Mapping Endangered Fish Habitat
in the Grand Canyon
M. Kathleen Webb
Watershed Science Program - Utah State University
GIS in Water Resources Term Paper - Fall 2001

 

INDEX

 

Background

Objectives and Project Overview

Study Area

Data Acquisition and Manipulation

Data Analysis

Results

Conclusions

Acknowledgements

References

 

 

 

 

 

 

 

 

 

Background

Operating a dam in order to manage for a healthy ecosystem was unheard of before the mid-1990s.  At that time the Glen Canyon Environmental Studies (GCES) culminated in the 1996 experimental flood in Grand Canyon.  The controlled flood generated high discharge flows of 45,000 cfs (beach/habitat building flows) to build sand bars above non-flood-stage river levels.  The goals of this flood were to deposit nutrients, to restore backwater channels, and to reinstate some of the dynamics that would be inherent in a natural system.  The flood was engineered to test the hypothesis that the dynamic nature of fluvial landforms and of aquatic and terrestrial habitats can be wholly or partially restored, by short-duration dam releases that are substantially greater than power plant capacity.  Other rationales for the flood were that it enabled measurement of geomorphic and ecological processes during flood passage and recession, provided data needed for predictive models, and helped establish an operational regime that could help maintain, manage and protect riparian and aquatic resources (Patten et al. 2001).  A similar flood was run in the fall of 2000, though at a lesser discharge of 31,500 cfs.  It is this flood on which my project was based.

Humpback chub, Gila cypha, is endemic to the Colorado River Basin, and is not found elsewhere.  This and other species native to this system are evolutionarily adapted to the flow conditions and characteristics of the Colorado and Green Rivers and their associated tributaries.  Humpback chub bodies are streamlined and have a high, pronounced hump immediately behind their heads.  This hump acts as a barrier to passing water, forcing the body against the bottom of the channel where currents are less strong, enabling them to maneuver through rapids to access preferred habitat within eddies.  They can grow up to eighteen inches and can weigh over 2 pounds.  They prefer deep, fast-moving, turbid waters and are often found at the base of large boulders and steep cliffs.  Humpback chub spawn between April and July in gravel bedded reaches of tributaries such as the Little Colorado River.  They feed predominantly on small aquatic insects, diatoms and filamentous algae (http://www.gf.state.az.us).   

 

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Objectives and Project Overview

 

My project objective was to monitor and analyze changes in the surface area of available backwater habitat due to high flow releases from Glen Canyon Dam.  Backwater habitat is crucial for the survival of the endangered fish species Gila cypha, Humpback chub.  My study area is the Colorado River within Grand Canyon National Park, AZ.  My project was accomplished using aerial photos and digital orthophotos from before, during, and after the spike flow of Fall, 2000.  This controlled spike flow release from the dam was designed to build and/or maintain critical habitat and beaches. 

 

 

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Study Area 

My study area included four reaches spanning from Glen Canyon below the dam, through Marble Canyon, and into the Grand Canyon at the confluence of the Colorado River and the Little Colorado River (LCR). 

Reaches:

·        Lees Ferry = 3.3 km

·        Redwall = 4.7 km

·        Pt. Hansbrough = 7.3 km

·        LCR/Tapeats Gorge = 10.2 km

Figure 4. Map of Study Area.
 


 

 

 

                                                             

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Data Acquisition and Manipulation

 

 

·        All images used in this study were obtained from the Grand Canyon Monitoring and Research Center, GCMRC, (http://www.gcmrc.gov/).  The images were on CDs and DVDs. 

·        Digital Orthoquads were available for the pre- and post-spike images.  These were cropped to include the river corridor only, using Mr. SID. 

·        Non-rectified aerial photos were available for the period during the spike flow.  Tic marks had to be added to non-rectified images that correlated with tic marks on the rectified images.  I did this using ArcINFO.  The spike flow images were then transformed to match the pre- and post-spike orthophotos.

 

 

The following images are comparison photos from before (August) and after (September) the flood.  One noticeable difference between the two sets of images is the turbidity of the water.  Before the flood, presumably due to natural sediment influx from the Paria River (located at the upstream end of the Lees Ferry reach), the water is less clear, a condition more akin to the pre-dam Colorado River.  However, the clarity of the water in the September images can benefit mapping procedures because submerged sand deposits are sometimes visible.  

 


                                          

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                    

 

 

 

 

 

                   

 

 

 

 

 

 

 

 

 

 

 

 

                       

 

 

 

 

 

 

 


                                 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 


For all of the newly rectified images of the river during the spike flow, the edge of water was digitized (on-screen) as arcs and then built into polygons that represented the river at peak flow, using ArcINFO.  These coverages were copied and overlaid on the pre- and post-spike images, additional arcs were digitized onto the new coverages and topology was built (this creates polygons with attribute tables).  The new polygons represented various surficial geologic features such as wet, dry, or submerged sand deposits in eddies or along channel margins, debris fans, talus, and mid-channel boulders.  These steps are outlined below.

 

 

Step one:  Locate deposits within designated reaches

 

 

This image is of an eddy sandbar, specifically a reattachment bar, formed by fluvial geomorphic processes of flow and sediment transport.  The recirculating flow of an eddy below a channel constriction deposits sand in a bar formation that is oriented in the upstream direction, and the water remaining behind the reattachment bar is the classic backwater habitat. 

These backwaters are characterized by slow-moving or stagnant water, warmer temperatures, large invertebrate populations, and abundant algal growth.  These characteristics are what make a backwater area an optimal place for a larval humpback chub to drift, and then to grow during its juvenile life.  Backwater channels provide a refuge from the fast-moving turbulent currents of the main channel, as well as providing abundant food sources. 

 

 

Step two: Digitize arcs outlining fluvial deposits, then build them as polygons and add attributes

 

Once the arcs are digitized using ArcEdit in ArcINFO (this process is too involved to describe in detail here), the polygon topology is built and labels are created. 

In ArcTools (within ArcINFO) the polygon coverages are opened and each polygon is attributed with its characteristics, i.e. a fluctuating flow (wet) reattachment bar, which indicates a low-level sandbar deposit in the downstream end of an eddy (shown by blue arrow).

 

 

 

 

Step three: create polygons that represent the backwater habitat area

 

An additional coverage is created that includes backwater areas delineated by the boundaries of wet sand and dry sand, represented in figure 6 by the parallel lines going from sandbar to shore. 

These newly created polygons are attributed with labels that designate what type of deposit formed the boundary of the backwater as well as the direction the backwater channel outlet is oriented, i.e. upstream or downstream. 

 

 

For comparison, here is the same eddy bar deposit and associated polygons from the pre-spike images of August 2000.  The changes in configuration of the reattachment bar formation are evident, as are the resulting changes in backwater area. 

 

 

 

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Data Analysis

 

·        Coverages were opened in ArcView for viewing and to access attribute tables

 

These two layouts represent a section of the LCR-TG reach comparing August to September deposits along the channel corridor.

 

    

 

 

 

 

·        Below are two graphics of the eddy sandbar deposit in the lower portion of the above maps.  They are located at the downstream end of the LCR-TG reach and demonstrate the changes in sandbar configuration after the flood. 

 

 

                       

Figure 8.  LCR-TG reattachment bar and backwater area from August 2000.

 

Figure 9.  LCR-TG reattachment bar and backwater areas from September 2000.

 
 

 

 

 

 

 

 


·        Attribute tables were exported into MS Excel where pivot tables were used to calculate backwater areas.

·        These pivot tables were copied to Kaleidograph for graphical representation.

 

The following graphs represent change in backwater area for various backwater types, for each of the four reaches:

a)     upst drysnd = backwaters with higher elevation sand deposits as their boundaries, opening oriented upstream

b)    upst wetsnd = backwaters with lower elevation sand deposits as their boundaries, opening oriented upstream

c)     dwnst drysnd = backwaters with higher elevation sand deposits as their boundaries, opening oriented downstream

d)    dwnst wetsnd = backwaters with lower elevation sand deposits as their boundaries, opening oriented downstream

 

 

 

 

 

 

 

 

 

 


                                                                                               

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Results

 

This final graph shows a comparison in the increase of total backwater areas with sand boundaries for each reach.  This demonstrates the longitudinal trend from upstream (Lees Ferry) to downstream (LCR-TG).  Glen Canyon Dam and the Lake Powell reservoir act as a sediment trap for all sediment transport in the Colorado River.  Because of this, there is an extreme lack of sediment available for transport and redistribution immediately below the dam.  Traveling downstream, more sediment gets added to the system due to tributary inputs (the Paria and Little Colorado Rivers) as well as scour from the bed.  (Note: pth-aug and -sept are out of order, spatially the Point Hansbrough reach is downstream of the Redwall Reach). 

 

 

 

 

Due to the dynamic nature of channel morphology, and the range in lengths between the 4 reaches, the number of eddies varies between reaches.   The greatest increase in backwater area occurred in the LCR-TG reach.  Characteristics of this reach that contribute to the increase are the overall length of the reach (10.2 km, the longest of the four reaches), the confluence of the Little Colorado River being in the middle of the reach - the largest tributary to the Colorado River in Grand Canyon.  The other reaches also show an increase after the flooding event, to varying degrees. 

 

 

 

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Conclusions

 

·        Controlled floods from Glen Canyon Dam succeeded at increasing overall area of backwaters. 

·        The longitudinal trend is of a larger percent increase in backwater area corresponding with increasing distance from Glen Canyon Dam.  This is a result of greater sediment availability due to scour of the bed and tributary inputs.

·        This study included an analysis of backwater change measured immediately before and after the controlled flood.  That time series does not provide information about the life span of these newly deposited and reconfigured sandbars.

·        Additional research questions deal with how long the new backwater areas last under regular dam operations, whether or not juvenile humpback chub are utilizing these new backwaters to a greater extent due to the increase in overall area, and what the ecological value of the habitat is in terms of food sources, i.e. algal growth and invertebrate biomass.

 

 

 

Figure 10.  Humpback chub, Gila cypha.  Endangered fish species native to the Colorado River Basin.
 
 

 

 

 

 

 

 


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Acknowledgements

 

My enormous thanks to

·        Hoda Sondossi - Department of Geography and Earth Resources, Utah State University, Logan UT

·        Jack Schmidt - Department of Geography and Earth Resources, Utah State University, Logan UT

·        Matthew Wilson - Department of Romance, Home Life and Food Consumption, Logan, UT

 

 

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References

 

Patten, Duncan T., David A. Harpman, Mary I. Voita, and Timothy J. Randle.  A Managed Flood on the Colorado River: Background, Objectives, Design, and Implementation.  Ecological Applications, 11(3): 635-643, 2001.

 

 

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