Final Project

Greys RIVER WATERSHED

The Greys River watershed is located in western Wyoming in the United States (fig. 1). It spans roughly 455 square miles. With its headwaters in the rugged Wyoming Range, the mainstem flows roughly 65 miles before its confluence with the Snake River near Alpine, WY. Shortly after the confluence, the Snake River is impounded to form the Palisades Reservoir.

Figure 1. Interactive location map of the Greys River watershed.

Figure 2. Geologic map of the Greys River watershed.

Geologic Setting

This geologic map provides a helpful view into the geologic setting of the watershed. Mudstone and shale a fairly dominant across the watershed, with limestone more common at the higher elevations (fig. 2).

Multiple landslides intersect the mainstem of the Greys River, which provide ample material for the river to work with.

Physiographic Setting

Figure 3a. The Middle Rockies level III ecoregion encompasses the entire Greys River watershed.

Figure 3b. The level IV ecoregions within the Greys River watershed and the drainage network.

Physiographic setting

Mapping the level III and IV ecoregions within the watershed is helpful for understanding its physiographic setting (figs. 3a, 3b). For example, reaches flowing through the alpine zone will receives large amounts of their flow from precipitation events and will be strongly influenced by snowpack. We can also observe that the ecoregion transitions to a high elevation valley just before the Greys River meets the Snake River, and that the mid-elevation sedimentary mountains ecoregion occupies the most area of the four types.

For more details on the level IV ecoregions in the Greys River, please explore the drop-down below:

MORE INFORMATION ON THIS WATERSHED's LEVEL IV ECOREGIONS

The following excerpts were sourced from the EPA / USGS Level IV Ecoregions of Wyoming map (Woods et al., 2001).

"The Sedimentary Subalpine Zone ecoregion is found southeast of Yellowstone National Park, in the overthrust belt, and in the northwest corner of the Bighorn Mountains in areas underlain by faulted and folded Mesozoic and Paleozoic sedimentary rocks (limestone, dolomite, shale, and sandstone). Elevations range between 8,500 feet and 10,000 feet. Relatively high precipitation amounts, areas of heavy snowpack, and snow with high water content provide enough moisture to support spruce-fir forest on the normally droughty sedimentary substrates. Potential natural vegetation includes subalpine fir and Engelmann spruce, with lodgepole pine as a seral species. However, tree growth is limited in fine-grained shale-derived soils which results in a landscape that alternates between forest groves and open grassy slopes."

"The Alpine Zone includes disjunct high elevation, glaciated mountainous areas above timberline across all of Ecoregion 17. The Alpine Zone is expressed physiographically as rockland, talus, tundra, and glacial lake (tarn) basins. Ecoregion 17h characteristically receives higher amounts of precipitation than surrounding mountains, as much as 50 to 70 inches annually. However, wind and snowdrift distribution are the limiting factors that influence upper tree line (defined as the elevational extent of upright trees). In alpine areas above 10,000 feet, krummholz vegetation clings to windswept areas above the mosaic of meadow and subalpine forest. Alpine tundra plants, including alpine forbs, sedges, and grasses, support nomadic groups of mountain bighorn sheep and mountain goats. "

"The Mid-Elevation Sedimentary Mountains ecoregion includes sections of the Teton and Wyoming ranges, and areas in the overthrust belt underlain by faulted and folded Mesozoic and Paleozoic sedimentary rocks (limestone, dolomite, shale, and sandstone). Stream water quality, water availability, and aquatic biota are affected by carbonate substrates that are soluble and nutrient rich. Soils are generally finer-textured than those found on granitic substrates such as those in Ecoregions 17c and 17k. Because of higher precipitation levels in Ecoregion 17g, the water-holding capacity of soil is higher than in the Dry Mid-Elevation Sedimentary Mountains (17m); as a result, some of the largest areas of Douglas-fir in Wyoming are found in this ecoregion."

"The High Elevation Valleys ecoregion includes Jackson Hole and Star Valley and to the north in the greater Yellowstone area, the Lamar, Hayden and Pelican valleys. These valleys include wet bottomlands, marshes, stream terraces, alluvial fans, and lower foothill slopes. Elevations range from 5,000 to 8,000 feet with an annual precipitation of 12 to 30 inches. Terraces and foothill slopes are covered with sagebrush (big, mountain, and some silver) with an understory of Idaho fescue and bearded wheatgrass. Douglas-fir with an understory of snowberry and pinegrass grows in mesic areas and on north-facing slopes. Some aspen groves also occur. Pasture, non-irrigated winter wheat, spring barley, hay, and livestock grazing are the most common land uses in developed areas. However, the cold climate and short growing season limits the extent of agriculture. In the Yellowstone region, elk and bison use the cold valleys as winter range. Elk still use Jackson Hole as a wintering ground, mostly within the confines of the National Elk Refuge."

To view the map, click here.

Vegetation

Trees are the dominant vegetation type in the Greys River watershed. There is also significant sparse, shrub, and herb cover. The vegetation cover is in agreement with the level VI ecoregions mapped for the watershed. Very little of the watershed is considered to be developed or agricultural. Produced using LANDFIRE dataset.

Climate

The Grey's River Watershed near Alpine, WY gets roughly is 22.6" of precipitation each year. On average, January receives the most precipitation and October receives the least. The average temperature of Alpine is 41.79 degrees Fahrenheit.

Flow Regime

The Little Greys River is the main tributary to the mainstem Greys River. The average flow is 817 cfs. Flows reach their highest (3,274 cfs) during spring snowmelt and their lowest (222 cfs) during the months of October to March (USGS, 2021).

RiverscapeS of INTEREST

I chose four riverscape reaches to explore: one confined, two partly confined, and one laterally unconfined (fig. 4).

Two of the reaches are part of the mainstem Greys River. One is on the Little Greys River and the other is on the South Fork of the Little Greys River.

Figure 4 (right). Location map of the riverscapes I will explore within the Greys River watershed.

A picture of a native Snake River fine-spotted cutthroat trout to break up my rambling

Site 1: Confined

Figure 6. Confined riverscape reach. Red arrow indicates location and direction of photo (fig. 8).

Figure 7. Conceptual cross-section of this riverscape in the Greys River watershed.

Figure 8. Looking downstream in the riverscape. See figure 6 for the location within the reach.

Characterization process

This reach is located on the mainstem of the Greys River. To characterize this reach, I first mapped the valley bottom (fig. 6) and the active channel to understand the valley setting. In this riverscape reach, >85% of the channel abuts the valley bottom margin, making it a confined valley setting. There is essentially no floodplain in this reach due to the fact that the banks rise up steeply from the sides of the active channel in many places. These steep slopes provide ample woody debris and bedload through natural slides that are closely coupled with the active channel. Next, I calculated the slope (2.4% - high) and the sinuosity (1 - straight).

In this reach, there is primarily one active channel, although there is one vegetated island that splits the flow temporarily (fig. 6). The river-left side of this reach has dense conifer forest with some intermittent riparian scrub at the active channel margin. The the primary bed material is large boulders and cobbles, which form a variety of structurally-forced geomorphic units including rapids and riffles.

My friend Dave catching his first fish on a fly rod.

Site 2: Partly Confined (Margin Controlled)

Figure 9. Conceptual cross-section of a partly confined margin-controlled riverscape in the Greys River watershed.

Figure 10. Partly confined margin-controlled riverscape reach.

Characterization Process

This reach is located on the mainstem of the Greys River. To characterize this reach, I first mapped the valley bottom (fig. 10) and the active channel to understand the valley setting. In this riverscape reach, 50-85% of the channel abuts the valley bottom margin, making it a partly confined margin-controlled valley setting. The valley bottom in this reach is really wide-- just under 800 m.

I chose this reach because it is one of the widest valley bottoms (if not the widest) in the catchment, and yet the channel is almost entirely pushed up against the margin. Aerial imagery tells us why-- people! Unlike the majority of the watershed, this area has infrastructure. There are at least four structures and their associated access roads, serviced by a bridge that crosses the Greys River at the south end of the reach. The fencing and corral indicate that this is likely a ranch and that the majority of this valley bottom is used for grazing. It seems plausible to me that this reach has been moved by the landowner to hug the valley bottom margin and provide the maximum area for grazing.

This reach is pretty boring in terms of GU's-- the channel has been pushed away from the western valley bottom margin and the pasture has no tree cover. On the other side of the channel, it hugs the gravel FS road. Between these two constraints, this reach is functionally prevented from recruiting material to work with. My guess is that as a result of this, there is very little structural forcing and this contributes to the homogeneity of GU's in this reach. It's basically one large glide/run.

I calculated the slope (0.8% - low) and the sinuosity (~1.1 - low sinuosity).

Another pretty picture to break up my rambling (not a picture of a selected reach)

Site 3: Partly Confined (Planform Controlled)

Figure 11. Partly confined riverscape reach on the Little Greys River. Areas where the active channel abuts the valley bottom margin are shown in red.

Figure 12. Location of areas of interest within the riverscape reach. Point bars, chute cutoffs, and diagonal bars are some of the prominent GUs in this section.

Figure 13. Landslide near the Little Greys River that has delivered a massive supply of material directly to the channel.

Characterization Process

This reach is located on the Little Greys River. To characterize this reach, I first mapped the valley bottom (fig. 11) and the active channel to understand the valley setting. In this riverscape reach, less than 50% of the channel abuts the valley bottom margin, making it a partly confined margin-controlled valley setting.

I chose this reach because it was interesting in terms of GUs that I could see from aerial imagery. For example, in figure 12c, I could see a chute cutoff and diagonal bars being formed. In 12B, a woody debris jam is forcing a mid-channel bar and large point bars are inside every bend. I calculated the slope (0.7% - low) and the sinuosity (1.3 - right on the border of low sinuosity and sinuous). I think this reach has more apparent bar forms because it has greater access to material. There are multiple places nearby where landslides intersect the valley bottom or are at least in close proximity (fig. 13). The terrain surrounding this section is extremely steep, and I'm sure large precipitation events, in addition to spring runoff, create pushy flows that help move and rework this material into the prominent bars I observed.

Site 4: Laterally Unconfined

Figure 14. Conceptual cross-section of the laterally unconfined riverscape reach.

Figure 15. Laterally unconfined riverscape reach on the South Fork of the Little Greys River.

Characterization PRocess

This reach is located on the South Fork of the Little Greys River. To characterize this reach, I first mapped the valley bottom (fig. 14) and the active channel to understand the valley setting. In this riverscape reach, <10% of the channel abuts the valley bottom margin, making it a laterally unconfined valley setting. Although I haven't seen this site in person, the vegetation in the aerial imagery, as well as the slope of the valley bottom and the sinuosity of the channel all point to the likelihood that this entire valley bottom is accessible and functional. Although the reach isn't recruiting sediment or woody debris from hillslopes, dense tree stands in nearby areas could served as sources for woody debris if beavers became active here. The slope of the reach is low (1%) and the sinuosity is high.

In this reach, there is a channel that carries the main flow. However, between extensive headcutting throughout the valley bottom and a planform that reflects past beaver dam activity, I would say that there are definitely multiple channels present. It's a bit messy and hard to quantify. The primary bed material appears to be sand and many sandy point bars are visible in the aerial imagery. Point bars appear to be the most common GU, although there are multiple plunge pools being forced by an abandoned beaver dams.

One more fish picture from this catchment... because why not?

Site Intercomparison / WRAPPING UP

Figure 16. Map showing location and elevation of selected riverscape reaches within the Greys River watershed.

The reaches that I chose to look at represent a variety of valley bottom confinement settings, upstream drainage areas, slopes, sediment supplies, and other characteristics that drive the geomorphic units and processes that I rambled about in this project.

The laterally unconfined reach on the South Fork of the Little Greys River at 2,220 m elevation was the highest up in the catchment and had the least upstream drainage area, but also had the greatest sinuosity. There are a lot of steep, straight, and confined mountain streams in this catchment, but as this reach shows, there are also areas high up in the catchment valley bottoms that are relatively steep and flat. The primary bed material was sand/silt.

After the South Fork meets the Little Greys, we come to one of the partly confined reaches at 1,970 m elevation. This reach has the second smallest upstream drainage area of the four reaches, was sinuous, and had a planform that suggests a high sediment supply. The primary bed material appears to gravel/cobbles.

The other partly confined reach, located on the mainstem Greys River at 2,010 m elevation, was unique in that it was the only reach that has obviously been altered by more than roadbuilding. This reach is homogenous in terms of GUs and occupies a relatively small area of its wide valley bottom. The lack of variety in GUs, which I believe is driven by its inability to recruit woody debris and other material, sets it apart from the other reaches. The primary bed material in this stretch it hard to determine, but I would guess gravel/cobbles with occasional boulders.

The confined reach, located furthest downstream among the selected reaches, was the steepest and straightest of them all. The large boulders dominated the bed and forced a lot of interesting geomorphic units. This section of the catchment comes just before the Greys River's setting transitions to a valley and meets the Snake River at the Palisades Reservoir. The steepness of the topography and the road in this section drives the planform, but GUs are determined by the large boulders and significant woody debris sourced from the adjoining hillslopes.

U.S. Geological Survey, 2021, National Water Information System data available on the World Wide Web (USGS Water Data for the Nation), accessed [April 29, 2021], at URL [http://waterdata.usgs.gov/nwis/].

Woods, A.J., Lammers, D.A., Bryce, S.A., Omernik, J.M., Denton, R.L., Domeier, M., and Comstock, J.A., 2001, Ecoregions of Utah (color poster with map, descriptive text, summary tables, and photographs): Reston, Virginia, U.S. Geological Survey (map scale 1:1,175,000).

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