Recent Vegetation Changes Along the Colorado River Between Glen Canyon Dam and Lake Mead, Arizona Recent Vegetation Changes Along the Colorado River Between Glen Canyon Dam and Lake Mead, Arizona By RAYMOND M. TURNER and MARTIN M. KARPISCAK GEOLOGICAL SURVEY PROFESSIONAL PAPER 1132 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Turner, Raymond M. Recent vegetation changes along the Colorado River between Glen Canyon Dam and Lake Mead, Arizona. (Geological Survey professional paper ; 1132) Bibliography: p. 22-24. 1. Botany-Arizona-Ecology. 2. Botany-Colorado Valley-Ecology. 3. Vegetation dynamics-Arizona. 4. Vegetation dynamics-Colorado Valley. I. Karpiscak, Martin M., joint author. II. Title. III. Series: United States. Geological Survey. Professional Paper ; 1132. QK147.T87 581.5 79-25928 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03269-4 CONTENTS Page Page List of common plant names and scientific equivalents ________ IV Distribution of major plant species—Continued Abstract _______________________________________ 1 Rabbitbrush ______________________________16 Introduction ____________________________________ 1 Desert broom._______________-________16 Acknowledgments ____________________________________ 3 Waterweed ______________—____________ ————— _17 Changes in Colorado River streamflow regime ________ 3 Seep willow __________——— ——— ——— ______ —— —— _17 Floods _________________________________ 3 Emory seep willow __________________ ____17 Daily stage.__________________________ 6 Apache plume _________________________________17 Annual discharge_________________________ 7 Netleaf hackberry __________________________ ———17 Monthly discharge __________ _______________________? Redbud ________________________________________—17 Changes in channel and alluvial deposits in the Colorado Cattail________________________________17 River below Glen Canyon Dam ______________________ 8 Reed——___________________________ _________________ -18 A history of photography on the Colorado River __________ 10 Spiny aster________________________ ————— ___18 Vegetation _______________________________________12 Sandbar willow _______________ —— ——— __ — _ ————— _18 Distribution of major plant species ___________________13 Goodding willow _______________________-18 Bermuda grass ________________________________14 Desert isocoma ____________________— —— ———18 Russian olive __________________________________14 Photographic documentation of changes ____———__ ——— ———18 Saltcedar____________________________________ 14 Summary of changes ______________________19 Elm __________________________________15 Zone of postdam fluvial sediments ____________—19 Camelthorn _________________________________15 Zone of predam fluvial sediments _______________—_19 Catclaw _________________________________15 Zone of predam flood terraces, eolian deposits, and Western honey mesquite ____________________15 stabilized talus slopes __________________21 Fremont cottonwood ____________________________16 General conclusion ____________________—-21 Arrowweed____________________ _____________16 References cited __________________- — ————— -22 Longleafbrickellia __________________________________ 16 ILLUSTRATIONS Page PLATE 1. Map showing Colorado River from Glen Canyon Dam to Lake Mead. _____________________ — — —— ___In pocket 2. Maps showing distribution of selected plant species along the Colorado River. [Includes figs. 7-15.] ___ ————In pocket 3. Maps showing distribution of selected plant species along the Colorado River. [Includes figs. 16-25.] _ —— — __In pocket FIGURE 1. Graph showing yearly range between minimum daily and maximum discharge and yearly range between minimum and maximum stage of Colorado River at Lees Ferry and near Grand Canyon _______-_ ——— — 4 2. Graph showing daily variation in river stage during two selected years, Colorado River at Lees Ferry ___ ——— ——— 6 3. Graph showing total annual streamflow (by calendar year) of Colorado River as recorded at Lees Ferry and near Grand Canyon ______________________________________- ——— ———— 7 4. Graph showing mean monthly discharge as a percentage of total annual discharge, Colorado River, Lees Ferry _ ___ 8 5. Graph showing regression analyses of annual (water year) discharge versus sediment yield _______ — — ———— _ 9 6. Graph showing accumulated volume of degradation below Glen Canyon Dam for various periods between 1956 and 1975 _____________________________________________________-______ 10 7-25. Maps showing distribution of selected plant species along the Colorado River from Glen Canyon Dam to Lake Mead: 7. Bermuda grass and elm ________________________ ———— ———————— ——— ——— _- —— -Plate 2 8. Russian olive and desert broom __________________________~——— — — — ———— - ——Plate 2 9. Saltcedar _________________________________________________________________——————-—Plate 2 10. Camelthorn and Apache plume ________________________ ——— ———————————————Plate 2 11. Catclaw._________________________________________________————————Plate 2 12. Western honey mesquite _____ _____________________ _———————————————————Plate 2 13. Fremont cottonwood __________________________________________———————Plate 2 14. Arrowweed ____________________________________________—————————————Plate 2 15. Longleaf brickellia ________________ ________________________________ ______________________—_„__ Plate 2 16. Rabbitbrush and waterweed _____________________—_ ——— ______ ——————— ____—Plate 3 III IV CONTENTS FIGURE 17. Seep willow _______________________________________________________Plate 3 18. Emory seep willow ______________________________________ _______________Plate 3 19. Netleaf hackberry ___________________________________________________________Plate 3 20. Redbud and desert isocoma _____________________________________________ ——— __————Plate 3 21. Cattail ______________________________________________________________Plate 3 22. Reed ___________________ ________________________________________Plate 3 23. Spiny aster _________________________________________________________Plate 3 24. Sandbar willow _____________._________________________ _______________Plate 3 25. Goodding willow ______________________________________________________Plate 3 26-73. Matched photographs showing changes in vegetation ___________________________— — — __ — — ____26-125 TABLES Page TABLE 1. Mean, standard deviation, and coefficient of variation of yearly maximum stage for the Colorado River at Lees Ferry and near Grand Canyon _________________________________________^________——— 5 2. Mean, standard deviation, and coefficient of variation of yearly minimum stage for the Colorado River at Lees Ferry and near Grand Canyon.________________________________ —— __ —— __ —— _ — _______ 5 3. Camera station descriptions, including dates, location, altitude, and photograph credits ——_____ —— ______ —— —— _ 20 LIST OF COMMON PLANT NAMES USED AND SCIENTIFIC EQUIVALENTS agave Agave spp. longleaf brickellia Brickellia longifolia Wats. Apache plume Fallugia paradoxa (D.Don) Endl. mistletoe Phoradendron californicum Nutt. arrowweed Pluchea sericea (Nutt.) Coville monkey flower Mimulus cardinalis Dougl. barrel cactus Ferocactus acanthodes (Lemaire) Britt. Mormon tea Ephedra trifurca Torr. & Rose netleaf hackberry Celtis reticulata Torr. beavertail cactus Opuntia basilaris Engelm. & Bigel. ocotillo Fouquieria splendens Engelm. bebbia Bebbia juncea (Benth.) Greene peppergrass Lepidium montanum Nutt. Bermuda grass Cynodon dactylon (L.) Pers. poison ivy Rhus radicans L. brittle bush Encelia farinosa Gray rabbitbrush Chysothamnus spp. camelthorn Alhagi camelorum Fisch. range ratany Krameria parvifolia Benth. carrizo (see reed) Phragmites communis Trin. red brome Bromus rubens L. catclaw Acacia greggii Gray redbud Cercis occidentalis Torr. cattail Typha spp. red willow Salix laevigata Bebb. coldenia Coldenia spp. reed (see carrizo) Phragmites communis Trin. creosote bush Larrea tridentata (DC.) Coville Russian olive Elaeagnus angustifolia L. desert broom Baccharis sarothroides Gray Russian thistle Salsola kali L. var. tenuifolia Tausch desert isocoma Haplopappus acradenius (Greene) Blake saltcedar Tamarix chinensis Lour. desert plume Stanleya pinnata (Pursh) Britton sandbar willow Salix exigua Nutt. desert trumpet Eriogonum inflation Torr. & Frem. sand verbena Abronia elliptica A. Nels. dicoria Dicoria spp. scouring rush Equisetum hiemale L. dogbane Apocynum spp. seepweed Suaeda torreyana Wats. dropseed Sporobolus spp. seep willow Baccharis glutinosa Pers. dyssodia Dyssodia spp. shrub liveoak Quercus turbinella Greene elm Ulmus minor Mill. slender poreleaf Porophyllum gracile Benth. Emory seep willow Baccharis emoryi Gray smooth horsetail Equisetum laevigatum A. Braun evening primrose Oenothera pallida Lindl. spiny aster Aster spinosus Benth. four-wing saltbush Atriplex canescens (Pursh) Nutt. watercress Rorippa nasturtium-aquaticum (L.) Fremont cottonwood Populus fremontii Wats. Schinz & Thell globemallow Sphaeralcea spp. waterweed Baccharis sergiloides Gray Goodding willow Salix gooddingii Ball western honey mesquite Prosopis glandulosa var. torreyana (Ben- great bulrush Scirpus validus Vahl. son) Johnston hackberry Celtis reticulata Torr. white bursage Ambrosia dumosa (A. Gray) Payne horsetail Equisetum spp. willow Salix spp. horseweed Conyza canadensis Cronquist wolfberry Lycium spp. jointfir Ephedra torreyana Wats. wire lettuce Stephanomeria pauciflora (Torr.) A. Nels. RECENT VEGETATION CHANGES ALONG THE COLORADO RIVER BETWEEN GLEN CANYON DAM AND LAKE MEAD, ARIZONA By RAYMOND M. TURNER and MARTIN M. KARPISCAK ABSTRACT Mississippi—less than the flow of such well-known riv­ Vegetation changes in the canyon of the Colorado River between ers as the Snake, the Missouri, and the Potomac, and Glen Canyon Dam and Lake Mead were studied by comparing photo­ less even than such little-known rivers as the Atch- graphs taken prior to the completion of the Glen Canyon Dam in afalaya (Louisiana), the Skagit (Washington), or the 1963 with those taken afterwards at the same sites. The old photo­ graphs, taken by J. K. Millers, T. H. O'Sullivan, William Bell, F. A. Apalachicola (Florida). Yet, if measured in terms of its Nims, R. B. Stanton, N. W. Carkhuff, N. H. Darton, L. R. Freeman, impact on regional and national requirements for rec­ E. C. LaRue, and others, document conditions as they were between reation, energy, and water, it is, for its size, of dispro­ 1872 and 1963. In general, the older pictures show an absence of portionate importance. Many of our national parks and riparian plants along the banks of the river. The new photographs of monuments lie within its scenic basin and the Colorado each pair were taken in 1972 through 1976. The most obvious vege­ tation change revealed by the photograph comparison is the in­ River is the erosional force that shaped the Grand creased density of many species. Exotic species, such as salteedar and Canyon, one of the great scenic wonders of the world camelthorn, and native riparian plants, such as sandbar willow, ar- and the main attraction in one of the more heavily rowweed, desert broom, and cattail, now form a new riparian com­ visited National Parks. The Colorado River serves as a munity along much of the channel of the Colorado River between major power source for the region's cities and indus­ Glen Canyon Dam and the Grand Wash Cliffs. The matched photographs also reveal that changes have occurred tries, and as a major water source for its domestic and in the amount of sand and silt deposited along the banks. The photo­ agricultural users. When measured in terms of the im­ graphs show that in some areas erosion has been significant since the pact it has upon the daily lives of the many human time of the earlier photograph while at other locations sediment has occupants of the arid southwestern United States and accumulated on river bars and terraces. northwestern Mexico, the Colorado River's presence Detailed maps are presented showing distribution of 25 plant species. Some of these, such as Russian olive and elm, were unknown must be accorded far more importance than would be along the Grand Canyon reach of the Colorado River before 1976. ascribed to it on the basis of discharge alone. Relevant data are presented to show changes in the hydrologic Until recently man was not a major factor affecting regime since completion of Glen Canyon Dam. Flooding, as expressed the vegetational and fluvial features along the Col­ by annual maximum stage, has decreased in amplitude, and its sea­ orado River within the Grand Canyon. But during the son of occurrence has changed from spring (May-June) to a longer period from April through October. Dam construction has had a last few decades the river has become one of the most- moderating influence on several other hydrologic variables. Com­ used rivers in America—both in terms of recreational pared to the predam era, discharge through the year now varies use and in terms of water consumption. To provide a within narrow limits, changing little from month to month or season perspective for viewing the changes along the river, to season; annual maximum discharges are now strikingly uniform, some examples are cited. During the first 86 years of and sediment load has materially decreased. Increases have occurred in some characteristics, however, such as daily variation in river river travel through the Grand Canyon (.1869-1955), stage and median discharge. only 185 persons traversed the canyon by boat (Wal­ The interaction of decreased flooding, decreased sediment load, lace, 1972); in the 1970's roughly 15,000 persons travel and increased riparian plant coverage makes the future of existing through the canyon by boat each year (Larson, 1974). river fans, bars, and terraces uncertain. The establishment of a new Coupled with the heavy recreational use has been an ecological equilibrium at the bottom of the Grand Canyon may re­ quire many decades. increasingly heavy use of the river's water. By the con­ struction of the four units of the Colorado River Stor­ age Project (Flaming Gorge Dam [1962], Navajo Dam INTRODUCTION [1962], Curecanti Unit dams [1965-66], and Glen Can­ yon Dam [1963]) in the Upper Colorado River Basin, When viewed as a water conveyance system, the man has been able to control the flow in the river, Colorado River is unspectacular. Its long-term average curbing the spring floods and distributing throughout annual discharge is 16,600 hm3 (Stockton and Jacoby, the year the water normally carried during these an­ 1976), only about one-thirtieth the flow of the nual events of high flow. The modifications in flow re- VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA gime have disrupted the equilibrium which formerly station, is taken as kilometer 0. All distances are based existed within the Grand Canyon, the reach of the upon this datum. Distances upstream are taken from river of primary concern in this report. But even before Glen Canyon National Recreation Area map (National major dams appeared in the Upper Colorado River Park Service, no date). The metric system is given Basin, the Lower Colorado River was impounded by a preference in this report; accordingly, distances are series of dams beginning with Hoover Dam (1935). By given in kilometers. Convention and long usage has far the greatest diversions from the river occur from firmly established proper names along the river such these impoundments lying below the Grand Canyon. as Seventyfive Mile Rapid and Two Hundred and Before the first diversions were made, approximately Ninemile Canyon. Where these place names appear 16,600 hm3 (Stockton and Jacoby, 1976) of water they are used alone without metric equivalents. For reached the Sea of Cortez (Gulf of California) annually. the few localities lying upstream from kilometer 0, dis­ Today, less than 1 percent of its virgin flow ever tance will be shown as kilometers above Lees Ferry. reaches the mouth of the Colorado River. The long-established convention of referring to river- Glen Canyon Dam probably has had greater impact banks as left or right when viewed downstream is fol­ on the riparian habitat within the Grand Canyon than lowed here. The altitude given in the captions refers to the combined effects of all other river system altitude at river level, regardless of the camera's posi­ modifications in the Upper Basin. It is difficult to con­ tion. ceive of a change in regional climate of sufficient mag­ Before examining the changes, certain features of nitude to reduce average annual maximum flows from the Colorado River and the canyon through which it 2,486 m3/s to 803m3/s, to increase the median discharge flows should be reviewed. The reach of the Colorado from 210 m3/s, to increase the average diurnal fluctua­ that we examine in this report lies between Glen Can­ tion in stage from a few centimeters to several meters, yon Dam, 26 km above Lees Ferry, Ariz., and Pearce to reduce the average annual water temperature from Ferry, Ariz., at kilometer 450 below Lees Ferry (pi. 1). a range of 0.2° to 28°C during the predam period This segment of the river is only a small reach of the (1949-1962) to a range of 5.5° to 18°C during the post- 2,700-km-long river, yet because it traverses the dam period (1962-1976), and to simultaneously reduce Grand Canyon, it is the best known and most famous the median sediment concentration from 1,500 to 7 portion. Strictly speaking, the Grand Canyon extends parts per million (ppm). Yet the foregoing changes from the mouth of the Paria River near Lees Ferry have all been recorded at Lees Ferry, 26 kilome­ downstream to the Grand Wash Cliffs; the reach above ters downstream from Glen Canyon Dam. Each of the Paria is part of Glen Canyon. The Grand Canyon is these changes, and others, has had an effect upon the 21 km across at its widest point, with a depth of as riparian ecosystem since the completion of Glen Can­ much as 1,800 meters. Various divisions of the Grand yon Dam, and inevitably, adjustments in the biota and Canyon have long been recognized: These include such the physical setting have occurred. Although estab­ reaches as Marble Canyon, Conquistador Aisle, and lishment of a new ecological balance requires many Upper, Middle, and Lower Granite Gorges. years in response to the new fluvial regime, sufficient In its course through the Grand Canyon, the Col­ time has elapsed since the dam's completion in 1963 to orado River moves through a narrow valley and is con­ reveal many vegetational and riverine shifts. fined by steep, high walls of mostly hard and resistant The study was undertaken to determine the nature of rock. There is no flood plain along much of the Col­ these transformations and to provide a basis for pre­ orado's course through the Grand Canyon, and the ab­ dicting future trends. Conditions existing prior to the sence of a flood plain broad enough to reduce the force regulation of flow in the Colorado River were estab­ of annual floods produced a predam valley devoid of the lished by examining photographs and hydrologic rec­ dense riparian community typical of other streams of ords made between 1872 and 1963. Postdam conditions the region. If one ignores the alternating pools and were documented by referring to recent hydrologic rec­ rapids, the river profile is smooth and nearly straight ords, by photographically matching scenes shown in (Leopold, 1969). The river gradient is under dominant the predam pictures, and by recording in detail the control of the tributary fans. The river's tendency to distribution of riparian plant species. move laterally is greatly reduced compared to rivers in In the following material, frequent reference will be unconfined channels (Leopold, 1969), and the river's made to localities along the Colorado River. In keeping poorly understood proclivity for vertical entrenchment with common practice, "mileage" designations rather than lateral movement has served to maintain downstream from Lees Ferry will follow those estab­ through millions of years the narrow gorge that today lished by the U.S. Geological Survey in 1923 (Birdseye is viewed annually by nearly 3 million people. and Burchard, 1924). Lees Ferry, at the stream gaging Although no river width and depth data exist for the CHANGES IN COLORADO RIVER STREAMFLOW REGIME entire reach described in this report, data are available Martin was an early observer of postdam vegetation for selected shorter segments (Leopold, 1969). The changes, and his shared insight and knowledge of the available depth measurements made in 1963 represent area is gratefully acknowledged. Finally, the patience predam conditions with a flow of 1,375 m3/s. Through of our companions on each of six photograph-matching the first 223.7 km below Lees Ferry, maximum river trips, as they waited while we tried to find the exact depth was 33.5 m (kilometer 183.9). Roughly 20 per­ camera position of some early-day photographer, is ac­ cent of the depth measurements, taken at 0.16-km knowledged, with appreciation intervals, equaled or exceeded 15.5 m, and 50 percent equaled or exceeded 11.0 m. Width measurements have CHANGES IN COLORADO RIVER STREAMFLOW been taken at 0.16-km intervals from 1965 aerial pho­ REGIME tographs for the reach between kilometer 45.1 and kilometer 177.0 (Leopold, 1969). Although only gen­ Construction of a dam across a river produces many eralized width values were given, these provide a broad changes in the hydrologic regime of the river system picture of river conditions. For example, of the approx­ both above and below the structure. The changes im­ imately 800 measurements, fewer than 5 percent were mediately upstream from the dam, such as water im­ less than 61.0 m, 50 percent of the observations poundment and silt accumulation are often the most equaled or exceeded 97.5 m, and 20 percent equaled or striking. The more subtle downstream hydrologic exceeded 125.0 m. In general terms this is the river modifications include smoothing the flow duration examined here. In following chapters specific charac­ curve, lowering maximum stages, and increasing base teristics of the river will be discussed with particular flow (Leopold and others, 1964). The downstream al­ emphasis on the impact of Glen Canyon Dam. terations in the discharge regime may directly affect riparian biotic communities. In the present chapter, pertinent streamflow records for the periods before and ACKNOWLEDGMENTS after construction of Glen Canyon Dam are presented Completion of this study required the assistance of as a basis for interpreting the vegetation changes that many people. We owe a major debt of gratitude to the will be noted in later sections. In the chapter following staff of the Museum of Northern Arizona, who provided this, we have also examined channel changes and some transportation and support of many kinds during the of the causes for the altered channel geometry. course of the study. Support by the Museum included FLOODS financial support for the junior author during the early months of his work on the study. Dr. Steven Carothers' Before the construction of dams along the Colorado knowledge of the Grand Canyon ecosystem and his River, flooding was commonplace. One of the better skill as a boatman were substantial contributions dur­ known floods occurred in November 1905, when the ing several trips through the canyon. Others from the Colorado River left its old channel via a manmade Museum to whom we are especially indebted include canal and flowed into the Salton Sink, thus forming the Dr. A. M. Phillips III, Dr. B. G. Phillips, Mr. George Salton Sea (Sykes, 1937). The river was not returned to Ruffner, Mr. S. H. Aitchison, and Mr. D. S. Tomko. To its original channel until February 1907 (Grunsky, the Phillips' we owe a special debt of gratitude for their 1907; LaRue, 1916). With completion of a series of assistance in augmenting our plant distribution data. dams along the lower Colorado River, a recurrence of We also wish to express our appreciation for the skill this event is unlikely. Similarly, floods through the and assistance of Mr. R. A. Heinz and Ms. Carroll Ben- Grand Canyon have been curtailed by the construction nett in obtaining some of the photographic matches. of Glen Canyon Dam. Librarians at the New York Public Library, Manu­ A river characteristic that is closely associated with script and Archives Division; the U.S. Geological Sur­ flooding is annual maximum stage (fig. L4). Stage rec­ vey Photographic Library, Denver; and Grand Canyon ords for the Colorado River at Lees Ferry and near National Park were important contributors aiding us Grand Canyon, Ariz., have been used to illustrate gen­ in the search for old photographs. The National Park eral changes in maximum stage throughout the reach Service has been a major collaborator, providing assis­ of the Colorado River examined in this report. (The tance in many ways, including the opportunity to ac­ stream gage "near Grand Canyon" is 0.4 km upstream company a Park Service crew on a boat trip through from Bright Angel Creek and 7.5 km northeast of the canyon, as well as providing boat transportation Grand Canyon, Ariz.) Because of variations in channel between Lees Ferry and Glen Canyon Dam. To Messrs. and valley geometry, the values provide only a relative M. A. Turner, M. S. Pierce, and Thomas Workman we measure of the height to which banks might be inun­ owe thanks for assistance while in the canyon. Dr. P.S. dated by flood waters. VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA 9500 ~1 I | \ | — 9000 A EXPLANATION ° Lees Ferry 8000 — 1 ~~ Oz 1 0 0« ft 7000 — ' Near Grand — CO Canyon DC LU 0co. 6000 — — DC LU LU o 5000 — — m DO • . 1 1 O-LD<UC 4000 — I I I 11 '1 i! iii iii '1 111i1 !|i , ~~ OCO 3000 — i I I i h I II 1 Q ! II 11 1 M hi li 1 , 1 1 11 , i'' 1i i - 2000 I 1 I i, ii i i 1 • I 1 I 1M Mn IN ii 1 1 1 1 H 1 I'lblilSIB 1000 — I I 1 1 1 1 Ml | 1 ||||t Hllllll|l| ||H|I|I| _ It 1895"1920 1930 1940 1950 1960 1970 198G WATER YEAR 12 n I • I | | _ _ B EXPLANATION o o1 Lees Ferry 1 1 • __ o • Near Grand 1 Canyon — — 1 — I I I 1 i ll — I II ' '.I '! 1 1 M — IIIII !,!! lilu1 l | lli| IM1!1' iii 1I I1 Iiii ii Ii'!Il l : ii i ii 111 II II 111 111 1 iI :: I MI i 1 II Ill i i ii 1 1 1 i 1 1 — -1 -/ <__ 1895 1920 1930 1940 1950 1960 1970 1980 WATER YEAR FIGURE 1.—Yearly range between minimum daily and maximum discharge (A) and yearly range between minimum and maximum stage (B) of Colorado River at Lees Ferry and near Grand Canyon. Excluding 1965, an anomalous year, the yearly (mean = 3.48 m; std. dev. = 0.19 m). (The high dis­ maximum stage (fig. 1A; table 1) has varied little at charge in 1965 resulted from the release of water Lees Ferry since Glen Canyon Dam was completed through a diversion tunnel and hollow jets as a means CHANGES IN COLORADO RIVER STREAMFLOW REGIME TABLE 1.—Mean, standard deviation, and coefficient of variation of TABLE 2.—Mean, standard deviation, and coefficient of variation of yearly maximum stage for the Colorado River at Lees Ferry and yearly minimum stage for the Colorado River at Lees Ferry and near Grand Canyon near Grand Canyon [Data, based on water years, are values for the total period of record, the predam period, and [Data, based on water years, are values for the total period of record, the predam period, and the postdam period] the postdam period] Period of record Predam Postdam Period of record Predam Postdam (1921-76) (1921-62) (1963-76) (1963-76, (1922-76) (1922-62) (1963-76) (1963-76, excl. 1965)' excl. 1965)' Lees Ferry Lees Ferry Number of years _____________56 42 14 13 55 41 14 13 _____________ 4.67 5.04 3.56 3.48 _____________ 1.68 1.76 1.46 1.47 Standard deviation (meters) ____________ 1.06 .96 .35 .19 _ _ _ _ .21 .17 .15 .15 _____________ .23 .19 .10 .06 _____________ .12 .09 .10 .10 Near Grand Canyon Near Grand Canyon (1923-76) (1923-62) (1963-76) (1963-76, (1924-76) (1924-62) (1963-76) (1963-76, excl. 1965)' excl. 1965)' _____________54 40 14 13 53 39 14 13 4.79 4.69 _____________ .52 .46 .67 .70 Standard deviation (meters) ________ _ _ 1.57 1.40 .85 .79 ____________ .31 .23 .45 .45 .18 .17 ___________ .61 .50 .68 .65 'The year 1965 was anomalous. See text. 'The year 1965 was anomalous. See text. of fulfilling downstream commitments before all the postdam period is slightly greater than the smallest generators were in operation. Since 1965, with all gen­ maximum value (3.37 m), recorded in 1934, for any erators operating, flow has been sufficient to meet predam year (fig. L4). Mean values for the Colorado downstream requirements (A.O. Dewey, U.S. Bureau River near Grand Canyon are also given in table 1 and of Reclamation, oral commun., 1975).) A similar but show that as a result of dam construction, mean stage less marked stability is apparent in the record from has fallen more than at Lees Ferry. This difference near Grand Canyon (mean = 4.69 m; std. dev. = 0.79 largely results from differences in valley and channel m). Annual minimum stage (fig. LA; table 2) appears configuration at the two sites. little changed at Lees Ferry (mean = 1.47 m; std. dev. The coefficients of variation (CV) of yearly maximum = 0.15 m) although the record from near Grand Can­ stages (table 1) during these two periods emphasize yon (mean = 0.70 m; std. dev. = 0.45 m) shows a slight further the postdam stability of streamflow. This increase in these minimal values.' (This increase is the statistic is a measure of the magnitude of the standard result of a December 1966 flood on Bright Angel Creek deviation relative to its mean. At Lees Ferry, for the which deposited new bouldery debris in the Colorado 42-year period before 1963, the CV is 0.19; for the 13- River channel, altering the control for the Colorado year postdam period, it is 0.06 (table 1). Near Grand River gaging station (Cooley and others, 1977).) At Canyon the CV's for the predam and postdam periods both gaging stations the range between annual max­ were 0.20 and 0.17, respectively. The decrease in CV is ima and annual minima has been narrowed, especially far less near Grand Canyon than at Lees Ferry, a fact because of reduced maxima. Thus, the effect of flowing largely attributable to flow from the Little Colorado water upon shore-zone plants is now characteristically River which enters the Colorado River between the two confined to a rather narrow band at these stations and gaging stations. Flow in the Little Colorado is erratic, presumably elsewhere in the canyon. there being no large dams along the river, and the The maximum stages in figure 1A clearly show the unregulated streamflow during flood stage is great reduction in the streamflow amplitude after comple­ enough to affect the flow of the Colorado mainstem. tion of Glen Canyon Dam. The stage, which had Prior to 1963, maximum discharges (fig. IB) were reached 11.43 m near Grand Canyon in 1921, has not almost always greater at Lees Ferry than farther exceeded 6.07 m since 1963. Proportional changes in downstream near Grand Canyon. This downstream de­ the maximum stage have also been recorded at Lees crease in maximum discharge probably occurred be­ Ferry. cause of channel storage along the 140.8-km channel Means based on the predam and postdam records are from Lees Ferry to the gage near Grand Canyon. In shown in table 1. The mean maximum stage at Lees almost all cases, the peaks at Lees Ferry and near Ferry for the postdam period is 3.48 m, excluding the Grand Canyon occurred during the same runoff event. anomalous values for 1965; the mean for the period Since completion of Glen Canyon Dam, annual peak prior to the dam is 5.04 m. The mean value for the flows at these stations have not been temporally corre­ lated, and annual peak flows near Grand Canyon have usually been greater than those at Lees Ferry. Before 'These values have been obtained, in part, from unpublished data, including recording 1963, annual peak flows near Grand Canyon exceeded charts. When no stage was recorded, estimates were made from available data.