Overview of Riparian Vegetation in Grand Canyon

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Riparian areas are conspicuous belts of dense, green vegetation along streams and rivers, and can be considered “ribbons of life”. Despite covering less than 2 percent of the land area in the southwestern U.S., riparian areas tend to have high species diversity and population density, making them valuable to managers, scientists, and the public. These unique ecosystems act as a link between dry, terrestrial uplands and aquatic ecosystems. Wildlife depend on riparian areas for foraging, nesting, and cover throughout their lives. Riparian vegetation can impact native and non-native fish populations by providing food and cover and influencing water temperature by shading. Also, more than 50 percent of 166 breeding bird species in the lowlands of the southwest are completely dependent on these habitats. Knowing the status of riparian vegetation along the Colorado River and how Glen Canyon Dam operations influences that vegetation is important because changes in riparian plant communities impact wildlife, the transport and deposition of sediment, the stability of sandbars, recreational camping and wilderness experiences, and provides metrics that inform management goals. 

Background and Importance: How Glen Canyon Dam has Impacted Riparian Vegetation Communities

Riparian plant research in Grand Canyon began in the early 1900s in the form of plant inventories and collections. Plant collections and research since then have varied in duration and intent, and USGS Grand Canyon Monitoring and Research Center scientists and their cooperators currently document the amount and types of vegetation found along the river corridor and how they may respond to changes in dam operations. In addition to long-term field-based monitoring, GCMRC uses remote sensing of very high resolution multispectral imagery and lidar acquired from fixed-wind airplanes and helicopters to monitor and research the short- and long-term dynamics of riparian vegetation. The long-term goal for monitoring riparian vegetation in the Colorado River is to use annual data on plant cover, species richness, and diversity in concert with semi-decadal vegetation mapping data to distinguish between the effects of dam operations and other environmental- or human-caused changes on riparian vegetation. Trend detection at the reach or regional scale will provide information about gains or losses in vegetated area and about the vegetation classes that change the most.

Prior to the establishment of the Glen Canyon Dam in 1963, the Colorado River through the Grand Canyon would seasonally flood with annual high flows of 85,000 cubic feet per second (f3/s) to the highest ever recorded of 210,000 cubic feet per second (f3/s). These flood events, typically from springtime snowmelt in the Rocky Mountains, would presumably scour the Colorado River corridor of plants, leaving sandy beaches and sparse patches of greenery. Without the scouring floods that came through the river before the Glen Canyon Dam, vegetation abundance has increased and is continually changing in response to varying flow regimes.

The floristic communities downstream along the Colorado River corridor have and continue to change in response to the Glen Canyon Dam’s regulated flow regimes that began in 1963. Operating criteria associated with the Environmental Impact Statement for Glen Canyon Dam completed in 1996 resulted in daily flows that fluctuate by 8,000 f3/s in a day and up to 25,000 f3/s in volume. These dam operations create distinct belts of vegetation that are botanically diverse.

Post- and pre-dam flood stages along the Colorado River

Diagram of a Colorado River cross section and associated floodplains before and after Glen Canyon Dam. (Credit: Sarah Sterner, USGS. Public domain.)

As illustrated in the diagram above, the area of riverbank where plants might be inundated on a daily basis is the active channel (flows up to 25,000 f3/s). High steady releases, also associated with High Flow Experiments (HFEs) implemented since 1996 (1996, 2004, 2008, 2012, 2013, 2016, 2017) have created an active floodplain that extends to elevations where discharges of 45,000 f3/s reach.  Plants in the active floodplain have less frequent access to water. Finally, without massive high flows from spring runoff, there is an inactive floodplain - a relic of a time before the dam when plants would receive once or twice yearly inundation above 45,000 f3/s. These three hydrologic zones created by the Glen Canyon Dam have distinct botanical differences. With the changes to riparian hydrologic zones brought about by the Glen Canyon Dam, riparian plant composition has subsequently changed throughout time.