|
Why Do We Exaggerate Stream Channel
Cross-Section Plots?
The Case for True-Scale Plotting
by John Potyondy and Larry Schmidt
Perception often governs reality. The way people perceive objects has a profound
influence on the way they conceptualize the objects. This is true for tangible objects that we identify
with our senses, but it applies equally well to concepts and ideas.
This article illustrates how typical displays of stream channel
cross-section data confuse perception and lead to misinterpretation
of stream channels and how they operate.
Most people recognize the characteristics of a square.
It is a familiar basic geometric shape learned at an early
age. But suppose for a moment
that you must illustrate a square for a person unfamiliar with the
concept. Your basic description
might say for example, that a square is a rectangle with four equal
sides (Figure 1).
Figure 1. A square.
Now consider the effect on the person’s perception
of a square if you provided an illustration of the square with a
2 to 1 vertical exaggeration (Figure 2).
Even though the exaggeration is clearly identified, it is
likely that the person will carry a warped view of what a square
is.
Figure 2. A square with 2:1
vertical exaggeration.
A rational person would reject the notion that this
is a good way to illustrate a square and would consider it a poor
way to teach a person the concept of a square.
As absurd as this example seems, this is the typical case
when plotting and displaying stream channel cross-sections.
Consider the following example of a cross-section plot
of the South Fork Cache la Poudre River in Colorado. The South Fork Cache la Poudre is a typical
mountain gravel-bed river, about 40 feet wide and 3 feet deep at
bankfull stage. The river
flows through a broad alluvial valley meadow and can be characterized
as a Rosgen C-3 stream type. The
plot in Figure 3 illustrates how the cross-section is typically
displayed when plotted with a standard computer spreadsheet, such
as Excel.
Figure 3. Plot of the South Fork Cache la Poudre River
cross-section as the data is directly plotted by a typical spreadsheet
plotting program. The vertical
axis is exaggerated by a factor of 5 making for a 5:1 vertical-to-horizontal
plotting ratio. The exaggerated
nature of the plot conveys a distorted view of the river’s true
cross-sectional characteristics.
One problem with this display is that it does not show
the cross-section as it appears in the natural world. This is because vertical elevation and horizontal
distance are not plotted at identical scales. For the cross-section to appear as it does
in reality, it would have to be plotted at a 1:1 vertical-to-horizontal
ratio. In this example,
the computer graphing software exaggerated the vertical scale by
a factor of 5 giving it a 5:1 vertical-to-horizontal ratio.
In other words, each foot on the vertical scale is 5 times
that of the distance used to show 1 foot on the horizontal scale.
In contrast, Figure 4 shows a true representation of
the actual stream cross-section (plotted at a 1:1 ratio); one that
is close to how we actually see and perceive the stream were we
to stand on its banks.
Figure 4. Plot of South Fork
Cache la Poudre River cross-section at a 1:1 ratio without vertical
exaggeration.
While
the astute viewer may be aware of the exaggerated nature of the
plot in Figure 3, the casual viewer will probably draw different
conclusions about river behavior from the exaggerated versus the
real-life scale plot.
In the exaggerated plot (Figure 3), the South
Fork Cache
la Poudre River appears to be a relatively deep river.
Depth appears to change dramatically over short distances
across the channel and one might be tempted to conceptually think
of this river as one which is fast and deep.
Because of the large vertical distances between low base
flows and flows which fill the channel, it looks like the river
fluctuates a great deal between wet and dry seasons.
The elevation of bankfull stage is identified, but the flat
depositional bankfull feature appears to be poorly defined and the
associated floodplain appears to be very narrow. Based on this depiction, it might be difficult
to convince someone that the active floodplain associated with bankfull
discharge is easy to identify.
It also looks like a lot of additional water will be needed
before the flow leaves the channel and inundates adjacent lands.
In contrast, the 1:1 cross-section plot (Figure 3)
conveys an alternative view of the river channel. From the true-scale plot, the South Fork Cache la Poudre River appears
to be fairly wide and shallow, rather than deep and narrow. The river appears to be easily wadeable with
gradual changes in depth. The
depositional flat of the floodplain associated with bankfull stage
is much more distinct and spans a wide band along the edge of the
river. Small increases in stage above bankfull will
quickly inundate adjacent riparian ecosystems.
In essence, this plot shows the dimensions of the river as
one would perceive it standing along its banks.
It is almost always wise to view cross-sections at
a 1:1 ratio to gain an appreciation for how they actually look spatially. Most spreadsheet graphics allow users to compress
the vertical scale of the plot and to manually resize the vertical
scale. Some software programs, such as WinXSPRO, allow
users to change the plotting ratio so that the cross-section can
be viewed at 1:1 and other scales of vertical exaggeration. When rivers are wide, it may not be possible
to show cross-sections at a 1:1 ratio.
In these cases, consider using the lowest ratio feasible
and note on the figure or in the caption the amount of vertical
exaggeration used.
Cameras are another effective way to show what channel
cross-sections look like. Panoramic
view cameras, including inexpensive disposable versions, are now
widely available and are ideal for capturing the width of the entire
cross-section. Figure 5
is an example of a panoramic view of the South Fork Cache la Poudre
cross-section.
Figure 5. Panoramic view of
the South Fork of the Cache
la Poudre River cross-section.
By
using these simple techniques, we can better communicate the true
nature of rivers to our various audiences.
John Potyondy and Larry Schmidt are hydrologist
and Program Manager, respectively, with the Rocky Mountain Research
Station, Stream Systems Technology Center; (970) 498-1731; jpotyondy/rmrs@fs.fed.us;
lschmidt/rmrs@fs.fed.us
| 
