Catchment-Scale Controls on the Logan River Watershed

Project Overview

Create a longitudinal profile for the Logan River and evaluate base-level controls through time
Calculate various morphometric values for the Logan River watershed using watershed boundaries and perennial stream flow paths
Determine stream order for multiple Logan River tributaries

Exercise instructions and data download can be found here. Data was sourced from the Riverscapes Consortium Data Exchange and the Utah Geological Survey.

Map of the Logan River Watershed

Map of the Logan River watershed. The watershed boundary is white; the Logan River and tributaries are marked in blue.

Longitudinal Profile

Figure 1 - Longitudinal profile along the main stem of the Logan River with knickpoints and current base-level indicated.

Figure 2 - Active faults within the Logan River Watershed. The Logan River main stem is orange; the fault lines are green; knickpoint locations are yellow; the watershed boundary is white. Fault line data was acquired for the Logan Quadrangle from geologic maps created by the Utah Geological Survey.

Knickpoints

As shown in Figure 1, the Logan River contains two knickpoints. Knickpoint 1 coincides with the location of a dam, known locally as Second Dam. Thus, Knickpoint 1 may be the result of rapid elevation change between the artificially raised water surface behind the dam and the water surface at the outlet of the dam.

Knickpoint 2 is located closer to the headwaters of the Logan River and close to where an active normal fault crosses the Logan River floodplain (see Figure 2). It is possible that fault activity has caused a knickpoint at this part of the river, though other locations along the Logan River that also coincide with active normal faults do not show the formation of a knickpoint. This suggests that, while the fault may have played a role in the formation of this knickpoint, other factors likely drove its creation.

Update: We discussed in class that the composition of bedrock changes at the location of this knickpoint. The abrupt change in underlying rock type leads to different rates of erosion on either side of the knickpoint.

Question: Is the fault at this location the reason the bedrock type changes, or is some other process responsible for the change in bedrock composition?

Figure 3 - Diagram of values used to calculate concavity of the Logan River main stem taken from its longitudinal profile. (Note: for ease of understanding, the longitudinal profile in this diagram has high vertical exaggeration.)

Concavity

The concavity of the Logan River was calculated using the following equation:

where C = concavity, A = the distance between the longitudinal profile and half the height of a right triangle created using the highest and lowest elevations of the longitudinal profile, and H = the highest elevation of the longitudinal profile (see Figure 3).

This value indicates that, in general, the longitudinal profile of the Logan River is moderately concave up.

Base-Level Controls

The slope at the lowest elevation of the Logan River is consistent for close to 20 km upstream, suggesting that the base-level control of the Logan River is close to this location and around 1,342 m elevation (see Figure 1). At its lowest elevation, Logan River has a confluence with the Bear River, which can be seen by zooming out of the Logan Watershed boundary in the map above. This indicates that the Bear River is the base-level control for the Logan River today.

Around 25,000 years ago, most of Utah was covered in an ancient inland lake named Lake Bonneville, depicted at its largest extent in the diagram to the right. The lake level of Lake Bonneville remained constant until the Bonneville Flood through Red Rock pass, which took place around 15,000 years ago (Utah History Encyclopedia). This flood led to a drastic reduction in lake level, creating distinct terraces at different shoreline elevations of Lake Bonneville that are still visible today.

The base-level of the Logan River 18,000 years ago (before the Bonneville Flood) would have been at the elevation of the Bonneville Shoreline terrace, the terrace corresponding to the largest extent of Lake Bonneville. The Bonneville Shoreline terrace elevation is 1,550 m (Utah History Encyclopedia), more than 200 m higher than current base-level elevation for the Logan River.

Depiction of Lake Bonneville at its greatest extent, courtesy of Utah Geological Survey.

Catchment Morphometrics

Figure 4 - Catchment length of the Logan River Watershed. White represents the boundary of the Logan River Watershed; the red rectangle represents the minimum rectangular bound that fully contains the Logan River watershed; yellow represents the catchment length used for calculations of catchment morphometrics.

Catchment Length

In some watersheds, the determination of catchment length is simple and straightforward. The Logan Watershed is not one of these watersheds. The topography of the Bear River Mountains surrounding the Logan River creates an inconsistently curved, elongate drainage area.

To attempt to incorporate the shape of the Logan River Watershed into the measurement of catchment length, I created a rectangular boundary around the Logan watershed representing the minimum rectangular boundary that could fully contain the Logan River Watershed (see Figure 4). The catchment length was then defined as the longest length that could be drawn between any two points of intersection between the watershed boundary and the minimum rectangular bound. This protocol yielded a catchment length of 50.91 km (Table 1). I don't know if this protocol would ever be used in a study, but it seemed to produce reasonable values for catchment morphometrics calculations. :)

Other Characteristics

The following equations were used to find the values listed in Table 1:

The drainage pattern of the Logan River appears to be dendritic, even though the drainage density of perennial streams suggests that it is a relatively young system. Incorporating both ephemeral and perennial streams into the drainage density calculation would likely yield a higher value for drainage density in the Logan River Watershed.

Table 1 - Various catchment morphometrics for the Logan River Watershed.

Stream Order

Figure 5 - Strahler stream order for the Logan River Watershed with consideration of unnamed perennial streams.

Figure 6 - Strahler stream order for the Logan River Watershed without consideration of unnamed perennial streams.

I decided to take two approaches using Strahler stream order to characterize the tributaries of the Logan River Watershed. The first approach, shown in Figure 5, incorporates all known perennial streams in the Logan River Watershed. This approach resulted in an overall stream order of 4 for the Logan River, 3 for Temple Fork, and 1 for Beaver Creek.

For the second approach, I excluded all perennial tributaries that were not named in the Logan River Watershed (see Figure 6). This approach resulted in an overall stream order of 3 for the Logan River, 2 for Temple Fork, and 1 for Beaver Creek.

The differences in these approaches demonstrates the importance of defining what tributaries are considered when determining stream order. Although the stream orders for headwater tributaries, like Beaver Creek, remained consistent through both approaches, the stream order of the Logan River was affected.

Hortonian Laws of Stream Network Composition

The Hortonian Laws of Stream Network Composition are listed below:

As stream order increases, the number of streams decrease (Law of Stream Numbers).
As stream order increases, stream length also increases (Law of Stream Length).
As stream order increases, catchment area also increases (Law of Catchment Area).

The Logan Watershed seems to follow the Law of Stream Numbers and Law of Catchment Area, but has some exceptions to the Law of Stream Length. Beaver Creek, a 1st order stream in both Figure 5 and Figure 6, is much longer than higher-order streams such as Temple Fork and Right Hand Fork. Other than this exception, the Logan River Watershed does seem to follow the Law of Stream Length in general.

Questions

The data was a little wonky in places. I'd like to know how to find out more about this data set and how the metrics in the .json file were calculated through the Riverscapes data exchange map viewer.
The coordinate system used for the calculations in this project was WGS 1984. Would UTM 12N be a more appropriate coordinate system to use for these measurements? Why or why not?
How do I create a point at an intersections between two line layers in ArcGis Pro or in QGIS? (Wanted to see intersection of faultlines with Logan river mainstem more precisely, rather than just eyeballing.)
How would I extract the elevation from an exact distance on an elevation profile?
I am confused about the form relief ratio calculation. In the .json file, it seemed to be calculated backwards from the book (as you'll see in my equations above). Does it matter which one you use as long as you define what is meant by the ratio somewhere?
What number of significant figures are appropriate when using areas/lengths calculated by mapping software?
I wasn't quite sure how to classify the Little Logan River in terms of stream order. Since it branches off the Logan River, which is 3rd order at that point in Figure 5, does that mean the Little Logan River is 3rd order? Or, since the Little Logan River re-joins the Logan River downstream, should it just be considered an extension of the Logan River (ie, NOT a tributary)? I am leaning towards the latter, but I'd love to discuss this idea further.
The Law of Catchment Area seemed a little vague to me. What catchment area is it referring to? Does it mean that a 5th order tributary will have a larger catchment area than a 3rd order tributary? Does it refer to how much of the given catchment area is upstream of a tributary of interest?

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