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6 Organisms and Biological Processes INTRODUCTION Organisms and biological processes were studied through Glen Canyon Environmental Studies (GCES) from three different perspectives: (1) as ecosystem components, (2) as resources of specific economic or recreational importance (trout), and (3) as a means of satisfying the requirements of the Endangered Species Act (humpback chub, Kanab ambersnail, bald eagle, southwestern willow flycatcher). These three perspectives, which were com- bined in the GCES study plan, had somewhat different objectives. Thus, the efforts of GCES were to some extent divided in three directions. In effect, ecosystem analysis, which was the unifying theme for GCES, was partially redirected by special concern over particular species. While the division of priorities is easily understandable in view of the societal value attached to trout and the legal requirements attached to endangered species, GCES was often diverted from its goal of understanding the entire system by strong focus of its resources on particular components of the system. This problem is inherent in government studies of ecological systems and can work against successful ecosystem analysis and prediction, as will be apparent from this chapter. The biological work of GCES can be divided into four parts: (1) lake studies, (2) studies of the Colorado River between the Glen Canyon Dam and the Paria River (26 km), (3) studies of the Colorado River below the Paria to Lake Meacl (450 km), and (4) studies of the riparian zone. These components are connected, of course, but each has distinctive communities and biological processes. 84
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Organisms and Biological Processes LAKE POWELL Characteristics of Lake Powell 85 Impoundment of the Colorado River by Glen Canyon Dam has created a qualitatively new kind of water source for the Colorado River below. Char- acteristics of Lake Powell, together with operation of the dam, determine the temperature, suspended and dissolved solids, nutrients, and organisms passing downstream. Thus, any comprehensive assessment of the Colorado River below Glen Canyon Dam must include Lake Powell (Figure 6.1~. Lake Powell has been studied by several organizations and individuals over more than 20 years. Stanford and Ward (1991) provide an excellent interpretive summary of work completed through 1990. The Bureau of Rec- lamation (BOR) has supported a range of studies on Lake Powell but has taken particular interest in its effect on the mean salinity of water in the Colorado River (BOR, 1987~. The National Science Foundation supported a decade-long system study of the lake that encompassed water quality, seasonal cycles, productivity, and aquatic community composition. This work is well summarized by Potter and Drake (1989~. Various individual projects beginning in the late 1 960 s have dealt with components of the biota or specific aspects of water quality (Carothers and Brown, 1991~. Thus, the starting point for GOES with respect to Lake Powell was an extensive but somewhat amorphous information base. Routine GOES studies of the res- ervoir did not begin until 1992, when quarterly synoptic surveys were initiated (unpublished as of 1995) and focused studies were initiated on layering of the upper water column and vertical distribution of oxygen (Marzolf, 1995~. Although the filling of Lake Powell began in 1963, the reservoir did not show strong development of stratification until about 1970 and did not fill to capacity until 1980. Thus, in a physical sense, the history of the reservoir can be divided into three intervals: 1963 to 1970, 1970 to 1980, and 1980 to the present. Optimal operation of the reservoir from the viewpoint of power production would require all water to pass through the generators at Glen Canyon Dam, and this was possible until 1980. Since 1980, however, operations entered a new era in which high runoff can require the release of water for the purpose of protecting the dam (Chapter 4~. For example, use of the dam's bypass system was necessary in 1983 in order to prevent water from passing over the dam. Releases above power plant capacity are called floods, although they differ in frequency and magnitude from natural floods.
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86 ~ Z He's .~ - _ ~ ~ r · . ~ Ol In' =~5 Jo ~ O Y ~ C O C l z r 0: L.. En z o ED a: En E~ 3 Car A:: a: \ I c,, it) I 7~ ~,_~ - ~ _ ~_ ~ ~O v a: LO 0~ Cal En ._ Cal - 3 o ELI
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Organisms and Biological Processes 87 A succession of dry years could, of course, suppress the mean volume of the reservoir well below capacity. The reservoir might then operate for a long time with no floods, as it did between 1963 and 1983. Although Lake Powell receives a very large amount of water from the Colorado and San Juan rivers, as well as a small amount from the Dirty Devil River, the large size of the reservoir allows a mean water retention time of approximately 2 years (Table 6.1~. The residence time of the reservoir is sufficiently long to allow complete sedimentation of the inorganic material that enters it at the upper end. Lake Powell receives much sediment from its water sources (Dawdy, 1991 ) but, because of its great volume, has lost only a small proportion of its storage capacity through sediment accumulation. A number of Lake Powell's features are characteristic of reservoirs in general, while others are more unusual. Like many reservoirs, Lake Powell has a dendritic shape and is much longer than broad, thus presenting the possibility of great longitudinal variations in water quality or biotic char- acteristics. Dendritic shape is accompanied by a high degree of shoreline development (length of shoreline). Unlike most reservoirs, however, Lake Powell has an essentially vertical shoreline around much of its perimeter. This characteristic minimizes the amount of shallow water and the potential for the development of macrophytes or other littoral communities and reduces the importance of littoral biogeochemical processes. As would be expected for most reservoirs, the water level of Lake Powell fluctuates by several meters per year (8 m on average) and from year to year (potentially 30 m or more). Because the water withdrawal point for Lake Powell is fixed, the depth of withdrawal relative to the water surface fluctuates seasonally and also changed as the lake filled between 1963 and 1980. Recently, water has been mostly drawn from a zone 50 to 70 m below the surface, depending on season and year. Lake Powell is divided throughout all seasons into two chemically distinct zones separated by a salinity gradient. The lower zone is consistently cold (about 7°C) and has about 50 percent more dissolved solids than the upper layer (Figure 6.2~. The upper zone shows a seasonal thermal cycle that is characteristic of a warm monomictic reservoir with a substantial amount of water withdrawal. This upper zone is warmest near the surface (exceeding 25°C in the summer) and coolest nearthe junction with the lower layer (about 7°C). Division of the upper zone into clearly identifiable layers that could be designated epilimnion, metalimnion, and hypolimnion would probably occur if it were not for the continuous withdrawal of large amounts of water from the
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88 River Resource Management in the Grand Canyon TABLE 6.1 Characteristics of Lake Powell Within 50 km of Glen Canyon Dam Item Amount Source Surface area, km2 Maximum depth, m Mean depth, m Typical depth of water withdrawal, m Approximate water retention time, years Maximum surface temperature, ° C Minimum surface temperature, ° C Botto m te m p e ratu re, ° C Suspended solids, mg/liter Total phosphorus,,ug/liter Dissolved phosphorus,,ug/liter Nitrate-N, ,ug/liter Soluble silica (S;O2), mg/liter Total dissolved solids, mg/liter Primary production, gC/m2/year Chlorophyll a,,ug/liter 653 171 51 55 2 28 7 15 10 300 7 630 190 Zooplankton abundance, individuals per liter 20 Potter and Drake (1989) Potter and Drake (1989) Potter and Drake (1989) GCES (unpublished) Potter and Drake (1989) GCES (unpublished) GCES (unpublished) GCES (unpublished) Stanford and Ward (1991) Gloss et al. (1980) Gloss et al. (1980) Gloss et al. (1980) Gloss et al. (1980) Gloss et al. (1981 ) Hansmann et al. (1974) Paulson and Baker (1983) and Sollberger et al. (1 9891a Sollberger et al. (1989) °Possibly much higher; see Ayers and McKinney (1995a). NOTE: Characteristics are for the upper 50 m of the water column unless otherwise specified. bottom of this zone and the large additions of water at the upper end of the reservoir. Continual voluminous water exchange of this type often blurs the boundaries between thermal layers in reservoirs. In Lake Powell the distinc- tion between metalimnion and hypolimnion istypicallyunclearforthis reason. The summer epilimnion (mixed layer) is typically well defined, however, and the overall thermal gradient causes stability of the water column during the summer. Wind-generated mixing of the entire upper zone (50 to 70 m) occurs only during winter. Thus, the lake shows summer stratification in the upper zone, even though it has a broad thermal gradient resulting from water exchange and a lower zone that does not mix fully at any time with the upper zone. Division of the water column into two zones is explained by seasonal fluctuations in water temperature and salinity. Water entering the reservoir during the winter is not only cold but also of high salinity relative to the water derivecl from snowmelt in early summer (Stanford and Ward, 1991~. During the winter, water entering Lake Powell flows into the bottom zone of the res
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Organisms and Biological Processes Temperature, °C 0 10 15 20 25 30 O ~' ' , E o - a) O o o - Q a) o O _ If Feb '95 i Specific Conductance, ~mholcm 600 700 ° ! ' ' ' , 900 1 000 1 1 00 Dissolved Oxygen, mg/liter 0 2 4 6 8 - o E s c, o O ./ f 89 Temperature, °C o E o a) ~ O 0 10 15 20 25 30 l ( Aug '94 Specific Conductance, ,umho/cm 600 700 800 900 1000 1100 O1 ~ 1 1 1 - Q a) 10 0 - a, O O _ it_ AL O 0, - Dissolved Oxygen, mg/liter 2 4 68 10 , , ,~ O ~ O _ O - f ·: FIGURE 6.2 Profiles of temperature, conductance, and dissolved oxygen for Lake Powell near the dam at Waheap (unpublished data from the BOR). (umho/cm = micromhosJcentimeter) ervoir and thus maintains the higher salinity of the bottom zone. During the summer, the Colorado River brings warm, less saline water derived mostly from snowmelt. This water enters the upper zone, which matches its density range. Recent studies by Marzolf (1995 and personal communication) in
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go River Resource Management in the Grand Canyon dicate that the summer inflow enters the upper zone at a depth of 15 to 20 m and remains unmixed with the rest of the upper zone as it passes laterally all the way to the dam. Winter mixing homogenizes the upper zone, but the upper and lower zones are so different in density that they do not mix at any time of the year. Thus, separation of the zones is maintained by seasonal alternations of density for the incoming water. Although the upper and lower zones of Lake Powell never mix fully under the influence of wind, there is a gradual interchange between the two. This interchange is quite important both to Lake Powell and to the downstream characteristics of the Colorado River. Slow exchange is caused by the addition of water to the lower layer during the winter, which increases its volume. Increase in volume of the lower layer is offset by erosion from the top of the layer under the influence of wind mixing and by withdrawal of water through the Glen Canyon Dam outlet structure. If water in the lower layer were not replaced, or were to be replaced more slowly than at present, the lower layerwould become anoxic, and anoxic water could pass downstream, where it could adversely affect trout and other organisms. Recent surveys of dissolved oxygen concentrations in Lake Powell have shown that the uppermost 10 m of the lake is often supersaturated (e.g., Figure 6.2), as would be expected for most lakes during daylight hours because of photosynthesis. There is often a metalimnetic minimum (20 to 30 m) of oxygen (e.g., Ayers and McKinney 1995a). Marzolf (1995) concludes that this minimum is caused by decomposition of organic matter in the water that enters with the spring and summer inflows at about this level. The lowermost portions of the bottom layer of Lake Powell show sub- stantial oxygen depletion with reference to saturation but, because of the continual replacement of water during the winter, do not become completely anoxic. Concentrations of oxygen do fall below 3 mg/liter near the bottom of the lake, however, and thus approach the limiting concentrations for the sustained presence of most kinds of aquatic life. Even minor changes in the oxygen dynamics of the reservoir could be significant. At present, however, water leaving the outlet structure is typically near saturation and experiences significant reaeration as it is discharged. The nutrient chemistry of Lake Powell is unusual because of the large amount of suspended inorganic matter that reaches the lake from the rivers entering its upper end. Concentrations of suspended sediment in waters entering the upper end of Luke Powell have a mean of approximately 1,500 mg/liter, although concentrations can increase seasonally to as much as 10 times this amount (Stanford and Ward, 1991~. Because phosphorus is readily
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Organisms and Biological Processes 91 adsorbed onto the surfaces of clay and silt particles, the total phosphorous load for Lake Powell is quite high. Although now about 20 years out of date, studies by Gloss et al. (1981 ) indicate that the total phosphorous loading for Lake Powell would be sufficient to maintain a hypereutrophic condition in the lake if a significant amount of the total load were in soluble form. In fact, however, over 90 percent of the phosphorous load is accounted for by phosphorus adsorbed onto inorganic particles, and this phosphorus is rapidly deposited as the reservoir sediment. Although Resorption of phosphorus from sediments is possible even at the bottom of the reservoir, the interchange between the large phosphorous reserve on the bottom of the reservoir and the overlying waters is sufficiently slow that phosphorous concentrations in the water column remain low. Studies by Gloss et al. (1980), which may still be applicable, indicate that the total phosphorus of the upper water column typically ranges between 10 and 20,ug/liter within 50 to 100 km of the dam, but may be two or three times higher in the upper water column of the reservoir arms. The same studies show that dissolved phos- phorus within 100 km of the dam has a mean concentration close to 10 g/liter in the upper water column. Concentrations of phosphorus generally decline from the upper end of the reservoir to the dam. During warm weather, dissolved phosphorus may be essentially depleted within 50 to 100 km of the dam in the upper water column. Thus, phosphorus probably limits primary production in Lake Powell near the dam, while shading caused by turbidity probably limits primary production in the upper portion of the lake (Gloss et al., 1980~. Nitrogen also reaches Lake Powell in substantial quantities, but is less associated with particulate material than phosphorus. Studies by Gloss et al. (1980, 1981 ) indicate concentrations of dissolved nitrogen in the upper water column of about 500 ~g/liter, of which approximately half is organic N and half is nitrate N. Concentrations may be twice this high nearthe bottom ofthe lake. Given that nitrate concentrations in Lake Powell are commonly in the vicinity of 250,ug/liter, significant nitrogen limitation of primary production seems unlikely. Gloss et al. (1980) did, however, demonstrate one instance of localized surface depletion of nitrate. Therefore, this issue is not fully re- solved. Although the most satisfactory information on nutrients is now quite old, it suggests that inorganic nitrogen passing through the outlet structure to the Colorado River is sufficient to support substantial algal growth downstream of the dam. Autotrophy just downstream of the dam seems to bear this out
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92 River Resource Management in the Grand Canyon (Ayers and McKinney, 1995b). Dissolved phosphorus in the outlet water would probably be in the vicinity of 10 ~g/liter, which could support significant production downstream but could be removed to biologically negligible concentrations by vigorous algal growth downstream (as sug- gested by preliminary data from Parnell and Bennett, 1995~. Concentrations of heavy metals in Lake Powell are potentially of interest with respect to the biota of the lake as well as aquatic communities down- stream. Concentrations of mercury in the water column of the lake appear to fall within the range that might inhibit growth of primary producers and may have other biological effects downstream (Graf, 1985; Blinn et al., 1977a). Seleniurr, may also be of biological significance both within the lake and downstream (Potter and Drake, 1989~. Sources of water for the reservoir contain significant amounts of radionuclides (Graf, 1985~. The effects of the lake on concentrations of these substances, and their passage downstream, are undocumented. The primary production of Lake Powell has not been estimated recently, but an estimate that is now approximately 20 years old suggests production of about 190 9 of carbon per square meter per year, which is consistent with an oligotrophic or mildly mesotrophic condition (Hansmann et al., 1974; Blinn et al., 1 977a). Phytoplankton abundance, as indicated by chlorophyll a, was between ~ and 2 ~g/liters during the 1980s (Paulson and Baker, 1983; Soll- berger et al., 1989~. This is also consistent with general oligotrophic status, although concentrations of chlorophyll tend to be higher in the upstream arms. Algae from Lake Powell are a potential source of food for macro- invertebrates downstream. Although studies of stable isotopes have clarified some of the trophic relations downstream of the dam (Angradi, 1994), the importance of algae in supporting the food web near the dam is still unclear. Phytoplankton composition of Lake Powell was studied by Stewart and Blinn (1976) and Blinn et al. (1 977b). Primary production and biomass appear to be dominated by diatoms, and the lake features a vernal bloom that is characteristic of oligotrophic lakes. Blue-green algae probably account for only small proportions of primary production or biomass, but the information on this subject is now quite old. Oligotrophic lakes often show significant primary production by attached algae (periphyton). The potential for periphyton growth in Lake Powell is reduced by the vertical nature of much of the shoreline, but studies by Potter and Pattison (1976) showthat illuminated substrates are heavily colonized by algae and show high production per unit area. The zooplankton community of Lake Powell was studied by Sollberger et
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Organisms and Biological Processes 93 al. (1989) and more recently by Ayers and McKinney (1 995a). These studies show an unremarkable species composition that includes copepods, clad- ocerans, and rotifers. Densities in the upperwater column were in the vicinity of 10 to 20 individuals per liter according to Sollberger et al. but were found by Ayers and McKinney (1995a) to be as high as 700 per liter (an extra- ordinary abundance that needs to be verified) in summer. Trout eat zoo- plankton that pass downstream (Angradi, 1994~. The benthic invertebrates of Lake Powell have been studied very little, but probably are dominated by chironomids (Potter and Louderbough, 1977~. Fishes have been studied much more extensively (Stanford and Warcl, 1991~. The fish community is dominated by introduced taxa, including numerous game species. The fish community has been remarkably unstable, however, in that the dominant forage species and dominant predator species have shifted almost continually since the reservoir fish populations were first established. The lake contains numerous species that are not present down- stream of the clam. Fish may survive transport through the dam, especially when some water is bypassing the turbines, but the frequency of fish passage is unknown. Influence of Dam Operations on the Lake The possibilities for influence of dam operations on organisms or biological processes of Lake Powell are much more limited than the reverse. The operation of a dam affects a reservoir primarily through changes in water level, mean depth, and hydraulic residence time. Because the requirements for delivery of water from Lake Powell are fixed, however, and because the reservoir is large, the flexibility for managing the water level or mean volume for any purpose otherthan water delivery is essentially nil. There is at present no reason to believe that any possible operating plan with the current facilities would have a distinctive influence on Lake Powell. If new facilities, such as a multiple outlet withdrawal structure or slurry pipeline, were to be installed, however, the possible effects of these structures on Lake Powell would become relevant to dam operations. Overvi ew of La ke St ud i es Although some studies of Luke Powell were part of GOES, they were
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94 River Resource Management in the Grand Canyon added too late and were too fragmentary to serve as an integral part of the studies. This is unfortunate, given that the water quality, temperature, and suspended organisms of Luke Powell set the initial conditions for the ecosystem downstream and may be changed either inadvertently by with- cirawal at higher points in the water column in the future following a suc- cession of ciry years or by manipulation through installation of a multiple outlet withdrawal structure that would allow control of temperature for environmental purposes (BOR, 1994~. Changes in the River Caused by the Dam Withdrawal of water from Lake Powell affects the abundance and community composition of fishes and other organisms downstream in several ways, which can be summarized as follows. · Hydrological changes. The Glen Canyon Dam has drastically reduced the amount of seasonal change in the flow of the Colorado River. Dam operations also cause the water level of the river to vary on a daily basis by an amount that greatly affects the inundation of backwaters and the distribution of current velocities in the channel (Figure 6.3~. The original Colorado River was subject to surges in flow and turbidity corresponding to the flooding of tributaries associated with convectional storms. Such surges can still occur, especially below the Little Colorado River, but are reduced in frequency because of the interception of storm flows above Glen Canyon Dam by Lake Powell. Also, the suppression of floods has led to net loss of beaches and siltation of backwaters, with associated changes in habitat for organisms that require these physical features (Kearnsley et al., 1994~. · Changes in water temperature. The river does not become warm in the summer as it did previously (Figure 6.4~. Because water is cirawn from a depth that lies belowthe upper mixed layer of Lake Powell, watertemperature is always cold near the dam (7° to 11 °C). Water warms progressively downstream in midsummer (about 1 °C per 30 river miles (RM); Valdez and Ryel, 1995) but reaches a summer maximum of only about 1 7°C near Lake Mead. · Changes in turbidity. Prior to impoundment, the Colorado River was very turbid; it allowed very little light penetration. At present, the reach between Glen Canyon Dam and the Paria River is nearly always transparent.
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Organisms and Biological Processes less abundant in the lower reach than in the upper reach (Minckley, 1991~. Tributaries 107 Not only the tributary junctions but also the tributaries themselves between Glen Canyon Dam and Lake Mead are refugia for the humpback chub and other warmwater fishes. The Little Colorado River is especially important in this respect. The tributaries are flooded on an irregular basis during summer by convectional storms. Such storms can raise the discharge of the Little Coloraclo, for example, to 20,000 cfs or more in a very short time. These summer storms are the key means by which coarse debris, sand, and fine sediments are transported through the tributaries to the main stem. In adclition, they appear to be related to the welfare of native fishes. Cir- cumstantial evidence (Minckley and Meffe, 1987; USFWS, 1993 and BOR, un- published) now suggests that summer floods of tributaries suppress exotic species, which are not well adapted to these conditions, and thus promote successful recruitment of native species, which are often suppressed by predation from exotics. Further study of these phenomena may illustrate the conditions under which native populations could be reinforced in tributaries other than the Little Colorado River and may highlight the importance of hydrological conditions in the Little Colorado River to the humpback chub of the Colorado River. Both the Paria and Little Colorado rivers have been drastically altered since the days of John Wesley Powell. The riparian habitats of both of these important tributaries have been overgrazed, and this has altered their flow regimes and physical characteristics. Because of water diversion, the Little Colorado River is now essentially spring fed (Hubbs 1985) except during convectional storms, whereas previously it had significant baseflow. The welfare of the humpback chub and other native species along the Colorado River will depend on environmental changes not only in the Grand Canyon but also in tributaries of the Colorado. Biotic Interactions Competitive and predatory interactions involving native and exotic species might well be influenced by the operation of Glen Canyon Dam. The outcomes of such interactions are notoriously difficult to predict. GOES
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108 River Resource Management in the Grand Canyon scarcely dealt with this subject, although biotic interactions will likely be critical in determining the final welfare of humpback chub and other native species. GCES documented the abundance and distribution of exotic fish species but primarily as an incidental matter to studies of humpback chub (\/aldez and Ryel, 1995~. At the community level, outcomes of operational alternatives are still highly uncertain. Overview of Biotic Studies on the Main Stem Studies of the aquatic foodweb by Blinn et al. (1994) and Angradi (1994), with emphasis on the area near the dam, and studies of the humpback chub in the main stem by BioWest (\/aldez and Ryel, 1995) are examples of well- executed projects with specific and useful outcomes. Other studies that have not yet been finalized may prove to be of this class. The main disap- pointments of the GCES biotic studies are that they showed uneven coverage of the biotic resources, were largely still in progress or unreported as of the end of GCES, were weakly knit together with each other, and did not reflect a master plan for integration with hydrological and geomorphic studies that would make the essential connection to operations. However, quite a number of findings, such as those related to the habitat affinities of the humpback chub, are directly useful. In fact, a publication by Clarkson et al. (1994) that discusses the welfare of native fishes in relation to the full range of management options is a fine example of the kind of bold and broad-ranging analysis that should have marked the culmination of GCES. Interestingly, Clarkson et al. published their paper privately-presumably because federal environmental analysis is still so immature that it cannot tolerate the publication of views or conclusions that are not already reflected in the plans of management agencies. The lesson for the future is the necessity for an integrated view from the earliest planning stages to final synthesis if scientific studies are to have their fullest value in the service of management. THE RIPARIAN ZONE The riparian zone of a river can be loosely defined as the area that is inundated by a 100-year flood. This zone is affected by the river through scouring or deposition of sediment. Riparian zones are also influenced by their close proximity to surface water, which provides habitat and food
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Organisms and Biological Processes 109 resources that may not be present in surrounding uplands. In addition, riparian zones can typically support an abundance of vascular plants in arid climates because of the proximity of ground water to the surface. In general, riparian zones are centers of biotic diversity, especially in arid climates. The riparian zone of the Colorado River was changed drastically by the installation of Glen Canyon Dam (Johnson, 1991~. The old riparian zone extended far above the mean water level of the Colorado because of the river's typically strong spring floods, which reached discharges as high as 300,000 ft3/second, or more than 10 times the mean discharge (Stevens and Ayers, 1993~. Installation of the dam has regulated the Colorado to such an extent that the new riparian zone, if identified on the basis of high water, is much restricted; it corresponcisto perhaps 90,000 cts (the 1983 flood was just over 90,000 cfs). In fact, the riparian zone is qualitatively different now because spring flooding does not occur at all in most years, whereas strong spring flooding would have been characteristic of all years under the natural hydrological regime of the Colorado River. The original riparian zone also was affected by summer floocis (convectional storm floods) and may still be under present circumstances below the Little Colorado River (e.g., Stevens and Ayers, 1993), but not between the Little Colorado and Glen Canyon Dam. Riparian characteristics associated with proximity to surface water and with the availability of ground water near the surface are the basis for maintenance of characteristic vegetation and associated riparian species below Glen Canyon Dam. In fact, ground water supply to the riparian zone is more constant now than before 1963 because the operation of the dam has stabilized the distribution of riparian ground water. This in part explains the general vegetative enrichment of the riparian zone following closure of the dam (Johnson, 1991~. The upper end of the riparian zone is characterized by hackberry, acacia, mesquite, and other species that were established by and tolerant of occasional flooding well above the mean annual flood. These species do not have access to phreatic water because they are too far from the river to reach the water table. Remaining individuals in this upper zone are old and probably will not be replaced because of the suppression of flooding. Flood- intolerant plants appear to be colonizing this zone now (Anderson and Ruffner, 1987~. Restoration of the conditions leading to the establishment of this portion of the riparian zone (old high-water zone) are not within the scope of any feasible operation for Glen Canyon Dam. The lower portion of the riparian zone contains native species, such as willow, and exotic species, such as tamarisk. In many locations, tamarisk
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110 River Resource Management in the Grand Canyon forms a riparian forest close to the water. Tamarisk now provides habitat for both native and exotic animals and figures importantly in the biology of the riparian zone. The woody taxa in general are dependent on sandy substrate deposited by the river and on phreatic water, which is also maintained by the river. The abundance of tamarisk and the general proliferation of woody species are attributable to the dam in the sense that the annual floods of the original riverwould have removed such vegetation, just es the 1983 flood did (Stevens and Waring, 1985; Pucherelli, 1988~. In addition, fluvial marshes have developed in places where they would have been swept away by the natural hydrological regime of the Colorado (Stevens et al., 1994~. GCES produced information on the composition of riparian vegetation (Stevens and Ayers, 1994) and lists of resident species of birds (Sogge et al., 1994), with particular attention to endangered species (Sogge and Tibbitts, 1994a,b). In fact, the majorfocus of GCES in the riparian zone was inventory. Functional relationships of organisms in the riparian zone and their responses to the operation of the Glen Canyon Dam were also studied by GCES but less completely and with a more variable outcome. For example, Stevens and KJine (1991) concluded that large daily fluctuations may be detrimental to waterfowl, while Stevens and Ayers (1 993 b) showed that moderation of daily fluctuations during the period of interim flows probably had no significant adverse effects on the riparian zone. Controlled or uncontrolled floods are of particular relevance to the riparian zone, but the forecasting of effects for such events is weak at present. For the future, responses of the riparian zone to operation of the dam should be an important consideration of adaptive management, but the scientific basis for this should be strengthened. OUTCOMES OF BIOLOGICAL STUDIES GCES provided much new information on the biotic resources of the Colorado River below Glen Canyon Dam. The abundances and distributions of many kinds of organisms were quantified satisfactorily for the first time. In addition, several kinds of functional relationships, such as the dependence of rainbow trout on amphipods and Cladophora near Glen Canyon Dam, were documented. Many functional relationships, however, were not explored satisfactorily or were not explored at all. Some of these relationships are critically connected to management options, but studies of them were not initiated or clicl not come to completion in a way that would be useful to management. Finally, the integration of biotic components with each other,
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Organisms and Biological Processes 111 and joint consideration of biological and physical aspects of the environment, particularly involving sediment dynamics, remained largely undeveloped as of the end of GCES. A few examples will illustrate some of the strengths and weaknesses of the biological component of GCES studies. Present management options include a range of potential discharge manipulations. Managers might askfor a list of fishes and other species that might be affected by such manip- ulations. GCES could easily provide a comprehensive list, along with relative abundances, probable habitat requirements, and general distribution down- stream of Glen Canyon Dam. Management might, however, also ask for a forecast of the effects on particular species. For example, howwould greater stability of discharge affect species and communities downstream? Here GCES might prove to be only partially satisfactory. For example, GCES showed that large rapid fluctuations in discharge interfere with spawning of trout near Glen Canyon Dam and strand some adult fish. At the same time, however, such variations appear to dislodge food items that in this way become vulnerable to trout. What is the relative importance of these two effects on the trout population? GCES does not provide an answer, although clearly management neecis at least a qualitative answer to such questions. In general, the success of GCES was greatest in the area of survey and inventory, impressive in the analysis of some (e.g., native fishes) but not all system components, and disappointing in the area of ecosystem analysis. REFERENCES Anderson, L.S. and G.A. Ruffner. 1987. Effects of Post-Glen Canyon Flow Regime on the 01d High Water Line Plant Community Along the Colorado River in the Grand Canyon. Glen Canyon Environmental Studies Tech- nical Report, Bureau of Reclamation, Salt Lake City, Utah. And rows, E.D. 1991. Sediment transport in the Colorado River Basin. Pp. 54- 74 in Colorado River Ecology and Dam Management. Washington, D.C.: National Academy Press. Angradi, T.R. 1994. Trophic linkages in the lower Colorado River: multiple stable isotope evidence. Journal of the North American Benthological Society 13:479-493. Angradi, T.R., and D.M. Kubly. 1993. Effects of atmospheric exposure on chlorophyll a, biomass and productivity of the epilithon of a tailwater reservoir. Regulated Rivers: Research and Management 8:345-358.
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112 River Resource Management in the Grand Canyon Ayers, A.D. and T. McKinney. 1995a. Water Chemistry and Zooplankton in the Lake Powell Forebay and the Glen Canyon Dam Tailwater. Draft final report, Arizona Game and Fish Department, Phoenix. Ayers, A.D., and T. McKinney. 1995b. Effects of Different Flow Regimes on Periphyton Standing Crop and Organic Matter and Nutrient Loading Rates for the Glen Canyon Dam Tailwater to Lee's Ferry. Draft final report. Arizona Game and Fish Department, Phoenix. Blinn, D.W. and G.A. Cole. 1991. Algal and invertebrate biota in the Colorado River: comparison of pre- and post-dam conditions. Pp. 102-123 in Colorado River Ecology and Dam Management. Washington, D.C.: National Academy Press. Biinn, D.W., T. Tompkins, and L. Zaleski. 1977a. Mercury inhibition on primary productivity using large volume plastic chambers in situ. Journal Phycology 13:58-61. Blinn, D.W., T. Tompkins, and A.~. Stewart. 1 977b. Seasonal light characteristics for a newly formed reservoir in southwestern USA. Hydrobiologia 51 :77-84. Blinn, D.W., L.E. Stevens, and J.P. Shannon. 1994. Interim Flow Effect from Glen Canyon Dam on the Aquatic Food Base in the Colorado River in the Grand Canyon National Park, Arizona. Cooperative Study Agreement CA824-8-0002. Blinn, D.W., J.P. Shannon, L.E. Stevens, and J.P. Carder. 1995. Con- sequences of fluctuating discharge for lotic communities. Journal of the North American Benthological Society 14:233-248. Bureau of Reclamation. 1987. Quality of Water, Colorado River Basin. Progress Report No. 13., U.S. Department of the Interior, Washington, D.C. Bureau of Reclamation. 1994. Glen Canyon Dam Discharge Temperature Control. Draft Appraisal Report, Bureau of Reclamation, Washington, D.C. Burr, B.E.M., and L.M. Page. 1986. Zoogeography of the fishes of the lower Ohio-upper Mississippi basin. Pp. 287-324 in The Zoogeography of North American Freshwater Fishes, C.H. Hocutt and E.O. Wiley, eds. New York: John Wiley. Carothers, S., and B. Brown. 1991. The Colorado River Through Grand Canyon: Natural History and Human Change. Tucson: University of Arizona Press.
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Organisms and Biological Processes 113 Carothers, S., and C. Minckley. 1981. A survey of the fishes, aquatic invertebrates and aquatic plants of the Colorado River and selected tributaries from Lee's Ferry to Separation Rapids. Prepared by Museum of Northern Arizona for Water and Power Resources Service, Lower Colorado Region, Boulder City, Nev. Clarkson, R.W., O.T. Gorman, D.M. Kubly, P.C. Marsh, and R.A. Valdez. 1994. Management of Discharge, Temperature, and Sediment in Grand Canyon for Native Fishes. Report privately published by the authors. Dawdy,D.R. 1991. Hydrology of Glen Canyon and the Grand Canyon. Pp. 40-53 in Colorado River Ecology and Dam Management. Washington, D.C.: National Academy Press. Gloss, S.P., L.M. Mayer, and D.E. Kidd. 1980. Advective control of nutrient dynamics in the epilimnion of a large reservoir. Limnology Ocean- ography. 25:219-228. Gloss, S.P., R.C. Reynolds, cr., L.M. Mayer, and D.E. Kidcl. 1981. Reservoir influences on salinity and nutrient fluxes in the arid Colorado River Basin. Pp. 1618-1629 in H.G. Stefan, ed. Proceedings of the Symposium on River Surface Water I Impoundments. New York: American Society of Civil Engineers. Graf, W.L. 1985. The Colorado River: Instability and Basin Management. Resource Publication in Geography, American Society of Geographers, Washington, D.C. Hansmann, E.W., D.E. Kidcl, and E. Gilbert. 1974. Man's impact on a newly formed reservoir. Hydrobiologia 45:185-197. Holroyd, E.W. 1995. Thermal infrared (FLIR) studies in eastern Grand Canyon. Technical Memorandum #8260-95-11. Bureau of Reclamation, Washington, D.C. Hubbs, C. 1995. Springs and S,oring Runs as Unique Aquatic Ecosystems. Copeia 1995:989-991. Johnson, R. R. 1 991 . Historic changes in vegetation along the Colorado River in the Grand Canyon. Pp. 178-206 in Colorado River Ecology and Dam Management. Washington, D.C.: National Academy Press. Kearnsley, L., d. Schmidt, and K. Warren. 1994. Effects of Glen Canyon Dam on Colorado River Sand Deposits Used as Campgrounds in Grand Canyon National Park, USA. Regulated Rivers 9:137-149. Liebfriecl, W.C. 1988. The utilization of Cladophora g/omerata and Epiphitic Diatoms as a Food Source by RainbowTrout in the Colorado River Below Glen Canyon Dam, Arizona. Master's thesis, Northern Arizona University, Flagstaff.
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114 River Resource Management in the Grand Canyon Maddox, H.R., D.M. Kubly, J.C. Devos, W.R. Persons, R. Staedicke, and R.L. Wright. 1987. The Effects of Varied Flow Regimes on Aquatic Resources of Glen and Grand Canyons. Arizona Game and Fish Department. Marzolf, R. 1995. Metalimnetic oxygen depletion in Luke Powell: a con- sequence of the inflow of the annual snowmelt water mass. Paper presented at national AS LO meeting, June 1 1-15, Reno, Nev. Melis, T.S., W.M. Phillips, R.H. Webb, and D.J. Bills. 1995. When the Blue- Green Waters Turn Red: A History of Flooding in Havasu Creek, Arizona. USGS Water-Resources Investigation Report, U.S. Geological Survey, Reston, Va. Miller, R.R. 1961. Man and the changing fish fauna of the American southwest. Michigan Academy of Science, Arts and Letters 46:365-404. Minckley, W.L. 1991. Native fishes of the Grand Canyon region: an obituary? Pp. 124-177 in Colorado River Ecology and Dam Management. Washington, D.C. National Academy Press. Minckley, W.L. and G.K. Meffe. 1987. Differential selection by flooding on stream-fish communities in the arid American Southwest. Pp. 93-104 in D.C. Heins and W.J. Matthews (eds), Evolutionary and Community Ecology in North American Stream Fishes. University of Oklahoma Press. Minckley, W.L., P.C. Marsh, J.E. Brooks, J.E. Johnson, and B.L. Jensen. 1991. Management toward recovery of the razorback sucker. Pp. 303-358 in Battle Against Extinction: Native Fish Management in the American West, W.L. Minckley and ~J.E. Deacon, eds. Tucson: University of Arizona Press. Montgomery, W.L., and K. Tinning. 1993. Impacts of fluctuating water levels on eggs arm fry of rainbow trout in the Colorado River below Glen Canyon Dam, Arizona. Unpublished manuscript. NationalResearch Council. 1987. River and Dam Management: A Review of the Bureau of Reclamation's Glen Canyon Environmental Studies. Washington, D.C. National Academy Press. Parnell, R.A., and J.B. Bennett. 1995. Influence of geochemical processes on nutrient spiralling within the recirculation zones of the Colorado River and the Grand Canyon. Quarterly Report, April. National Park Service Cooperative Agreement CA8000-8-0002, National Park Service, Wash- ington, D.C. Paulson, L.~. and J.R. Baker. 1983. Limnology in reservoirs on the Colorado River. Technical Compl. Report OWRT-B-121-NEV-1, Nevada Water Resources Research Center, Las Vegas.
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Organisms and Biological Processes 115 Potter, L.D., and C. Drake. 1989. Lake Powell: Virgin Flow to Dynamo. Albuquerque: University of New Mexico Press. Potter, L.D., and E.T. Louderbough. 1977. Macroinvertebrates and diatoms on submerged bottom substrates, Lake Powell. Lake Powell Research Project Bulletin 37, Institute of Geophysics and Planetary Physics, University of California, Los Angeles. Potter, L.D., and N.B. Pattison. 1976. Shoreline ecology of Lake Powell. Luke Powell Research Project Bulletin 29, Institute of Geophysics and Planetary Physics, University of California, Los Angeles. Pucherelli, M.J. 1988. Evaluation of riparian vegetation trends in the Grand Canyon using multitemporal remote sensing techniques. Pp. 217-228 in Glen Canyon Environmental Studies: Executive Summaries of Technical Reports. U.S. Department of the Interior, Bureau of Reclamation, Salt Lake City. Reger, S., C. Benedict, and D. Wayne. 1993. Statewide Fisheries In- vestigations: Survey of Aquatic Resources, Colorado River, Lee's Ferry. Draft Fish Management Report 1989-1993, Federal Aid Project F-7-M-36. Shannon, J.P., D.W. Blinn, and L.E. Stevens. 1994. Trophic interactions and benthic animal community structure in the Colorado River, Arizona, U.S.A. Freshwater Biology31:213-220. Sogge, M.K., and T.J. Tibbitts. 1994a. Wintering Bald Eagles in the Grand Canyon: 1993-1994. Summary Report, National Biological Survey Colorado Plateau Research Station/NorthernArizona Universityand U.S. Fish and Wildlife Service, Phoenix. Sogge, M.K., and T.). Tibbitts. 1994b. Distribution and Status of the Southwestern Willow Flycatcher Along the Colorado River in the Grand Canyon-1994. Summary Report, National Biological Survey Colorado Plateau Research Station/Northern Arizona Universityand U.S. Fish and Wildlife Service, Phoenix. Sogge, M.K., D. Felley, P. Hodgetts, and H. Yard. 1994. Grand Canyon Avian Community Monitoring: 1993-94 Progress Report. National Biological Survey Colorado Plateau Research Station Report. Sollberger,].P.,P.D.Vaux,andL.J.Paulson. 1989. Investigation of Vertical and Seasonal Distribution, Abundance and Size Structure of Zooplankton in Lake Powell. Lake Mead Limnological Research Center, University of Nevacla, Las Vegas. Stanford, J.A., and J.V. Ward. 1991. Limnology of Lake Powell and the chemistry of the Colorado River. Pp. 75-101 in Colorado River Ecology and Dam Management. Washington, D.C.: National Academy Press.
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116 River Resource Management in the Grand Canyon Stevens, L.E., and T.J. Ayers. 1993a. The Impacts of Glen Canyon Dam on Riparian Vegetation and Soil Stability in the Colorado River Corridor, Grand Canyon, Arizona. 1992 Final Administrative Report, National Park Service Cooperative Studies Unit, Northern Arizona University, Flagstaff, Aria. Stevens, L.E., and T.J. Ayers. 1 993b. The Effects of Interim Flows from Glen Canyon Dam on Riparian Vegetation Along the Colorado River in Grand Canyon National Park, Arizona. Draft 1992 Annual Report, NPS Co- operative Work Order No. CA 8021-8-0002, National Park Service, Grand Canyon, Ariz. Stevens, L.E., and T.J. Ayers. 1994. The Effects of Interim Flows from Glen Canyon Dam on Riparian Vegetation Along the Colorado River in Grand Canyon National Park, Arizona. Draft 1994 Annual Report, NPS Co- operative Work Order No. CA 8021-8-0002, National Park Service, Grand Canyon, Ariz. Stevens, L.E., and N. Kline. 1991. 1991 Aquatic and semi-aquatic avifauna in the Colorado River corridor in the Grand Canyon, Arizona. Draft report. Stevens, L.E., and G. Waring. 1985. The effects of prolonged flooding on the riparian plant community in Grand Canyon. Pp. 81-86 in Riparian Ecosystems and Their Management: Reconciling Conflicting Uses. USDA Forest Service General Technical Report RM-120. Rocky Mountain Forest and Range Experiment Station, USDA Forest Service, Ft. Collins, CO. Stevens, L.E., J.C. Schmidt, T.J. Ayers, and B.T. Brown. 1994. Fluvial marsh development along the dam-regulated Colorado River in the Grand Canyon, Arizona. Unpublished manuscript. Stewart, Am., and D.W. Blinn. 1976. Studies on Lake Powell, U.S.A.: environmental factors influencing phytoplankton succession in a high desert warm monomictic lake. Arch. Hydrobiology 78: 139-164. Tyus, H.M. 1991. Ecology and management ofColoradosquawfish. Pp.379- 404 in BattleAgainst Extinction: Native Fish Management intheAmerican West, W.L. Minckley and J.E. Deacon, eds. Tucson: University of Arizona Press. U.S. Fish and Wildlife Service. 1978. Biological Opinion on the Effects of Glen Canyon Dam on the Colorado River as It Affects Endangered Species. Memorandum issued from Albuquerque, N.Mex. U.S. Fish and Wildlife Service. 1993. Habitat Use by Humpback Chub, Gila cypha, in the Little Colorado River and Other Tributaries of the Colorado River.
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Organisms and Biological Processes 117 Valdez, R.A., and Pal. Ryel. 1995. Life History and Ecology of the Humpback Chub (Gila cypha) in the Colorado River, Grand Canyon, Arizona. Draft Final Report, BioWest, Inc., Logan, Utah. Yard, N.D., G.A. Hayden, and W.S. Vernieu. 1993. Photosynthetically Available Radiation (PAR) in the Colorado River: Glen and Grand Canyons. Glen Canyon Environmental Studies Technical Report.
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