Hydrologic alteration affects aquatic plant assemblages in an arid-land river.
The diversity and distribution of riverine hydrophytes is regulated by light availability and other biophysical factors including bed sediment, flow velocity, and disturbance (Westlake, 1973). Biomass and species occurrences of attached algae, mosses, and macrophytes often increase downstream from dams in response to reductions in peak annual flow, higher winter base flows, and reduced ice cover (e.g., Ward, 1976; Holmes and Whitton, 1977; Rprslett, 1988; Rprslett et al., 1989; Blinn and Cole, 1991). Overall there have been few studies of riverine hydrophytes across well-defined gradients of flow alteration to allow for much generalization as to the responses of these communities to altered flow regimes (reviewed by Walker, 1985; Bunn and Arthington, 2002; Poff and Zimmerman, 2010).
In this paper, we summarize long-term changes in submerged vegetation in the Green and Yampa rivers in Dinosaur National Park, Colorado and Utah. Historically, the Green and Yampa rivers had similar hydrologic and ecological characteristics (Woodbury, 1963; Irons et al., 1965; Andrews, 1986). Conditions in the Green River changed dramatically after 1962 when Flaming Gorge Dam was constructed upstream from the confluence of the two rivers. The Yampa River remains free flowing. The Green River downstream from the confluence with the Yampa River has blended hydrological characteristics of both systems. We compare historical hydrophyte collections made by three survey teams in 1961 and 1962 just before the closure of Flaming Gorge Dam (Holmgren, 1962; Hagen and Banks, 1963; Woodbury, 1963) with contemporary collections we made in 2009-2010. We also compare contemporary aquatic plant occurrences in the regulated Green River upstream and downstream from the confluence with the Yampa River with the unregulated Yampa River. We relate these findings to long-term changes in the hydrologic regime of the Green River. The results are useful for examining the long-term effects of flow alterations on aquatic plant communities and for designing more natural flow regimes downstream from large dams to accomplish river restoration goals.
METHODS--This study was conducted on the Green and Yampa rivers in the arid western United States. The Green River is the largest tributary of the Colorado River, and the Yampa River is the largest tributary of the Green River. The Green River originates in the Wind River Mountains in Wyoming and flows south to its confluence with the Yampa River within Dinosaur National Monument in northwestern Colorado (40.5[degrees]N, 108.9[degrees]W; Fig. 1). Flaming Gorge Dam is a 149-m-high dam with a multilevel discharge structure located 100 km upstream from the confluence with the Yampa River. The Yampa River originates in northwestern Colorado and flows west 400 km before meeting the Green River. The Yampa River is one of the last large rivers in the United States without a major water control structure. A few small dams and diversions occur in the headwaters.
Stream Flow and Turbidity--Stream discharge and turbidity data for the Green River upstream from the Yampa River were obtained from U.S. Geological Survey stream gauges near Linwood, Utah (Station 09225500) located 50 km upstream from the dam for the years 1928-1950, and near Greendale, Utah (Station 092345000), 0.8 km downstream from the dam for the years 1950-2010 (Fig. 1). Only small tributaries enter the Green River between the two gauges, so differences in discharge were slight, 7% (Vinson, 2001; Grams and Schmidt, 2002). Data for the Yampa River are from the gauge at Maybell, Colorado (Station 09260050, 1916-2010). Data for the Green River downstream from the Yampa River are from the U.S. Geological Survey gauge atJensen, Utah (Station 09261000, 1947-2010, Fig. 1). The Maybell gauge is located about 130 km upstream from the confluence with the Green River. The Jensen gauge is located about 45 km downstream from the confluence of the two rivers. Maximum annual and mean monthly discharge data were summarized for each station for the predam and postdam periods. Water turbidity data were limited and none of the measurement dates overlapped, but data were available for the Green River at the Greendale gauge for 1978-1986 (67 measurements), for the Green River at Jensen gauge for 200-62007 (246 measurements), and for the Yampa River at the Maybell gauge for 1978-1993 (85 measurements). In addition to presenting turbidity data, we related turbidity data to Secchi depths on the basis of Davies-Colley and Smith (2001), as hydrophyte light limitations have been reported in relation to Secchi depth rather than turbidity (i.e., nephelometric turbidity units [NTUs]). Maximum annual discharge for the three stream reaches was compared between predam and postdam periods, as were differences between the Green and Yampa rivers before 1963 with a t-test. We graphically compare mean monthly values for both measures of water clarity among the three reaches.
Historical Predam Plant Surveys--In all surveys, vegetation was surveyed on the Green River from Gates of Lodore (40[degrees]44'N, 108[degrees]53'W) 70 km downstream through Split Mountain Gorge (40[degrees]27'N, 109[degrees]15'W), and on the Yampa River near the Green River confluence (40[degrees]32'N, 108[degrees]59'W) and near Deer Lodge Park (40[degrees]27'N, 108[degrees]31'W, Fig. 1). Floristic collections were made following standard hydrophyte survey methods (e.g., Holmes and Whitton, 1977). Holmgren (1962) surveyed terrestrial, riparian, and in-stream vegetation in the summer of 1961. Hagen and Banks (1963) surveyed in-stream vegetation in the spring and fall 1961. Woodbury (1963) surveyed riparian and in-stream plants including algae from 20 July to 6 August 1961. They also surveyed several tributary streams. All of the surveys traveled the Green River via rafts or canoes. Sampling sites were located about every 1-3 km and all representative habitats were inventoried.
Contemporary Postdam Plant Surveys--We surveyed the Green River from Gates of Lodore through Split Mountain Gorge and the Yampa River from Deer Lodge Park to the confluence with the Green River (ca. 74 km, Fig. 1). Sampling sites were located about every 1-3 km and matched the location and general distribution of sites sampled in the 1960s. Sites were selected to be representative of all habitat types (eddies, runs, rapids, riffles, and pools). We surveyed the Green River by raft on three separate trips in June and July 2009 and again in July 2010. We surveyed the Yampa River by canoe in August in both 2009 and 2010.
We looked for and collected vascular plants, macroalgae, Chara and Cladophora, and the aquatic bryophyte, Amblystegium riparium, following standard methods used in the 1960s surveys. Specimens were identified using Fassett (1957) and from comparisons with voucher specimens housed at the Intermountain Herbarium at Utah State University including Holmgren's (1962) 1961 collections. Vouchers of our collections are housed at the Intermountain Herbarium at Utah State University.
RESULTS--Green River Hydrology--Before dam construction in 1962, annual and seasonal variation in discharge was high. At Greendale, annual peak discharge normally exceeded 400 [m.sup.3] [s.up.-1] during spring runoff (Fig. 2a) and minimum discharge was less than 10 [m.sup.3] [s.up.-1] during winter (Fig. 3). At Jensen, downstream from the Yampa River, annual peak discharge ranged from 340 to 1,043 [m.sup.3] [s.up.-1] (Fig. 2b) and winter minimum flow was about 30 [m.sup.3] s(Fig. 3b). Historical measurements of water clarity are lacking, but descriptions of the predam Green River and the Yampa River describe them as highly turbid even at low flows (Hagen and Banks, 1963; Woodbury, 1963).
After 1962, the effects of the dam on the hydrology of the Green River upstream from confluence with the Yampa River included large reductions in the annual maximum flow (Fig. 2a), higher winter flow (Fig. 3), and reduced sediment transport. At Greendale, the average maximum annual flow declined from 309 [m.sup.3] [s.up.-1] predam to 144 [m.sup.3] [s.up.-1] postdam (Fig. 2a, t-test, P < 0.0001, [F.sub.1,80]=72.2). Since 1962, flow has exceeded 130 [m.sup.3] [s.up.-1] (power plant capacity) only seven times (Fig. 2a). Minimum winter flow increased from about 20 [m.sup.3] [s.up.-1] to about 50 [m.sup.3] [s.up.-1] (Fig. 3a). Overall, mean monthly flows varied little from the annual average of 54 [m.sup.3] [s.up.-1] (Fig. 3a). Mean monthly turbidity ranged from 0 to 2 NTU with a median of 0.7 NTU after dam closure (Fig. 4).
Downstream from the Yampa River, the maximum annual flow at Jensen ranged from 203 to 1,143 [m.sup.3] [s.up.-1] (Fig. 2b). Maximum annual flow decreased significantly from an average of 633 [m.sup.3] [s.up.-1] pre-dam to 500 [m.sup.3] [s.up.-1] postdam (t-test, P = 0.004, [F.sub.1,62] = 8.8). Mean monthly flows were less during the spring and greater during the winter after 1962 (Fig. 3b). Mean monthly turbidity ranged from 0 to 332 NTU with a median for all records of 91 NTU during 2006-2007 after dam closure (Fig. 4).
Yampa River Hydrology--The present hydrologic regime of the Yampa River is similar to historical conditions (Fig. 2c,) and similar to the predam Green River (Fig. 2a and 2c, t-test, P = 0.5, [F.sub.1,79] = 0.5). At Maybell, maximum annual discharge normally exceeded 297 [m.sup.3] [s.up.-1] during spring runoff (Fig. 2), and minimum annual discharge was usually less than 9 [m.sup.3] [s.up.-1] and occurred in winter (Fig. 3). There was no difference in maximum annual discharge between the pre-1962 (291 [m.sup.3] [s.up.-1]) and post-1962 period (298 [m.sup.3] [s.up.-1], Fig. 2c, t-test, P = 0.7, [F.sub.1,93] = 0.1). Mean monthly turbidity ranged from 3 to 112 NTU with a median for all records of 8 NTU from 1978 to 1993 (Fig. 4).
Aquatic Plants--Hagen and Banks (1963) reported "small amounts of filamentous algae clinging to rubble" in the Green and Yampa rivers. They did not report observing or collecting any submerged vascular plants in the Green or Yampa rivers. Woodbury (1963) collected the macroalga Cladophora in the Yampa River and the Green River downstream from the Yampa River, and the macroalga Chara in the Yampa River at Deer Lodge Park. The moss Didymodon tofaceus and the liverwort Riccia frostii were collected on floodplains. Holmgren (1962) collected six submerged vascular species, Potamogeton alpinus, Potamogeton nodosus, Potamogeton pectinatus, Zannichellia palustris, Ranunculus gmelinii, and Nasturtium officinale, as well as the macro-algae Cladophora and Chara. Holmgren was not specific as to the location of the vascular species, but noted that all six species were present only at Echo Park, near the confluence of the Green and Yampa rivers. We evaluated Holmgren's (1962) specimens and the original description tag attached to the archived specimens at the Intermountain Herbarium. The tag does not specify the exact collection location (alongside or within the river channel) from which these species were collected. The lack of specificity of this collection as compared with that recorded for other species and the findings of the other two 1960s surveys suggests that these specimens were collected in off-channel ponds. Small floodplain ponds are still present along the rivers in this area because of seeps and impounded floodwaters. Aquatic plants collected in tributary streams by Holmgren (1962) and Woodbury (1963) included Chara, N. officinale, and Ranunculus. Alga species collected in the Green and Yampa rivers included 12 Cyanophyta species, 3 Chrysophyta species, 10 Chlorophyta species, and 1 Euglenophyta species. This represented <50% of the algal species collected in tributary streams.
In 2009-2010, we collected nine submerged plant species upstream and two species downstream of the confluence with the Yampa River and one species in the Yampa River (Table 1). Submerged species we collected in the Green River upstream from the Yampa River included the vascular plants Elodea canadensis, Myriophyllum sibiricum, N. officinale, Potamogeton crispus, P. pectinatus, and Ranunculus aquatilis; the macro-algae Chara and Cladophora; and the bryophyte, A. riparium. Cobble substrates were typically carpeted with Chara, Cladophora, and A. riparium throughout the channel bed. Nasturtium officinale occurred in more isolated patches on the downstream end of cobble bars and was typically associated with springs or seeps. The other five vascular hydrophyte species were restricted to pools and backwaters. In the Green River downstream from the Yampa River and in the Yampa River we collected Chara and Cladophora, but no hydrophytes. The macroalgae were scattered throughout both river reaches. Cladophora was observed as a fringe on boulders in the base-flow splash zone and was not present below the waterline. Chara only occurred in small patches on cobbles and boulders at the downstream end of debris fans where side canyons entered the river.
DISCUSSION--The predam hydrologic regime of the Green River was typical of unregulated Rocky Mountain rivers (Poff and Ward, 1989). Annual and seasonal variation was high for all measured hydrologic attributes (discharge, temperature, suspended sediment, organic matter transport, and specific conductance (Irons et al., 1965; Vinson, 2001). Macrophytic vegetation is not typically present in unregulated Rocky Mountain rivers not dominated by groundwater (Matsumura and Harrington, 1955). The major effects of the installation and operation of the dam on hydrophyte habitat were an elimination of scouring flows and an increase in water transparency. Before dam closure, suspended sediment was estimated to be 3.6 million tons per year for the Green River at Greendale (Andrews, 1986) and entrainment of bed particles occurred most years in this section of the river (Graf, 1980; Andrews, 1986; Grams and Schmidt, 2002). After dam closure, the suspended sediment load decreased to near zero at the Greendale gauge (Andrews, 1986), and bed scour was mostly eliminated upstream from the Yampa River (Graf, 1980). Downstream from the Yampa River at the Jensen gauge, sediment transport decreased from 6.9 to 3.2 million tons per year (Andrews, 1986). The concentrations of nutrients increased immediately after dam closure (1962-1968; Madison and Waddell, 1973), but declined soon afterward (Bolke and Waddell, 1975; Vinson, 2001). Present differences in nutrients between the three stream reaches are slight (U.S. Geological Survey, in litt.). Upstream from the Yampa River, substrates in erosional habitats are more armored than in the predam channel (Williams and Wolman, 1984) and seasonal fine sediment deposition in the channel has been reduced (Andrews, 1986). Conversely, eddies and low-velocity runs have accumulated fine sediments because they are no longer flushed by high flows (Grams and Schmidt, 2002) and have been colonized by five of the six hydrophytes (E.canadensis, M. sibiricum, P. crispus, P. pectinatus, and R. aquatilis) that were found upstream from the Yampa River. In contrast, in the unregulated Yampa River, hydrologic conditions (i.e., water transparency, sediment transport, and the timing, magnitude, and duration of high flow events) remain similar to historic conditions. On the basis of historical descriptions of the environment (Hagen and Banks, 1963; Woodbury, 1963; Irons et al., 1965; Andrews, 1986; Grams and Schmidt, 2002) and our recent observations, we conclude that the Yampa River as an environment for hydrophytes has changed little in the past 60 years.
In 1962, Woodbury (1963) found that, with the exception of a few side streams and springs of fresh clear water, the algal, hydrophyte, and bryophyte flora of Dinosaur National Monument was notoriously poor, due, he speculated, to the annual spring flooding by the Green and Yampa rivers and the heavy sediment loads they carry. Hagen and Banks (1963) likewise reported no aquatic macrophytic vegetation in the Green and Yampa rivers and only sparse algae in the Yampa River near Echo Park. In 2009-2010, submerged vegetation was abundant and widespread in the Green River upstream from the confluence with the Yampa River. Conversely, there were no differences in species occurrences between the two time-periods for the Yampa River or in the Green River downstream from the Yampa River.
In a review of the effects of river regulation on aquatic macrophytes in Norwegian Rivers, Rorslett et al. (1989) stated that "whenever macrophytes develop profusely [where they were formerly rare], we always relate this phenomenon to changes within the hydrology regime." Elevated winter flows, lack of ice cover, and a reduction in scouring flows were the primary factors leading to increased growth of aquatic plants in Norwegian regulated rivers (Rorslett, 1988; Rorslett et al., 1989). Similar hydrophyte responses after dam construction have been observed in Africa (Davies, 1979), France (Decamps et al., 1979), and in the tropics (Boon, 1979). These rivers were in regions that lacked frigid winters, and hydrophyte responses were attributed to an increase in water transparency and a reduction in scouring flows. The proliferation of submerged aquatic plants in the Green River downstream from Flaming Gorge Dam appears to be a response similar to that observed in Norway as well as a large change in sediment transport and water transparency. Turbidity data were not available for all sites for the same time periods, but the data available indicate a large reduction in turbidity in the Green River postdam and little change in turbidity in the Yampa River or the Green River downstream from the Yampa River. In Australia, Walker (1979) attributed the lack of prolific hydrophyte growth in upstream sections of the River Murray to high turbidities, unstable sediments, and variable flows. In lower reaches of the River Murray and in other Australian regulated rivers without high turbidities, water hyacinth (Eichhornia crassipes), salvinia (Salvinia molesta), E. canadensis, Potamogeton, and Myriophullum growth greatly exceed that in unregulated rivers (Walker, 1979).
Bernez et al. (2002) compared riverine plants (macroalgae, bryophytes, hydrophytes, and emergent rhizophytes) in the River Rance in France in 1861-1865 with those present in 1995-1996 downstream from a dam constructed in 1938. After 133 years, they found few compositional differences in the assemblage between the two time periods and no change in species richness. They related this lack of floristic change to the ubiquity and adaptability of aquatic flora. Information on the predam river hydrology was not presented, so it is unknown to what extent the environment changed after dam construction or over the 133 years between surveys. The source of the prolific vegetation we observed in the Green River upstream from the Yampa River likely originated from springs or clear-water tributary streams along the Green River. We found no aquatic plants in the Green River that were not also found by Holmgren (1962) or Woodbury (1963) in springs and tributaries.
The lack of vegetative change we observed in the Green River downstream of the confluence with Yampa River suggests that the Yampa River ameliorates the effects of Flaming Gorge Dam sufficiently to prevent occurrence of macrophytes and bryophytes. These results support the serial discontinuity concept that predicts that recovery of natural conditions downstream from a dam is dependent upon the contribution of tributary streams (Ward and Stanford, 1983). Tributary inputs can alter flow regime, temperature, nutrient concentration, and sediment load. The Yampa River's postdam contribution of flow and sediment reduces the impact of Flaming Gorge Dam, but there still remains a 21% reduction in maximum annual discharge (Fig. 2b) and a 54% decrease in suspended sediment transport in the Green River downstream from Flaming Gorge as compared with predam (Andrews, 1986). Even though the overall sediment load of the Green River downstream from the Yampa River is presently half of what it was before the construction of Flaming Gorge Dam (Andrews, 1986; Grams and Schmidt, 2002), the current sediment load appears to reduce water transparency sufficiently to inhibit hydrophyte colonization.
Squires et al. (2002) reported that the Secchi threshold for macrophyte colonization was about 1 m. We estimated that the mean monthly Secchi depth was consistently >2 m in the Green River upstream of the Yampa River and <0.5 m in the Green River downstream from the Yampa River and in the Yampa River during the hydrophyte growing season (April-October, Fig. 4). In addition to limiting macrophytes, the lack of light penetration may explain why Cladophora was only found in the splash zone around large boulders and not underwater in the Green River downstream from the Yampa River and in the Yampa River. In the Green River upstream of the Yampa River, Cladophora and A. riparium carpet submerged cobbles and boulders.
We found ecosystem recovery to predam conditions for aquatic hydrophytes immediately downstream from the Yampa River. The same recovery effect has been observed to a less dramatic extent for native aquatic invertebrates (Vinson, 2001) and fish (Bestgen et al., 2007), which suggests that the Yampa River ameliorates some but not all of the ecological effects of Flaming Gorge Dam. To restore habitat for fish and invertebrates, changes in dam operations with respect to other hydrologic variables such as temperature or high flow releases that modify river habitats will also likely be needed, though the specific flow and thermal regimes required for these biota are unknown.
Funding for this study was provided by the U.S. National Park Service. T. Naumann provided resources and logistics that made this project possible. J. Kotynek and C. Vinson assisted with field collections. L. Anderton examined Holmgren's voucher specimens. T. Angradi enhanced the clarity of the manuscript. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. This article is contribution 1828 of the U.S. Geological Survey Great Lakes Science Center.
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Submitted 3 September 2013.
Acceptance recommended by Associate Editor, James E. Moore, 30 January 2014.
MARK R. VINSON, * BENNETT HESTMARK, AND MARY E. BARKWORTH
U.S. Geological Survey, Great Lakes Science Center, Lake Superior Biological Station, 2800 Lakeshore Drive East, Ashland, WI54806 (MRV) Department of Watershed Science, Utah State University, Logan, UT 84322 (BH) Intermountain Herbarium, Department of Biology, Utah State University, 5305 Old Main Hill, Logan, UT 84322 (MEB)
* Correspondent: email@example.com
TABLE 1--Aquatic plants collected in the Green River and Yampa River channels in Dinosaur National Monument before the closure of Flaming Gorge Dam in 1962 (Predam) by Holmgren (1962), Hagen and Banks (1963), and Woodbury (1963) and in 2009 and 2010 in the Green River upstream and downstream from the Yampa River and in the Yampa River. A = absent; P = present. Taxon Predam Green River upstream from the Yampa River (regulated) Amblystegium riparium A P Chara P P Cladophora P P Elodea canadensis A P Myriophyllum sibiricum A P Nasturtium officinale A P Potamogeton crispus A P Potamogeton pectinatus A P Ranunculus aquatilis A P Total species 2 9 Taxon Green River Yampa River downstream (unregulated) from the Yampa River (partially regulated) Amblystegium riparium A A Chara P P Cladophora P P Elodea canadensis A A Myriophyllum sibiricum A A Nasturtium officinale A A Potamogeton crispus A A Potamogeton pectinatus A A Ranunculus aquatilis A A Total species 2 2 In addition to the species occurrences listed, Holmgren (1962) collected five submerged vascular species, Potamogeton alpines, Potamogeton nodosus, Potamogeton pectinatus, Zannichellia palustris, and Ranunculus gmelinii near the confluence of the Green and Yampa Rivers. They may have been collected in an off-channel pond rather than in the river (see text.) Nasturtium officinale was collected by Holmgren (1962) adjacent to the Green River near the Jensen stream gauge, where a small spring enters the river and not in the main river channel (see text).
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|Author:||Vinson, Mark R.; Hestmark, Bennett; Barkworth, Mary E.|
|Date:||Dec 1, 2014|
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