Jens Ammon
Instream Geomorphic Units via GUT Analysis
Asotin Watershed, WA
For the following exercise, we examine the North Fork Asotin Creek in the Asotin Watershed in Southeast Washington. The North Fork Asotin Creek is located near the center of the Asotin Watershed and is a moderately sinuous 3rd order stream. Using the GUT program (http://gut.riverscapes.xyz/) we will compare the outputs of in-channel geomorphic units for a relatively devoid reach and a relatively complex reach.
2. North Fork Asotin F4
2.1 This reach is generally structureless. It is a straight, single channel with with large areas of featureless flat runs.
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2.2 I am comparing year 2011 and 2015. Since the channel has little structure, it probably doesn't change much year to year so I want to give multiple years of development for the comparison.
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2.3 Similar Tier 2 units are present, such as the channel being dominated by troughs and planar features running down the center of the channel. However, the features of the 2015 survey are much more connected than lengthwise down the stream.
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2.4 This reach is dominated through time by concave and planar features. Troughs, bowl transitions and planes are common in both years.
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2.5 As with the Tier 2 units, the reach is dominated by flat Tier 3 units. A glide-run vastly outweighs any other type of in channel unit.
2.6 Yes, geomorphic units are in reasonable locations in the channel. A pool precedes the riffle and are oriented along where a thalweg might be located. Margin bars seem to be opposite pools.
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2.7 GUT seems to be doing fairly well at classifying the units. There are some areas where there are both "pools" and "pocket pools" of the same size and in similar positions along the channel, so maybe GUT is having a hard time differentiating the two. As far as the overall trends of the channel units and types of units that would be present in the channel, it seems like GUT is succeeding.
2.7A Pool
GU Forcing: By Bar/Structure
GU Orientation: Streamwise
GU Position: Bank/Side of channel
Low Flow WS Slope: Flat
Low Flow Relative Roughness: Low
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Bar-Forced Pool
As determined by the GU forcing from the bar
GUT does not classify pools into what is creating
them, so while it did identify the pool, it did
not identify that it was bar-forced.
2.7B Bar
GU Forcing: Not forced
GU Orientation: Streamwise
GU Position: Bank
Low Flow WS Slope: Shallow
Low Flow Relative Roughness: Moderate
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Eddy Pool
As determined by the bank-attached location
and forced shape
GUT just classifies this as a margin-attached bar,
and does not go into further refinement of what
type of margin-attached bar.
2.7B Planar
GU Forcing: Not forced
GU Orientation: Streamwise
GU Position: Mid-channel
Low Flow WS Slope: Shallow
Low Flow Relative Roughness: Low
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Glide
As determined by the low relative roughness
GUT groups glides and runs into one category,
and with the given data this is a fair assumption.
In order to identify it specifically as a Glide, I
had to guess at the low relative roughness.
2.8
A: Based on the topo lines, this is a pool. From the bar structure next to it, it is a Bar-forced pool.
B: I included the smaller concavity upstream of the larger pool all as one large pool. GUT
separated the two and called the smaller one a rapid.
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C: GUT classified part of what I considered a pool as a rapid. From the steep gradient, it makes
sense why GUT classified it as a rapid, but it would make more sense to include it as part of the
pool.
3.1 This reach has much more complexity. It is more sinuous, has a much larger range of geomorphic units, and has convexities in the middle of the channel rather than limited to the sides.
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3.2 I will be comparing 2011 to 2012 for this reach. Based on the greater structural complexity, I assume there will be more changes from year to year in channel structure than in a structurally devoid channel.
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3.3 The layout of this reach changes much more than the previous structureless reach, despite less time between the compared years. General trends are convexities along the edges and concavities near the center/where the thalweg would be.
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3.4 No Tier 2 form is dominant in this reach, and the reach remains complex through time.
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3.5 As with the Tier 2 units, there is a larger variety of Tier 3 units present in each year. Between years there are mid channel bars and margin attached bars in the same general location, but their exact extents change. The lowest point in the reach also gets much more complex in 2012.
3.6 Yes, geomorphic units are in reasonable locations in the channel. Bars seem to occur on inside bends and pools opposite of the bars. Units are elongate in the direction of the thalweg/flow.
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3.7 GUT seems to be doing fairly well at classifying the units. There are occasional units that do not make sense, such as mid-channel bars being present on the sides of the channel, or small pockets of rapids where it is unlikely rapids would develop. But the general distribution of bars and pools seems to be logical.
3.7A Pool
GU Forcing: Channel Bend/Bar
GU Orientation: Streamwise
GU Position: Mid/Side
Low Flow WS Slope: Low
Low Flow Relative Roughness: Low
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Bar-Forced Pool
As determined by the GU forcing from the bar
GUT does not classify pools into what is creating
them, so while it did identify the pool, it did
not identify that it was bar-forced.
3.7B Bar
GU Forcing: By planform
GU Orientation: Streamwise
GU Position: Bank-attached
Low Flow WS Slope: Moderate
Low Flow Relative Roughness: Moderate
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Point Bar
As determined by the Planform GU forcing
GUT only goes down to classifying the bars as
margin-attached, so while it is correct, refining
further to Point bar has to be manually done.
3.7B Plane
GU Forcing: By flow width
GU Orientation: Streamwise
GU Position: Channel spanning
Low Flow WS Slope: Shallow/Moderate
Low Flow Relative Roughness: Moderate
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Run or Rapid
As determined by the GU position and slope
Here, without being able to refine the WS Slope
and Roughness in person, GUT does as good
of a job of classifying as I can do
3.8
A: Based on the topo lines, there is either a chute cutoff or an eddy bar.
B: The area I included for the bar being cut off by the chute cutoff is much bigger than what GUT
classified it as.
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C: As with the previous examples, my classifications have more modifying terms attached to
them. GUT fills in a lot of space with glide-run or riffle classifications rather than pinpointing
where the chute cutoff occurs or the riffle crest occurs.
4.1 GUT places much of the channel into the trough/bowl/concave category at the Tier 2 scale. While this is technically accurate, I might classify some of these areas as planar from a field study. The general shape of a channel is a trough, so I would be more lenient on the cutoff between a planar feature and a convex feature. GUT seems to do fairly well delineating the convex features.
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4.2 It seems that GUT groups a lot of mid channel features into the Glide-Run category. GUT also does not account for channel constrictions or channel widening, a common attribute we would use in the field. Since GUT is based on the topography/channel bed surface, it does not consider water surface slope or roughness in its classification. Overall, it seems that GUT is and effective way to get an approximate layout of the whole reach, but if it is necessary to pinpoint individual features, a field study is still necessary.
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4.3 Some Tier 4 attributes could be assigned. An estimate of the Tier 4 vegetation could be made from landcover data, but attributes such as grain size would have to be identified in field. The vegetation classification would likely have a large amount of error depending on the landcover raster resolution.
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4.4 Desktop mapping through topography is efficient and accurate enough for wider scope studies. When comparing reaches such as what we were doing in this exercise, the units area plenty refined to draw conclusions about the health of each reach. However, if you were doing a study to determine which bars need to be excavated, etc. you would want to do a field visit and manually refine the GUT boundaries further.