WMV& im fvctA yvJtP THE POTENTIAL FOR RESTORATION ALONG THE VIRGIN RIVER IN ZION NATIONAL PARK VirginRiver, ZionNationalPark, PhotobyDavePettit GRAND CANYON T R U S T Grand Canyon Trust 199 North Main St. St. George, UT 84770 September 2001 Digitized by the Internet Archive in 2012 with funding from LYRASIS Members and Sloan Foundation http://archive.org/details/potentialforrestOOgran FINAL REPORT THE POTENTIAL FOR RESTORATION ALONG THE VIRGIN RIVER IN ZION NATIONAL PARK An evaluation of restoration alternatives with focus on the potential to regenerate cottonwood {Populus fremontii) along the North Fork of the Virgin River in Zion National Park Technical Collaboration provided by: James P. McMahon, Southwest Director Grand Canyon Trust 199 North Main St. St. George, UT Tom Moody, P.E. Natural Channel Design, Inc. 33 East Mohawk Drive 1 Flagstaff, AZ 86001 Steven Apfelbaum and Susan M. Lehnhardt, Research Ecologists I. Douglas Eppich, P.E. Applied Ecological Services, Inc. 17921 Smith Rd. Brodhead, WI 53520 Vern LaGesse, Restoration Specialist 1619 South Pasfield Springfield, IL 62704 Under a Cooperative Agreement with Zion National Park UT Springdale, September 2001 TABLE OF CONTENTS Table ofcontents i List of Figures and tables ii Executive Summary iii Introduction 1 Background 2 National Park Service Objectives 3 Project tasks and objectives 3 Methods 4 Summary ofprevious studies 5 Results and discussions 8 Summary ofreach observations 9 Zion Canyon and East Fork cottonwood conditions 20 Competing Vegetation 22 Explaining the changing woody vegetation condition in Zion National Park 24 Channel geomorphology 29 Geomorphic conditions for successful recruitment 32 Hydrology 34 Condition ofexisting revetments 36 Design channel/floodplain geometry 37 Assessing design flooding potential 39 Historic meander 40 Design summary 42 Results of Preliminary Design Workshop 42 Evaluation ofmanagement alternatives 43 Summary & conclusions 53 Recommendations 55 References cited 57 Final Report i Virgin RiverRestoration 1 LIST OF FIGURES AND TABLES Figure 1. Mature cottonwood trees in Zion Canyon iii Figure 2. Rock and wire revetments channelize Virgin River in Zion National Park vi Figure 3. Geomorphic evolution ofthe Virgin River. v Figure 4. Map ofZion Canyon and project reaches 1 Figure 5. Lack ofyoung cottonwood trees along valley floor Figure 6. Summary ofgeneral relationships with fluvial aggradation and erosion processes (Hereford, et al, 1995). 7 Figure 7a,b. Cross-section and photo at Narrows survey site. 10 Figure 8a,b. Cross-section and photo at Hereford survey site 1 Figure 9a,b. Photo and cross-section at Big Bend survey site 13 Figure 10. Cross-section at Great White Throne survey site 15 Figure 1 la,b. Cross-section and photo at Grotto footbridge survey site 16 Figure 12a,b. Photo and cross-section #1 in Birch Creek reach 17 Figure 13. Surveyed Cross-section #2 in Birch Creek reach 18 Figure 14. Cross-section at East Fork survey site 20 Figure 15 a, b, c. Outline ofthe relationships between flood events, scour, deposition, moisture, substrate conditions, and cottonwood recruitment and survival 28 Figure 16 Historic channel geomorphology a. -1880-1930 30 b. 1930-1950 31 c. 1950 -Present 31 Figure 17 Preliminary correlation ofstorm discharge, fluvial features, and cottonwood establishment 33 Figure 18. All peak flows greater than 400 cfs at North Fork Virgin River at Springdale, UT gage. (1913-14, 1926-98) 34 Figure 19 a, b, c. Peak flow events greater than 400 cfs by season. North Fork Virgin River at Springdale, UT gage station. (1913-14, 1926-98) 35 Figure 20. Rock revetments in disrepair near Zion Lodge 36 Figure 21. Design cross-section overlaid on existing cross-section near Zion Lodge reach 37 Figure 22. Design cross-section overlaid on existing cross-section near Birch Creek reach 38 Figure 23. Photo of Big Bend reach 38 Figure 24. Modeled water surface elevations for 50-year flood (7,500 cfs) in existing and design cross-sections near Zion Park Lodge 39 Table 1. Summary ofchannel and floodplain dimensions for survey sites 19 Table 2. Summary ofdimension, pattern, and profile data for survey sites. 19 Table 3. Relationship of floodprone stage to related discharges and return intervals 19 Table 4. Cottonwood regeneration comparison between the North Fork in Zion Canyon and East Fork (Parunaweap) Virgin River, in Zion National Park. 21 Table 5. Terrace features along North Fork Virgin River (Hereford, et al, 1995) 29 Table 6. Stage-Discharge relationship at survey sites. 39 Table 7. Meander widths values through Zion Canyon 40 Table 8. Historic meander width data by transect and reach 41 Table 9. Alternatives Matrix: Resource Impacts 51 Table 10. Alternatives Matrix: Management Considerations 52 Final Report ii Virgin RiverRestoration EXECUTIVE SUMMARY Without active intervention the magnificent canopy ofcottonwood trees, part ofthe historic landscape within Zion Canyon, will vanish in the next few decades. Pioneer accounts describe a canyon filled with many mature trees as early as 1860 but today there is insufficient successful reproduction to replace those aging trees. Cottonwood reproduction is reduced throughout the Canyon, the lack ofyoung trees is especially apparent in the areas near Zion Lodge where virtually none are present. Previous studies suggest that the reduction in cottonwood recruitment is greatest in areas where man-made rock and wire revetments were constructed in the 1930's. These aging revetments along the Virgin River will soon have to be replaced or removed as their deterioration creates an increasing public safety hazard. Cottonwood reproduction is dependent on bare, moist soils commonly created during flooding. It is apparent in the areas lined with revetments that cottonwood trees are not being replaced due to the separation ofthe river from the floodplain. Concern over the loss ofthe historic cottonwood forest and the need to address the deteriorating revetments caused the National Park Service to explore options including the possible restoration ofa naturally functioning river channel and floodplain. The purpose ofthis study was to provide the park with an assessment ofexisting conditions and potential for restoring riparian function with its associated benefits to the river and the cottonwood community. ^l^Sitl FIGURE 1. Mature cottonwood trees in Zion Canyon are nearing the end oftheir biological life. Lack ofsuitable conditions limits the recruitment ofyoung cottonwoods to take their place. Note lack ofyoung or middle age trees. Final Report in Virgin RiverRestoration