Asotin Watershed, WA

26 Mar. 2021

1. North Fork Asotin Creek

The Asotin Watershed is located in southeastern Washington State and is part of an Intensively Monitored Watershed (IMW), meaning the whole watershed is researched and monitored on a consistent basis. This gives us some great data, including detailed surveys at two sites where we will take a closer look at instream geomorphic units. Survey sites are marked in Fig. 1., and are both located along the North Fork Asotin Creek. More information on the Asotin IMW is available at joewheaton.org, through ArcGIS, and in this helpful website published by Washington State Recreation and Conservation Office (Fig. 2).

2. North Fork Asotin Creek F4

2.1 In-Channel Geomorphology

The first reach is a straighter section of channel described as a "wandering gravel bed" site. The majority of the site is a glide-run with intermittent riffles, pocket pools, and several margin attached bars. These elements are illustrated in the geomorphic units from the GUT Analysis for the site, shown in Fig. 3 for the F4 Reach from 2016.

2.2 In-Channel Geomorphology: 2016 & 2017

I took a look at two survey years of this site, shown in Fig. 3, from 2016 and 2017. It's easy to see how the instream geomorphology changed over time, even within the short period of a year. The reason I chose to look at 2016 and 2017 was because I was interested in how the stream had changed more recently, so I can be more informed of recent events if I ever get a chance to explore Asotin Creek in coming years.

In-stream geomorphic units present in both years include glide-runs that extend along the entirety of the channel, as well as pools and bar development. The survey from 2017 has a large riffle, and several bars where an upstream riffle from 2016 was broken up. The geomorphology from the 2016 survey appears to be more bank-attached, while the 2017 survey shows more mid-channel or channel spanning units, such as riffles and the emergence of some small mid-channel bars.

2.3 Tier 2 Forms & Difference

Tier 2 from the GUT Analysis shows the unit forms in the survey area. These forms are based on the shape of the river bed as a plane, a concavity, or a convexity. Fig. 4 shows the Tier 2 forms broken down into more distinct unit forms in the channel between 2016 and 2017.

In 2016, the channel has a large trough-shaped "bowl transition" through most of the survey area. There are a few convexities, which are located only along the edges of the channel (bank-attached) in the form of mounds, mound transitions, and saddles. In 2017, the unit forms become more pronounced. The bowl transition and trough sections that made up most of the survey from 2016 are interspersed with more pronounced bowls. In 2017, the convexities begin to take over more of the channel, including two saddle/mound forms that extend across the entire channel (Fig. 4b). The main unit forms that stay the same between 2016 and 2017 are the mounds located at the banks of the channel. There are very few planar unit forms in either survey, however, the survey from 2017 shows more planar forms between transitions a transition from a trough to a saddle.

2.4 Do any of the Tier 2 Forms dominate the assemblage or is it fairly mixed? Is this true through time?

The dominating Tier 2 forms in 2016 and 2017 include bowl transitions and troughs. This makes sense because the overall channel shape is concave. It does appear that over time, the geomorphic unit forms are in flux and the most change is seen in convex units. The amount and shape of saddles seems to be the biggest difference over time. Interestingly, the major saddle seen in the 2017 survey (Fig. 4b) appears to be historically found in the 2013 survey, only slightly downstream in the same section of the channel.

2.5 Tier 3: In Channel Geomorphic Units

In-stream geomorphic units present in 2016 and 2017 include glide-runs that extend along the entire length of the channel, where the Tier 2 concave forms create the perfect conditions for this (Fig. 3). Margin-attached bars also appear to be somewhat consistent between the 2016 and 2017 surveys. In 2017, there are two riffles and several bars and transition zones where the upstream riffle from 2016 was broken up. The orientation of geomorphic units between 2016 and 2017 is very different; in 2016 the units are primarily diagonally oriented in the channel (intersecting the channel at an angle), while in 2017 the orientation of geomorphic units is overwhelmingly longitudinal (along the length of the channel). This is evidenced somewhat in the fact that geomorphology from the 2016 survey appears to be more bank-attached, while the 2017 survey shows more mid-channel units, such as the riffle and some small mid-channel bars beginning to form that direct water longitudinally through the channel.

3. North Fork Asotin Creek F6

3.1 In-Channel Geomorphology

This second reach is about 3.5 km downstream of the F4 reach. It is also considered a "wandering gravel bed river" in good condition. This section of the river has a small bend in it, and is characterized by a variety of mid channel and margin attached bars. The main flow follows a glide-run path meandering between these accretions, as shown in the GUT Analysis in Fig. 9.

3.2 In-Channel Geomorphology: 2016 & 2017

The two survey years I chose to look at are shown in Fig. 9, from 2016 and 2017. The reason I chose to look at 2016 and 2017 was because I was interested in how the stream had changed more recently, so I can be more informed of recent events if I ever get a chance to explore Asotin Creek in coming years. I also wanted to be consistent with the study I did in the F4 section of the river, explored above. It's easy to see how the instream geomorphology changed over time, even within the short period of a year.

In-stream geomorphic units present in both years include mid channel and margin attached bars that the stream glides around through the entirety of the channel. This section of the river has a nicely sized island, as well. The survey from 2016 shows more margin attached bars that in 2017 show up as mid channel bars, where they have become disconnected from the banks. Similar to the survey from the F4 site on Asotin Creek, the geomorphology from the 2016 survey appears to be more bank-attached, while the 2017 survey shows more mid-channel units, including the emergence of several large mid-channel bars.

3.3 Tier 2 Forms & Difference

Tier 2 from the GUT Analysis shows the unit forms in the survey area. These forms are based on the shape of the river bed as a plane, a concavity, or a convexity. Fig. 10 shows the Tier 2 forms broken down into more distinct unit forms in the channel between 2016 and 2017.

In 2016, the channel has more planar features, however, both years show a considerable amount of mound formations and large trough-shaped "bowl transition" forms through most of the survey area. In 2017, there is a noticeable saddle feature that creates a riffle (Figs. 10b, 9b). The main unit forms that stay the same between 2016 and 2017 are the mounds located throughout the channel.

3.4 Do any of the Tier 2 Forms dominate the assemblage or is it fairly mixed? Is this true through time?

The dominating Tier 2 forms in both 2016 and 2017 are mounds and bowl transitions. The bowl transition forms make sense because the overall channel shape is concave. Over time, the geomorphic unit forms are in flux, with the most change seen in convex units including mounds, mound transitions, and saddles. Both surveys show diversity in the types of unit forms, and the largest difference is in placement and shape of the forms. The sum of the areas of each form seems to be relatively stable between these two years.

3.5 Tier 3: In Channel Geomorphic Units

In-stream geomorphic units present in 2016 and 2017 include glide-runs that extend along most of the channel length, through the Tier 2 concave forms of troughs and bowls (Fig. 10). In 2016, there is a significant amount of margin-attached bars, which by 2017 become mid-channel bars through channel avulsions and natural meandering. There is also a large riffle present in 2017 which was a shallow pool surrounded by margin-attached bars only the year before. By 2017, there are not as many pools along this river reach as there were in 2016. Overall, there are many different types of tier 3 geomorphic units present in both years, indicating that the geomorphology at this site changes more rapidly and drastically than the previous F4 Asotin Basin site.

4. Differences Between F4 & F6

4. 1: What are the primary differences from exploring GUT between these two sites that you noticed between the in channel geomorphic units of these two sites?

Between the North Fork Asotin Creek sites F4 and F6, there were several differences including channel shape and geomorphic unit placement. Site F4 is a relatively straight gravel bed, with tier 2 trough and concave units through the center of the channel indicating a long, homogeneous trough through the whole site. Site F6, on the other hand, had several different geomorphic units throughout, creating more mixed forms of geomorphology through the reach. Site F6 also had larger, more prevalent bars (both margin-attached and in-channel) than site F4, where bars were thin and generally margin-attached.

4. 2: What inferences can you make about geomorphic processes and behavior when contrasting GUT outputs between these two sites?

One inference about geomorphic processes that stands out to me from the comparison between Asotin Creek sites F4 and F6 over the course of just one year is the speed at which these processes take place. Particularly, the change from a shallow pocket pool to a riffle in site F6 is quite interesting to me. This is a change entirely from a concavity to a convexity, likely due to surrounding sediment filling it in as a riffle. But the quick behavior of the river changing is quite interesting, in both site F4 and F6.

4.3: If you only had one GUT output from each of these two sites (i.e. one snap shot) how representative would be your inferences about geomorphic processes? If you did not have the luxury of six or seven surveys, but just one, would your conclusions above be different?

I believe my inferences wouldn't be quite as representative of the geomorphic processes given only one GUT output. Given the major changes that happen over the course of only a year, it would be hard to determine what might be the past or future of the site. In site F6, there are several margin-attached bars in 2016, while these same bars seemingly dissipate or become in-channel bars by 2017. For more complex sites such as F6, it would be very difficult to determine geomorphic processes; I'm sure my conclusions would have been that bar formation might have continued or simply shifted the following year, rather than decreasing. The F4 site, on the other hand, might have been a little easier to analyze given only one snapshot of GUT output. This is due to the relative simplicity of the F4 site, and also from my observations that this site does not change as dramatically as the F6 site, and many of the tier 2 forms are intact between the two years.

5. Synthesis

5. 1: How are the Tier 2 Forms from GUT different than what we discussed in field?

The Tier 2 forms from GUT differ from what we discussed in the field as they consider transitions into and out of concave, convex, and planar units. In the field, we mainly considered the units themselves, rather than covering the transitions between each of them.

5. 2: How are the Tier 3 GUs GUT exports different then the ones we discussed? Why do you think GUT does not output the same things?

Tier 3 geomorphic units seem to be less specific than those our class discussed in the field. GUT combines some geomorphic units such as glides and runs into the same unit, a "glide-run." Geomorphic units we discussed in the field also had more specific

5.3: How would you apply Tier 4? Do you have enough information here to do that?

Tier 4 (I'm assuming this is Tier 3b, where geomorphic units are sub-classified) can be applied by looking at Tier 3 unit positions and orientations. From the GUT Analysis, I think there is enough information to sub-classify some of the units, but not all of them. For example, it may be difficult to identify the low flow relative roughness of the region based on survey data alone. Also, many of the Tier 4 units are classified by geomorphic unit position and water slope, which may be "varied" based on Table 1 from the field trip.

5.4: Does exploring these GUT outputs give you more or less confidence in applying the fluvial taxonomy through manual mapping off of topography versus identification in the field?

I think it gives me more confidence in fluvial taxonomy through manual mapping than I went into the exercise with. I still really think that being in the field to identify topography is the best way to identify geomorphic units. The mapping, however, is an adequate substitute that I think is really helpful for quick analysis and may be tweaked or verified by field identification to become powerful tools for fluvial taxonomy.