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Virgin River Watershed, UT
14 April, 2021
I love the Virgin River- surrounded by steep, red cliffs the river is a beautiful respite from the surrounding desert. The Virgin River is 13 million years old and was named by the Spanish for the Virgin Mary (originally la Rio de la Virgen). For my final project, I chose to take a closer look at the Lower Basin watershed and la Rio de la Virgen through my personal experience with the river from Zion's Narrows through small-town Utah, and into the growing metropolis of St. George.
Introduction
The Virgin River is home to some of the most scenic red-rock and water features of Utah, Arizona, and Nevada (Fig. 1). It is part of the Lower Basin Watershed region, and has its own drainage basin of approximately 12,250 sq mi (31,700 sq km) [1], shown in green in Figs. 2, 3. The Virgin River begins at the confluence of the North and East Fork of the Virgin River in Zion Canyon and eventually joins the Colorado River as it drains into Lake Mead (Nevada).
Climate, Flow & Geologic Setting
Located in a desert climate, the Virgin River sees low flows (~70 cfs/ 2 cms) in the summer and peak flows (~330 cfs/ 9.3 cms) in the spring. The average flow through the Virgin River in St. George is about 180 cfs (5.2 cms) [2].
The Virgin River cuts through some fascinating geology, from red walls of Navajo Sandstone to black fields of more recent lava flows. Figs. 4 shows some of the main geologic formations exposed by the incising (cutting) of the Virgin River through Zion Canyon and beyond to Lake Mead.
Most of the rock the Virgin river cuts through is sedimentary, although it also flows through the currently active Santa Clara volcanic field (last recorded eruption <10,000 years ago). The Utah Geologic Survey produced a fascinating interactive map for further exploration of geology along the Virgin and throughout Utah.
Fig. 4. Sedimentary rock layers the Virgin River cuts through are found throughout Zion National Park. At the top of these formations there is sometimes a deposit of newer volcanic rock [3].
Fig. 5. Level IV EPA Ecoregions encompassing the Virgin River Watershed include the Mojave Basin & Range and the Colorado Plateau.
Fig. 1. The beautiful Virgin River runs through Zion National Park and scenic Southern Utah.
Figs. 2, 3. The Virgin River watershed (shown here) is part of the greater Lower Basin Watershed, where it joins the Colorado River. The Upper Virgin Catchment (green) contains the study sites examined on the Virgin River.
Physiographic Setting
The Virgin River is situated in the Level III Mojave Basin and Range region, as defined by the EPA (see Fig. 5 for Level IV classification). Although the Upper Virgin River Basin includes some eco-regions that may be considered part of the Wasatch and Uinta Mountains, the mainstem of the river runs primarily through the Colorado Plateaus and Mojave Basin and Range.
This region is characterized by warm deserts with lower mountains than the Central Basin and Range to the North. Vegetation is sparse, and little to no water exists outside drainage areas.
Vegetation
Vegetation along the Virgin River is native to the desert, and plants commonly found along the banks of the river include coyote willow (salix exigua), indian ricegrass (oryzopsis hymenoides), and cottonwood trees (populus fremontii). Invasive salt cedar/tamarisk (tamarix spp) are also found along the Virgin River, and scientists and engineers throughout the southwest are constantly working to remove tamarisk and replant native species.
As the Virgin enters Washington and St. George, the landscape becomes primarily developed and agricultural land. Problem salt cedar has taken over much of the riverbed, and they are a constant source of frustration at local golf courses and parks.
Outside St. George, the Virgin River enters a canyon and the floodplain is once again sparsely vegetated with native and invasive desert species all the way to Lake Mead.
Catchment Context
The total Catchment for the Virgin River and its tributaries (Fig. 2, Fig. 5) is roughly 3,785,341 acres (15,325 sq km), a little more than the size of Connecticut. The shape of the catchment is circular, with an elongatoin ratio of 0.82, where 1.0 would be a circular area. Within the catchment, the Virgin River begins at the confluence of the North Fork and East Fork, roughly halfway through the Upper Virgin Catchment. Notable tributaries that join the Virgin include La Verkin Creek and the Santa Clara River (Fig. 6)
The site visits conducted for this project are all located in the Upper Virgin Catchment. This catchment is also more circular, covering 1,397,207 acres (5,654 sq km; Er = 0.83), or a little less than the state of Delaware.
Study Reaches
The reaches I chose to study show varying geologic and confinement settings of the Virgin River. The reaches and are all located in the Upper Virgin Catchment (Fig. 2, 5), and are places I am personally familiar with. A map of site locations within the context of the Virgin River Drainage Basin is shown in Fig. 6.
Site 1: The Narrows, Zion National Park
The first reach is a famous area of high confinement along the Virgin River, located in Zion's Narrows. The Narrows are located along the North Fork tributary to the Virgin River, and are a popular hiking destination for many visitors to the National Park.
Site 2: Confluence Park, La Verkin, UT
The second study reach is located as the Virgin River drops out of Zion Canyon and into the volcanic deposit areas between Hurricane and La Verkin, UT. In this area the river is margin confined by steep basalt cliffs, with some room to meander as it approaches Confluence Park. Fig. 7 shows a shot of this area from a field study conducted in July 2020/Feb. 2021 on geomorphic units.
Site 3: Santa Clara Confluence, St. George, UT
The third study reach is located near downtown St. George, Utah. It's difficult to say today whether any areas of the Virgin can still be considered fully laterally unconfined, especially due to anthropogenic influences along the river. In St. George, the Virgin River seems laterally unconfined within a small valley bottom that has been carefully engineered. The floodplain is controlled by the City, which strategically places golf courses and parks in areas that are likely to flood.
Fig. 6. Study sites along the Virgin River in southern Utah. Site 1: The Narrows, Zion National Park. Site 2: Confluence Park, La Verkin, UT. Site 3: Santa Clara Confluence, St. George, UT.
Fig. 7. The Virgin River at Confluence Park, La Verkin, UT taken during field observations in Feb. 2021
Site 1: The Narrows, Zion National Park
Reach Description
Zion's famous Narrows (Fig. 8) are a popular hiking destination. This reach of the Virgin River is highly margin confined, meaning that the river has essentially no room to meander between the surrounding landforms.
Channel/Floodplain - In the highly confined Narrows, the Virgin River channel and floodplain are essentially one and the same. During low flows, point bars or margin-attached bars may be found along the channel, and these comprise the floodplain when there is one (see Fig. 8). This makes hiking in the Narrows, like any slot canyon, dangerous during flood events. Because there is no room for water to flood laterally, the Virgin River barrels down the Narrows, moving sediment and debris at high velocities.
Geology/Sediment - The Narrows are cut into an expansive stretch of Navajo Sandstone (see Fig. 10), which is known for its strength (hence not crumbling as the river cuts into it) and beautiful red/orange color. Sediment in this reach of the Virgin River is fine sand mixed with cobbles and boulders. Fig. 8 shows how these sediments are found along the bed as well as suspended in the river. The fine sediment is moved easily through the canyon, while boulders and cobbles are moved only during higher flood events.
Geomorphic Units - Geomorphic Units are sparse in this reach of the channel. When the river runs at low flows, point bars and margin-attached bars are evident at the sides of the canyon walls. These are made up from the sand and cobbles/boulders found in the channel and throughout the canyon. Because this area is highly confined, there are no geomorphic units found in the floodplain.
Vegetation - Vegetation in this reach is sparse, as seen in Fig. 8. The trees and plants that manage to grow in a dry, shaded environment are hardy, but never grow to be very large. Vegetation grows mainly in the canyon walls, with hardly any vegetation present in the channel or on bar formations.
Flows - Flows in this reach are low during the summer and fall, hence the popularity of hiking the Narrow. Flash floods, usually caused by summer rain storms or more extreme floods in the spring are the cause of most of the sediment transport in the Narrows. Because there is no valley margin for floods to inundate, the shape and size of the channel generally stay constant. Although the movement of larger sediment may have an effect on geomorphic units, the few bars present rarely see major changes.
Fig. 8. The Narrows are a popular hike in Zion National Park. This photo shows how confined this reach of the Virgin River is, as well as the sand and boulders that comprise the sediment in the canyon, which create bars that make up the floodplain.
Fig. 9. The Narrows, Zion National Park, are a highly confined section of the Virgin River. Confinement along the reach is shown in pink, with the channel/floodplain shown in blue.
Fig. 10. The Utah Interactive Geologic Map shows that most of the areas of high confinement in the upstream portion of the North Fork Virgin River are found in the strong Navajo Sandstone formation, shown here in gray-green.
Site 2: Confluence Park, La Verkin, UT
Fig. 11. The Virgin River at Confluence Park is partially confined both marginally and anthropogenically. Margin confinement of tall, basalt cliffs is shown in pink, while partial anthropogenic confinement (dirt roads that may be washed out) are shown in yellow.
Reach Description
Confluence Park, adjacent to the small towns of Hurricane and La Verkin, Utah, is a great place to cool down in the summer. This section of the river may be considered partially unconfined, due to the way the channel meanders between steep, basalt walls.
As an intern, I performed an ordinary high water mark delineation in this area (July 2020) and an earlier post gives in-depth descriptions of geomorphic units within the channel (Feb. 2021).
Channel/Floodplain - This partially confined section of the Virgin River is free to roam between tall, basalt cliffs. This is called margin confinement, because the river is forced by the margins of the valley bottom. Fig. 11 shows the areas where the Virgin River is margin confined in bright pink, and the valley bottom margins are shown in blue. Yellow lines indicate areas of partial anthropogenic confinement, or dirt roads that may be washed out in a flood event, but nonetheless may have in impact on the flow of the river.
Geology/Sediment - This section of the Virgin River is by far the most geologically diverse of the three reaches. Fig. 12 shows a snapshot of the different geologic formations exposed as the Virgin drops out of the canyon and into the towns of Hurricane and La Verkin. Within this specific study reach, the main exposed deposit is a basalt volcanic flow, shown in purple in Fig. 12. The river itself runs through old river and stream deposits, with a lighter suspended sediment load. The channel bottom is medium sand with cobbles and some boulders, shown in site sketches in Figs. 13 and 14.
Fig. 12. The Virgin River near Confluence Park, La Verkin, UT (Site 2). This snapshot from the Utah Interactive Geologic Map shows the Virgin River coming out of Zion Canyon and into the towns of Hurricane & La Verkin. The river flows through the Moenkopi (dark green) and Kaibab (Teal) sedimentary formations, enters a section of Volcanic Flows (purple), and continues through Navajo Sandstone (light grey/green).
Fig. 13. Hand sketch of study reach, with hydraulic features and cross section A-A' marked
Reach Sketch & Cross Section
This reach extends along a dirt road with a steep cliff marking the leftmost boundary of the river, and a sandy beach marking the right side. Several boulders fill the channel, and a USGS Stream Gage has been installed on the left of the channel, shown in a field map sketch and cross-sectional view of the channel (Figs. 13, 14).
Fig. 14. Cross section at A-A' including relative flow depth and OHWM from previous delineation
Geomorphic Units - This site has several in-stream geomorphic units, including a notable hydraulic jump (Fig. 15) located just downstream of a deep glide. These units are further explored in a previous post on the Virgin River at Confluence Park and the video in Fig. 15. Outside of the channel, the floodplain is a flat, medium sand with vegetation up to the basalt cliffs of the valley bottom.
Vegetation - This reach has much more vegetation in the floodplain and up near the banks of the channel. The same hardy desert plants are present: large cottonwood trees are located in the floodplain north of the river, and smaller willows and Russian olive trees are also present on either side of the channel. Other vegetation present includes desert grasses and small shrubs.
Flows - Thanks to a USGS Stream Gage located right in this reach, there is plentiful information on the flow regime of the Virgin River for this area. The mean flow for the past 14 years is roughly 93.8 cfs (2.6 cms). Flow velocity is much higher near the center (thalweg), which is located along the basalt cliff. Flow is convergent at the cascade (hydraulic jump), shown in Fig. 15. Upstream of the USGS gage, boulders create a more turbulent flow field, while downstream of the hydraulic jump flow becomes uniform and there is little turbulence.
Fig. 15. Geomorphic units and notable features at the Virgin River at Confluence Park.
Site 3: Santa Clara Confluence, St. George, UT
Reach Description
The Santa Clara River is one of the main tributaries to the Virgin, and they meet just south of downtown St. George, Utah. Although this section of the river is highly confined by anthropogenic influences, within the little valley margin that is left for it, the river is free to be "laterally unconfined". Fig. 16 shows the valley bottom margins of the river and the alluvial fans of incoming tributaries: Fort Pearce Wash (joining from the east) and the Santa Clara River (west).
Channel/Floodplain - The channel is unconfined within the valley bottom margins bordered by encroaching development. One way humans have made their mark on the floodplain is by developing parks and golf courses on the alluvial fans and other areas that frequently flood. In general, this protects homes and other property along the river from becoming inundated by floods. This is not always successful, however, as severe flooding in 2005 proved (see Fig. 17).
Geology/Sediment - The sediment in this reach is coarse sand with gravel and some cobbles. Fig. 18 shows that the Virgin River runs on a bed of alluvium, and interestingly this alluvium extends under most of the City of St. George and around other geologic features in the area. The presence of alluvium throughout the area suggests that at one point, the river or another body of water was unconfined within the margins of the surrounding cliffs (shown in purple in Fig.16).
Geomorphic Units - Bar formations are constantly present in the channel, including during flood events, such as Fig. 17. The most prevalent bars appear to be mid-channel or diagonal bars, as well as point bars or braiding the channel. Due to the fact that there was not a recent field visit associated with this site, there is little information on other in-channel geomorphic units. Outside the valley bottom margins of the Virgin River (but within the Valley Bottom overall) the alluvial fans associated with the incoming tributaries of the Santa Clara River and Fort Pearce Wash also encroach on the Virgin River's ability to meander. These fans are shown in orange on the Site Map in Fig. 16.
Vegetation - This area of the Virgin River is highly vegetated with native plants as well as invasive salt cedar. This reach also covers some areas that have been landscaped with either native plants or non-native grass (i.e. parks and golf courses). Just north of this reach, as the Virgin River flows through a similar reach in Washington and western St. George, there is a significant amount of agricultural land.
Flows - Recent USGS Stream Gage Data show that flows in this area of the Virgin River average 100-200 cfs (2.83-5.66 cms) in normal years. The all-time record mean flow is recorded as 758.5 during 2005, the year of the flooding pictured in Fig. 17.
Fig. 16. The Virgin River at the Santa Clara confluence. The valley bottom is shown in blue, with yellow borders denoting anthropogenic confinement. Orange denotes alluvial fans from incoming tributaries, the Fort Pearce Wash and the Santa Clara River. If there were no development in the area, the river would be more truly laterally unconfined, hemmed in by minimal margin confinement, shown here in purple lines.
Fig. 17. This is a view of the Virgin River just north of the Santa Clara River confluence, looking downstream (The upstream/northern portion of this study reach). Floods in 2005 caused flows of roughly 12,000 cfs (340 cms).
Fig. 18. The Utah Interactive Geologic Map shows that the majority of the city of St. George is built on older alluvial and eolian deposits, indicating that the Virgin River or an older body of water might have roamed freely and laterally unconfined in this area.
Inter-comparison: Sites 1, 2, 3
Longitudinal Profile
Although the three reaches considered are part of the same watershed, they each have unique characteristics that make them distinct. Besides confinement, the geology at each site is unique, as well as conditions during flood events. Similarities among the three sites include sparse and native desert vegetation and relatively flashy flood regimes, outside normal spring runoff highs and summer lows.
Putting the sites in context of their location along the entirety of the Virgin River, Figs. 19 and 20 show longitudinal profiles of the North Fork of the Virgin River and the Virgin River, respectively. The profiles also show the elevations of sites 1 and 3, near the top of the watershed.
In the Virgin River longitudinal profile (Fig. 20), the first major nick-point occurs just upstream of Site 2, at Confluence Park. This occurs right as the river comes out of the highly confined, sedimentary canyons of Zion and drops into the alluvial and lava flow deposits in Hurricane and La Verkin. Farther along the Virgin River, the slope again becomes slightly steeper, right outside of St. George, as the river again enters an area of high confinement. The final nick-points occurs at the end of this confinement, where the slope inexplicably increases dramatically. After this, the slope returns to base-line as the Virgin River enters Lake Mead.
Fig. 19. North Fork Virgin River Profile, showing the elevation of Site 1: The Narrows (confined region of the watershed).
Fig. 20. The Virgin River Profile, showing the elevation of Site 2: Confluence Park, La Verkin, UT and areas of confinement and partial confinement along the river.
[MORE] Catchment Context
As explained in the introduction, the total Catchment for the Virgin River and its tributaries (Fig. 2, Fig. 5) is roughly 3,785,341 acres (15,325 sq km). Within the catchment, the Virgin River begins at the confluence of the North Fork and East Fork, roughly halfway through the Upper Virgin Catchment. In 2009, the Virgin River was named Utah's first "wild and scenic" river. At just over 160 miles, roughly 145 miles or 90% of the total length of the Virgin River is designated wild.
Along the study reaches I chose, there are many instances of both direct and unintentional human disturbances on the river. Zion is a National Park and attracts millions of visitors from around the world. The Narrows is by far the most famous hike in the park, perhaps with the exception of Angel's Landing, and is constantly trafficked by people who, even unintentionally, leave a mark on the riverscape.
The second site, located in Confluence Park, is also a popular spot among locals. The large cottonwood near the USGS Stream Gage has a rope swing, and park amenities including a picnic table and climbing bolts in a nearby basalt deposit make their mark on the floodplain. In the future, a pedestrian bridge will be constructed right in this area. This will increase traffic in the area and cause more major changes to the riverscape.
By far, the most disturbed site is the Santa Clara River Confluence. The confluence is located just south of downtown St. George and this section of the river is home to a lot of city trash, sees a lot of recreation, and encroaching development on either side of the channel. Perhaps as an act of rebellion, during large storms or when the Virgin River floods it deposits sediment all along main streets in St. George. A healthy reminder that this is a natural system, not to be trifled with.
Hydrology
Catchment hydrology is highly dependent on whether the area is in drought. According to the USGS, the water supply in Utah is generally in 'feast or famine' [4]. This is evidenced in the high and low flows of the Virgin River. From record lows in 2004 to record high flows in 2005, the Virgin River proved its unpredictability. The USGS Stream Stats and information from USGS Stream Gages used throughout this report are useful for determining precise flow rates, water quality, and other characteristics for monitoring water in the Virgin River.
Sources
[1] Virgin River. (2021). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Virgin_River&oldid=998143063
[2] USGS Surface Water data for USA: USGS Surface-Water Monthly Statistics. (n.d.). Retrieved April 26, 2021, from https://waterdata.usgs.gov/nwis/monthly?referred_module=sw&site_no=09413500&por_09413500_143365=448777,00060,143365,1950-10,2021-03&start_dt=1951-01&end_dt=2021- 01&format=html_table&date_format=YYYY-MM-DD&rdb_compression=file&submitted_form=parameter_selection_list
[3] Rock Layers. (2021, January 17). [Gov]. Zion National Park Utah. https://www.nps.gov/zion/learn/nature/rock-layers.htm
[4] Flooding and streamflow in Utah during water year 2005. (2006, May). [Gov]. U.S. Department of the Interior U.S. Geological Survey. https://pubs.usgs.gov/fs/2006/3085/PDF/FS2006-3085.pdf
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