Riparian monitoring of wadeable streams at Armstrong Canyon, Natural Bridges National Monument: Summary report, 2010–2018

Witwicki, Dana; Weissinger, Rebecca

The goal of Northern Colorado Plateau Network (NCPN) riparian monitoring of wadeable streams is to provide park managers with status and trend information on integrated hydrologic, geomorphologic, and vegetative attributes of these areas. This information is intended to provide early warning of resource degradation and determine natural variability of wadeable streams. This report summarizes NCPN monitoring of Armstrong Canyon in Natural Bridges National Monument (NM) from 2010 to 2018. It outlines goals, methods, sampling design, and objectives of the network’s wadeable streams monitoring. The goals of this report are to (1) present baseline vegetation and geomorphology data from four reaches (1, 2, 4, and 5) monitored in Armstrong Canyon from 2010 to 2014, (2) examine changes in geomorphology and vegetation resulting from multiple large flood events in September 2013 at the two reaches monitored before and after the floods, and (3) evaluate flood-stage data collected from 2014 to 2018. Armstrong Canyon is a primarily ephemeral drainage the cuts through the Cedar Mesa sand-stone, with bedrock often constraining the channel on at least one side. Straighter, narrower sections of canyon are common; these reaches have higher flood power and limited alluvial and vegetation development. Most of our repeat measurements in Armstrong Canyon occurred at this type of reach, similar to reaches 1 and 5. Changes at these locations tend to be stochastic, making longer-term trends more difficult to detect. More-substantial terraces and less bedrock are present at large bends in the canyon, similar to reaches 2 and 4, but there are relatively few areas like this in Armstrong Canyon. Surveys of cross-section transects at Reach 5 conducted before and after the September 2013 floods showed some cutting in the channel (widening, deepening, or both) and some deposition on the floodplain along most transects. However, much of the surveyed cross-sections remained the same between years. Pebble counts showed that sand and gravels were also scoured from this reach. Repeat geomorphic surveys were not conducted at any other reaches, but repeat pebble counts at Reach 1 showed little to no change in particle-size distribution before and after the floods. Overall, changes in vegetation after the flash floods of September 2013 seemed minor, considering that flows were powerful enough to wash large trees down the canyon and perch one of them over three meters above the channel. Vegetation was monitored before and after the floods at two reaches, and changes varied by reach. The main differences detected after the September 2013 floods were a reduction in the number of forb species present at Reach 5, a reduction of forb cover at Reach 1, and a reduction in the frequency of exotic species at Reach 5. Sediment deposition on the flood-plain probably buried some forbs and exotic grasses, while flood waters may have washed away plants or part of the seedbank. A relatively rapid recovery of these species is expected because they are adapted to disturbance. Larger floods are well known to promote germination of cottonwood seedlings in riparian areas of the Southwest, and density of cottonwood seedlings increased 60 times after the floods at Reach 1—but not at all at Reach 5. However, cover of narrow-leaved cottonwood doubled after the floods at Reach 5, most likely due to increases in foliage on established seedlings and saplings. Hydrologic data collection did not begin until spring 2014, and the data are highly variable in response to local weather patterns. Flash floods were common from 2014 to 2018, while more sustained flows from snowmelt were uncommon. The longest dry period within a water year at Reach 5 ranged from 23 to 214 days. The highest stage recorded in September 2017 was about 2.7 meters higher than the channel thalweg.

Riparian monitoring of wadeable streams at Armstrong Canyon, Natural Bridges National Monument: Summary report, 2010–2018

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