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.

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.

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.

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.