the volume of water flowing through a channel per unit time. So discharge is increasing downstream. And by its definition, discharge is velocity times area. So as discharge is increasing, the velocity and area must increase!
Another thing to think of is the amount of drag along the channel and turbulence. Headwater channels often look like they are going very fast, but they also contain a high amount of turbulence and eddies that prevent flow from going downstream in a straight, simple fashion. You also have more frictional drag from the banks and bottom because most of the water is close to a bank. Whereas downstream, you loose the high turbulence and the deeper channels allow for more water to flow along without experiencing much drag. 2

8. The fluvial system
The volume of water increases downstream as more tributaries and run-off join
This volume of water is termed discharge, and is the volume of water flowing through a channel per time unit. 1
Therefore, Q = v * A
So as Q increases, velocity and area increase!
However, one thing to think of for our purposes is that discharge is pretty constant at the reach scale, where no tributaries (or irrigation channels) take water in or out of the system. So in our study area, Q should be constant, so v*A will balance each other out. Does this makes sense – low velocity areas are pools, which are deeper, so have higher A’s to make up for lower v. 2
Note: At the reach scale, such as BSC, Q is constant

9. The fluvial system
How do we talk about it’s components?
Stream order: A numerical classification of stream segment size within a larger network
Headwaters 1
Straight channel
Tributary 1
Meandering channel
OK so now that quiz is done and you can impress all your friends with that piece of knowledge. Now, when we talk about streams, how do we compare channels – not all are the same size. One way to do this is through the stream order. This is a way to number streams based on how many tributaries/junctions it has. A headwater stream is 1, and when two 1s join, the resulting stream will be a 2. When two 2s join, a 3 results. And so on. 2
Delta 3

10. The fluvial system Straight channels: What do these streams look like?
We also classify channels based on their shape. First, we have straight channels, which tend to form in the mountainous headwaters.

11. The fluvial system Meandering channels:
What do these streams look like?
Meandering channels:
In the lowlands, we have meandering channels, which we will discuss in more depth later. These meanders are often self-propagating, and migrate across the floodplain. Often, they have one steep bank and one gentle bank.

12. The fluvial system Braided channels: What do these streams look like?
Braided channels form at deltas but also where streams come out of the mountains and are still carrying high amounts of sediment. They have multiply channels, which are non-vegetated. These multiple channels are confined to an active channel, across which the channels migrate.

13. The fluvial system
What do these streams look like?

14. The fluvial system
Pool-riffle features in meandering or sinuous rivers
Pool: over-deepened channel from scour or high discharge events
Riffle: shallow channel with rough low-flow water surface
Bar: above flow, loci of sediment deposition during high flows
More specific features: in meandering rivers, a typical morphology will be pool riffle – we’ll probably see some of this at BSC.
Thalweg: German for valley, deepest, and often fastest, part of the channel

15. The fluvial system
Pool-riffle features in meandering or sinuous rivers
An example – unknown location.

16. Hydrology Q = area * velocity = width * depth * velocity
As we discussed earlier, discharge is the cross sectional area times the flow velocity, which is equivalent to the width times the depth times the velocity. That assumes a rectangular channel.

17. Water velocity 0.6 depth = depth average velocity
Laminar sub-layer (very very thin)
The water velocity is not the same throughout the water column – why might this be??
Drag. Frictional resistance from the bed slows the water near the bed so we get a velocity profile that increases towards the top of the water column. It’s average is found at 0.6 times the depth. Flow is always turbulent, even though some streams may look smooth, they are not!
The only laminar part is near imaginary – the laminar sub-layer which is fractions of a millimeter thin and is used mainly in sediment transport equations. It’s not something you’ll notice in the field.
With the exception of the laminar sublayer, flow is turbulent!

18. Water velocity 0.6 depth = depth average velocity
Laminar sub-layer (very very thin)
So since the flow is turbulent, our velocity lines should actually look like this – if we were following a sediment particle moving through the water, it would not be flowing in a straight line at all, but get caught in eddies.
With the exception of the laminar sublayer, flow is turbulent!

19. Velocity in a meander Factors: Drag from the bed
Drag from banks produces a parabolic velocity shape across the width
Super-elevation and pressure gradients?!
Not only does velocity vary with depth, but it also varies across bends in rivers. Some reasons for velocity differences across the channel width are: bed and bank frictional resistance, centrifugal force of the bend, and superelevation and pressure gradients. As you can see with this diagram of the meander flow, water is higher on the outside of the bend, due to centrifugal forces. This creates a pressure gradient that results in flow moving towards the inside of the bend.

20. Velocity in a meander
Higher elevations = higher pressure = pressure gradient inwards = helical flow
Lower velocity on inside = deposition of gravel bar
Higher velocity on outside = erosion of bank
The result of all this is helical flow patterns in river bends. Water is flowing downstream but also reacting to pressure gradients and it gets all sorts of crazy. However, despite the flow being helical, you still have higher velocity on the outside of the bend and lower velocity on the inside, which leads to erosion on the outside and deposition on the inner bank. We saw the result of this on the field trip – the cut bank and point bar.

21. Meander migration Cut banks erode New material on bar Meander migrates
Planes off a valley
As meander erosion continutes, the meander will migrate outwards towards that eroded bank. This carves away the floodplain, as you can see in the diagrams. From meander migration, we get a planed off valley surface that is very important in creating terraces, as we’ll discuss in a few minutes.

22. Incision
Planation forms the valley but how does a river erode down?
Rivers are at a “graded equilibrium”, where their profiles approximate an inverse log curve
A change to the shape of this curve – i.e. fault uplift – can provoke a reaction – such as incision and knickpoint formation
First though, how does a river erode down – the process we discussed moves the river back and forth, but why might it erode down into its bed?
First, we need to understand the concept of a ‘graded equilibrium’ – this is the profile rivers prefer, due to a balance of gravitational forces and discharge. By profile, we mean the elevation of the channel with downstream distance.
A change or perturbation to this profile puts the river out of whack, and it adjusts itself to regain the graded equilibrium.

23. Incision
Planation forms the valley but how does a river erode down?
So for example, if we have a fault across the river profile, and it suddenly offsets the river.
Fault

24. Incision
Planation forms the valley but how does a river erode down?
We get a locally steep point, and the river profile is disturbed.
Fault

25. Incision
Planation forms the valley but how does a river erode down?
The river upstream of the fault is now going to erode down to return to its previous equilibrium, and there is now a knickpoint (zone of steepened slope)
The locally steepened area is called a knickpoint – which might be called a waterfall, but we use the term knickpoint in fluvial geomorphology. At this point, erosion will be focused, and the river will start to erode here until, propagating the knickpoint upstream until the profile returns to its smooth shape.
Knickpoint
Fault

26. Incision Knickpoint will move upstream like a wave
Planation forms the valley but how does a river erode down?
Knickpoint will move upstream like a wave
Migration style of knickpoint will depend on bedrock and substrate
Pool generate at base of knickpoint
As the knickpoint retreats upstream, it can do so in a couple different fashions. The migration style depends on the bedrock type and fracture style, but in all these cases, a pool will form at the base of the knickpoint (just as you’ve observed pools at the base of waterfalls).

27. Knickpoints
There are multiple ways to generate a knickpoint, including:
Differential bedrock strength creating zones of different erosion
Base level drop from sea level changes or rock uplift
Base level drop from faulting across the river
Changes in discharge from stream capture, piracy, or beheading
Anthropogenic influences – redirecting streams, changing surrounding surfaces
Natural or artificial damming
In that example, the knickpoint, which we focus on because it’s the loci for erosion into the riverbed, was formed from tectonic uplift. It can also form though these reasons….

28. Landforms
Floodplain: the lowest surface adjacent to the river that is composed of overbank flood deposits overlying laterally accreted alluvium
Levee: overbank deposits immediate to the river formed during rapid settling during floods – builds up bank height
Terrace: abandoned floodplain, product of incision and lateral planation
Floodplain
Oxbow: abandoned meander bend in the floodplain
As knickpoints erode down, and meanders erode laterally, we get a variety of landforms produced. Floodplains form….
Alluvial fan (not shown): conical in shape, loose, coarse sediments from
small channels on steep hillslopes

29. Terraces Alluvium only, bedrock buried Fill terraces
Erosion & planation into bedrock with thin gravel cap
Strath terraces
Incision and planation occur at different times
Paired terraces
One landform we focus on in order to gage the history of the stream valley, are terraces. They come in a variety of shapes and sizes, from fill to strath and paired to unpaired. From a fill terrace, we know the valley must have filled with sediment, then eroded through that sediment. Strath terraces tell a story of erosion only. By interpreting a sequence of fill and strath terraces, we can figure out the valley’s history.
Incision and planation occur together (more common)
Un-paired terraces

30. Terraces What happened in this valley?
A record of the incision and planation history
Let’s give it a try in this example. The dark grey is bedrock. What happened in this valley? (Draw on board)
What happened in this valley?

31. Terraces
A record of the incision and planation history
Well, what we see are a lot of incision and aggradation events, as well as periods of planation. For BSC, we ask you to do something similar, so let me know if you have questions about this!
What happened in this valley? – 6 or more incision, 9+ planation, and 3 aggradation periods!
You will be asked to do this, or something similar, for Assignment 4!

32. Response timescales
Terraces form due to incision, which we learned has multiple triggers
Thus, a younger terrace might record recent 103 year fault movement, while an older terrace might be due to climate variations on a 104 to 105 timescale
Terrace forming mechanisms occur over different timescales, so we can’t interpret terraces to be formed due to the same process or trigger.

33. Response timescales
Terraces aren’t the only features left from the adjustment of rivers to environmental changes
In fact, the whole river is going to react differently due to different triggers and the timescale of that trigger.

34. Your site map: Read HARRELSON ET AL. 1994
Recording all the features we just talked about will help you constrain the geomorphic history: oxbows, slumps, modified areas, terraces, alluvial fans…
Channel features such as pools, riffles, knickpoints, strath exposure, will also help your history – note these on a site map or on the long profile notes!
So what implication will all this have for you? Mainly the site map, where noting floodplain features such as oxbows, terraces, etc will help tell you the story of BSC! And those channel features, such as knickpoints, bedrock exposure, strath exposure, will help detail how the Red Rock fault and BSC interact.

35. Additional readings ESS326 – Geomorphology (WTR)
If you are curious and want to learn more, here are some good references. I have the left 2 and you are welcome to borrow them if you want to learn more. Geomorphology and Tectonic Geomorphology are both available online through the UW libraries. There are also some courses you can take if you want to learn more!
ESS326 – Geomorphology (WTR)
ESS426 – Fluvial geomorphology (SPR)
ESS541 – Applied fluvial geomorphology (AUT)