Riparian Zones—It’s all about the Water

What are riparian zones? Riparian zones, or areas, are lands that occur along the edges of rivers, streams, lakes, and other water bodies. Examples include streambanks, riverbanks, and flood plains. They’re different from the surrounding uplands because their soils and vegetation are shaped by the presence of water. Riparian zones in the southwestern United States make up less than two percent of the land area, but they support the highest density and abundance of plants and animals of any habitat type there.

Riparian zones provide many important functions and benefits, including:

• Providing habitat (including migration routes and habitat connectors) for a diversity of wildlife,
• Helping to maintain water quality, because riparian vegetation can remove excess nutrients and sediment from surface runoff,
• Stabilizing stream banks and reducing floodwater velocity (thanks to riparian vegetation), and
• Providing recreation opportunities and scenic beauty.

Also, in addition to riparian plants providing habitat and stabilizing stream banks, overhanging vegetation shades streams, which reduces water temperatures for fish.

The Glen Canyon riparian zone

A riparian area in Harris Wash.

NPS Photo

More than 3,000 miles of waterways flow through Glen Canyon National Recreation Area (NRA)—that’s longer than the distance between Los Angeles and Washington, D.C.! These waterway miles are distributed among five major rivers (with year-round flow), five smaller streams (with year-round flow), and 32 streams with intermittent or temporary flow, with most of the miles falling into the last category.

The amount of water in streams depends mainly on the amount of annual snowpack accumulated in the surrounding mountains and the rate of melt in spring and summer, but there are other contributors to flow (such as monsoon rains and intermittent precipitation). Riparian plant communities in Glen Canyon range from cattail/rush/sedge marshes and streamside species such as willows, to deciduous forests.

What are the biggest threats to riparian zones at Glen Canyon?

• Climate change: Climate change is bringing warmer temperatures and drier conditions, as well as potential shifts in the amount and timing of precipitation. Precipitation is a particularly important driver of the riparian zone. Warmer and drier conditions also stress native riparian plant communities while giving a competitive advantage to some invasive plants.

• Non-native invasive plants: The invasion of non-native plants is one of the largest stressors of riparian zones. Although the park and its partners have worked to control invasive plants along the Escalante River, other riparian zones (such as along the Dirty Devil River) are heavily invaded by non-native plants.

Assessing conditions of riparian zones and other resources at Glen Canyon

A recent Natural Resource Condition Assessment (NRCA) through the NPS NRCA Program focused on water-related resources located away from Lake Powell and the mainstem Colorado River. NRCAs evaluate natural resource conditions so that parks can use the best available science to manage their resources. The riparian zone was one of five focal resources selected for this project.

The riparian zone assessment study area included four major tributaries to the Colorado River (Dirty Devil River, Escalante River, Halls Creek, and Lake Canyon), as well as two tributaries to the Escalante River (Coyote Gulch and Harris Wash) and one tributary to the San Juan River (Wilson Creek). Ecologists at Utah State University used three indicators of condition for the assessment: water quantity and availability, stream/watershed health, and vegetation. The indicators, measures, and reference criteria were derived from the scientific literature, which characterized healthy riparian systems on the Colorado Plateau as those with high flow variability resulting in plant communities dominated by native species.

Some of what we learned:

• No impairments to water quality were reported for the streams studied in the assessment.

• Some of the subwatersheds studied were rated as low or very low for aquatic intactness, which considered human impacts from water demand, dams, diversions, developments, etc.

• There were no significant declines in annual discharge (flow) for the Escalante (1943—2020) or Dirty Devil (1948—2020) rivers. However, for the Escalante River, there has been a moderate decrease in average annual flow, annual variability in flow, and monthly variability in flow.

• Vegetation in riparian zones has remained relatively stable, but non-native species comprise many of the riparian plants along some stream corridors, particularly along the Dirty Devil River. These species include some of Glen Canyon’s most problematic invasive species.

• Overall, for the three indicators of condition used, two (water quantity and availability, and stream/watershed health) were given ratings of “fair,” and one (vegetation) was given a rating of “poor” (on a scale of “good” to “poor”).

What can park managers do with this information?

This study can be used to inform park planning and management actions that involve the riparian zone. For example, even though control of some non-native invasive species (such as tamarisk [Tamarix ramosissima]) has been effective, riparian zones continue to be under significant threat from non-native plants, and the effects of climate change will increase this stress; during development of the NRCA, both continued invasive plant control and research activities on non-native plants were identified as management needs.

Additional Information

• For more information on non-native species, see this article on the Glen Canyon NRA website: https://www.nps.gov/glca/learn/nature/nonnativespecies.htm

• For more information on water quality in Glen Canyon NRA, see this article focusing on water in the park’s rivers and Lake Powell: https://www.nps.gov/glca/learn/nature/waterquality.htm

Information in this article was summarized from: Albright J and Others. 2022. Natural resource conditions at Glen Canyon National Recreation Area: Findings & management considerations for selected resources. Natural Resource Report. NPS/SCPN/NRR—2022/2374. National Park Service. Fort Collins, Colorado. https://doi.org/10.36967/nrr-2293112

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