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

132 Pages·2001·4.9 MB·English

by  Grand Canyon Trust

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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