Final Project

Introduction

Geographic Location

For my final project I chose to analyze three sites on the Greybull River in Wyoming (Figure 1). The Greybull river is a 90 mile long tributary to the Bighorn river which flows into the Yellowstone, then into the Missouri, then into the Mississippi and finally what little is left into the Gulf of Mexico. The Greybull is contained in the sub basin HUC 8 1004009 but my study sites are within the the watershed HUC 10 100900402 (Figure 2). This HUC 10 cuts off where the Wood river meets the Greybull near Dumbell Wyoming. The Greybull has a cold semi-arid with mean temperatures of 20 degrees celsius, summer temps of 30 celsius, and winter temperatures as low as -20 celsius.

Figure 1: Interactive map of the Greybull River

Figure 2: Left Shows the extents of the sub-basen and watershed of the Greybull river. Right shows my study sites in the upper Greybull river watershed.

Geological Setting and Ecoregions

The Absorkas where the headwaters of the Greybull river originate were formed by volcanic activity during the Eocene then carved and shaped by glaciers. The Absorkas are primarily made up by igneous and sedimentary rock and the peaks near the Greybull look like mounds of unconsolidated talus. The highest peak is Francs peak at 4,011 m. As the Greybull continues down stream it runs into the Bighorn basin.

The ecoregions of the Greybull is alpine near the headwaters. As the river continues it turns into subalpine then shrubland finally running into the bighorn basin.

Figure 3: Shows a map of the dominate vegetation type.

Vegetation:

The dominant vegetation type is dwarf shrubland which is consistent for the geographic are of this watershed. A map of the vegetation types can be seen in figure 3.

Catchment Metrics

I calculated the catchment metrics for the upper section of the Greybull due to the location of my sites. Using QGIS I calculated the following metrics.

Catchment area=1034.17 km
Mainstem length= 67.73 km
Catchment Relief= 2,124m
Relife Ratio=0.03
Drainage Pattern=Dendritic

Flow Regime

Sadly the only USGS stream guage on the Greybull is no longer in service. However I was able to find some flow data from 2017. The Greybull is a snowpack dominated river experiencing peak flows during the spring and summer then tapering off during the fall and winter.

Figure 4: Hydrograph of the Greybull at Meeteetse Wyoming. Measurements are in cfs.

Figure 5: Longitudinal profile of the upper section of the Greybull.

Study Sites

Reach 1:Confined, bedrock margin-controlled, gorge, boulder bed.

Valley bottom length: 487m

Channel length: 507m

Sinuosity: 1.04

Average riverscape width: 7m

Confinement:100%

Constriction 100%

Figure 6: An aerial view of reach 1. Yellow denotes the valley bottom, blue the channel.

Reach one is dominated by cascades and is constricted on both sides by bedrock. The substrate seems to be large boulders. This reach is in the headwaters of the Graybull. From the imagery this section looks to be rather high gradient. The steep gradient mixed with the large substrate size could explain why there is not much complexity in this reach. I doubt there is much vertical or lateral migration of this reach.

Figure 7: A cross section of reach 1.

I took a cross section of reach 1 in google earth pro and found that the channel is rather shallow. This could be caused by poor resolution or because it is near the headwaters of the system.

Reach 2: Partially confined, bedrock margin controlled, braided, cobble bed.

Valley bottom length: 1534m

Channel length: 1637m

Sinuosity: 1.06

Average riverscape width: 351m

Confinement:50-85%

Constriction 0%

Braiding Index: 3-9

Figure 8: A aerial view of reach 2. Yellow denotes the valley bottom, blue the channel, and red is where the channel is being confined.

Reach 2 was hard to read without the ability to go to the site. It is a wide braid plane with as many as 9 or more channels at one cross section. This reach adjusts laterally carving new channels. This site is also found near the headwaters of the Greybull and similar braid plains can be found in this area. While the reach is a braid plain it is still partally confined or fully confined. The left side of this reach could have a channel meaning this reach could very well be 100 percent confined.

The instream geomorphic units in this section are mid channel bars which is characteristic of braid planes. The lack of vegetation growing on these mid channel bars suggests that they are turned over frequently. From aerial imagery I would assume that the substrate size is cobbles or even gravel.

Figure 9: A cross section of reach 2.

I used google earth pro to create a cross section of reach two. Surprisingly the channels were not denoted very well which could mean they are shallow or it could be caused by the resolution of the DEM.

Reach 3: Partially confined, bedrock planform controlled, anastomosing, cobble bed.

Valley bottom length: 877m

Channel length: 981m

Sinuosity: 1.12

Average riverscape width: 122m

Confinement:10-50%

Constriction 0%

Figure 10: A aerial view of reach 3. Yellow denotes the valley bottom, blue the channel, and red is where the channel is being confined.

Reach 3 is lower in the water shed near the town of Dumbell Wyoming. As you can see in the aerial imagery this area has been developed for agriculture. Howerever, this reach is one of the least confined sections of the Greybull and the next 30 or so miles of the Greybull closely resemble this reach. The river seems like it would adjust latterally in this section.

The instream geomorphic units for this section are very complex with anastomosing channels, diagonal bars, point bars, riffles, and pools. This section bed seems to be made up of gravel or small cobble. The diagnal bars are very easily seen in the middle of the channel with two offset riffle crests on either side. Generally this section follows a riffle-pool-riffle sequence.

Figure 11: Cross section of reach 3.

The cross section for reach three shows how steep the banks on the north side of the channel are. Additionally, you can see the depth is consistent with the planar riffle feature in both channels.

Reach Intercomparison

The upper Greybull (within HUC 100900402) in general is a partially confined riverscape. The headwaters of this system are partially confined to confined with some sections of multithreaded channel. Generally the headwaters of the Greybull has one to three channels until it reaches a confined bedrock controlled canyon. As the Greybull continues it reaches sections of wide braid plain connected by canyon or even sections of constricted gorge. The middle section of the Greybull aslo runs through sections of canyon then turns into partially confined ,bedrock, margin controlled, multi threaded or anastomosing channel. The bottom section of the Greybull runs through farmland and its hard to tell if it is naturally partially confined or if it is due to anthropogenic causes.

Through the Greybull there is limited connectivity to its floodplain. The only places with floodplain connectivity are near the tailwaters of the watershed and even then the floodplain is discontinuous.

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