Terrestrial Riparian Vegetation Monitoring: How One Square Meter Can Tell the Story of 245 River Miles

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The goal of Grand Canyon Monitoring and Research Center’s (part of the Southwest Biological Science Center) riparian vegetation monitoring program is to assess changes and trends in plant species composition and cover and relate those changes to Glen Canyon Dam operations, river hydrology, climate, and geomorphology. Monitoring is done by annual field-data collection on plant cover and diversity in concert with semi-decadal remote sensing to distinguish the effects of dam operations from other environmental and human-caused changes to riparian vegetation. Between 2012 and 2017, the GCMRC vegetation program sampled 392 sites (43 of them repeated every year and only counted once in that total), and has sampled approximately 16,524 unique 1m2 (about 3.3’ x 3.3’) plots. Remote sensing analyses are synoptic image classification and change detection assessments of the entire 475 km (295 miles) river corridor between Glen Canyon Dam and Lake Mead or shorter reaches within the corridor. These robust data sets can tell the story of a changing riparian landscape.

Diagram of vegetation monitoring sites along the Colorado River

Image depicting the vegetation program’s field methods and data collection. These techniques can show patterns, trends, and changes throughout the Colorado River corridor downstream of the Glen Canyon Dam. ​​​​​​​Click image to enlarge it. (Credit: Sarah Sterner, USGS. Public domain.)

General Methods:

Monitoring riparian vegetation along the Colorado River

Monitoring riparian vegetation along the Colorado River downstream of Glen Canyon Dam. (Credit: Amy Washuta, NPS/USGS. Public domain.)

Each year, the Grand Canyon Monitoring and Research Center (GCMRC) terrestrial vegetation program monitors vegetation between the Glen Canyon Dam and Spencer Canyon at river mile 245—the location that the hydrology and botanical communities begin to be impacted by the Hoover Dam and Lake Mead’s sediment deposition. Each year, the vegetation program monitors up to 90 randomly selected sites that are divided between three predominant geomorphic features—sandbars, debris fans, and channel margins—as well as 43 sites that are at fixed (repeated) sandbars. Sites are stratified at three nested scales—river segment, geomorphic feature, and hydrologic zone. The samples encompass differences in vegetation due to geography, geomorphology, and the frequency of inundation.At each monitoring site, nine 1 meter-squared (1m2 or 3.3’ x 3.3’) vegetation plots are sampled along three transects that are oriented perpendicular to the river, totaling 27 one meter square plots per site. The vegetation plots are arranged on each transect to cover the three hydrologic zones and captures the vegetation variability of the site.

The GCMRC vegetation program collaborates with the National Park Service (NPS) Northern Colorado Plateau Inventorying and Monitoring Network’s big-river program that monitors large rivers upstream of the Glen Canyon Dam—the Upper Colorado, Green, and Yampa Rivers. These collaborations link processes occurring in the rivers upstream to the Colorado River in Glen and Grand Canyons. For information on the NPS Northern Colorado Plateau Inventorying and Monitoring Network, please visit their website: https://science.nature.nps.gov/im/units/ncpn/.

For more detailed sampling methods, reference:

Palmquist, E.C., Ralston, B.E., Sarr, D.A., and Johnson, T.C., 2018, Monitoring riparian-vegetation composition and cover along the Colorado River downstream of Glen Canyon Dam, Arizona: U.S. Geological Survey Techniques and Methods, book 2, chap. A14, 65 p., https://doi.org/10.3133/tm2A14.

Important Results:

Floristic Community Changes in Grand Canyon (published in Palmquist et al. (2018)):

Table of vegetation along Colorado River

Table of common species found in the riparian area along Colorado River downstream of Glen Canyon Dam. See paragraph above for more infomation. Click on table to enlarge. (Credit: Emily Palmquist, USGS. Public domain.)

Floristic communities change and species richness declines between the Glen Canyon Dam and river mile 245. Riparian vegetation sampling along the Colorado River through Grand Canyon by GCMRC’s vegetation program identified how richness and functional diversity vary, and described the implications of our results for river management (Palmquist et al. 2018, see "Publications" tab near top of this page). Cluster analysis identified three divergent floristic groups that are distributed longitudinally along the river. These groups were distributed along gradients of elevation, temperature and seasonal precipitation, but were not associated with annual precipitation or local-scale factors. Species richness and functional diversity decreased as a function of distance downstream showing that changing landscape-scale factors result in changes to ecosystem characteristics. Species composition and distribution remain closely linked to seasonal precipitation and temperature. River segments were delineated by floristic groups, and are bound by three large tributaries—the Paria River at river mile 0, the Little Colorado River at river mile 60, and National Canyon at river mile 160. The table below illustrates the floristic groups by river segment.

These patterns in floristic composition in a semiarid system inform management and provide insight into potential future changes as a result of shifts in climate and changes in flow management. 

Graph of vegetation communities along the Colorado River

Graph of three common species along the Colorado River downstream of Glen Canyon Dam illustrating species turnover. Baccharis emoryi, Baccharis salicifolia, and Baccharis sarothroides relative cover distribution between river mile 0 to river mile 245. Vertical dotted lines on each graph delineate Marble Canyon, Eastern Grand Canyon, and Western Grand Canyon. Graph modified from Palmquist et al. (2018). Photos of each species are located to the right of their distribution. ​​​​​​​Click on graph to enlarge. (Credit: Emily Palmquist & Sarah Sterner, USGS. Public domain.)