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NATHANIEL BOOTH, DAVID KASMER, and DIANE MCKNIGHT, Institute for Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80302
Glacial meltwater streams are common features of the McMurdo Dry Valleys (Vincent, Howard-Williams, and Broady 1993). Stream research has focused primarily on the Taylor and Wright Valleys with occasional studies in Miers Valley and the Alph River. The record of lake-level rise and the long-term record for the Onyx River in Wright Valley indicate that a warming trend has occurred over the last several decades in the McMurdo Dry Valleys (Chinn 1993). To understand the effects of climate and landscape position on dry valley streams, it is useful to know more about the other streams in other valleys. Several descriptors, such as flow, chemical, physical, and biological characteristics, are helpful in comparing stream systems. Garwood Valley is located approximately 56 kilometers south-southeast of Taylor Valley and provides a stream system containing a relatively long second-order stream.
Two site visits to the Garwood Valley occurred on 24 January 1995 and 18 January 1997 as part of the McMurdo Dry Valleys Long-Term Ecological Research project. Both visits included flow- and field-meter measurements and sample collection at various locations along the four main reaches of the river. Measurements were made following methods described by Alger et al. (1997) and Von Guerard et al. (1994). The results are presented in tables 1-3.
The Garwood River is approximately 8 kilometers long, starting at the Garwood Glacier and discharging into McMurdo Sound. The Garwood Valley is a wide, steep valley composed mainly of sand and gravel-sized till. It is open to maritime air masses. The river has relatively high flow and can be characterized into four main reaches: the headwater tributaries, the confluence to the flats, the flats, and the ponds. All reaches are relatively straight, except for a meander before the flats. Two tributaries, the north and the south forks, join approximately 1.6 kilometers below the glacier. The reaches of the tributaries between the glacier and the confluence are steep-sloped narrow channels of cobble and gravel substrates, and flow is turbulent with high velocities. The green alga Prasiola sp. was found growing as mats attached to the underside of the rocks in the south fork of the Garwood River. These Prasiola streamers are common in steep rocky reaches of Bohner Stream and several other streams in Taylor Valley (Alger et al. 1997).
From the confluence to the flats, the river continues down a steep, narrow channel with very high set-in banks of fine gravel. This loose material is at its angle of repose on both sides of the stream. Two small streams drain high snow fields through the steep southern valley wall into this reach. These extremely low-flow streams have stable cobble substrates with diverse algae including Prasiola streamers, orange-colored mats of filamentous cyanobacteria, and dark red-colored algal mats on the rocks. These orange- and red-colored mats appear to be similar to orange mats found in Taylor Valley streams in that they occur in flowing-water habitat (Alger et al. 1997).
The steep, narrow channel opens up on to a flats area of sand and gravel. The gradient is low and evidence of stream meandering within this area is apparent. Some orange-colored algal mats are found in bands on the sides of the widened channel. The final reach of the river is characterized by several ice-covered ponds terraced along the last 100 meters of flow to the ocean. Diverse types of algal mats occur in the ponds.
The flow in the Garwood River system was several times greater than streams in Taylor Valley at the same time. The climate may be more moderate and sunnier in Garwood Valley. Comparison of the south fork and confluence reaches in tables 1 and 2 shows that the north and the south forks contribute similar flow.
The south fork has higher ionic concentrations, in particular sulfate and chlorine. The sodium-to-chloride ratios given in table 3 are higher than those of sea water, which indicates that, in addition to dissolution of marine aerosols, mineral weathering may be a source for sodium in this stream system. Comparison of the results from the two field trips shows that chloride, sulfate, sodium, and calcium concentrations are higher at low flow in January 1995, whereas the ratios of major ions were similar. These results indicate that dilution by the glacial meltwater is one factor, in addition to mineral weathering, that controls the streamwater chemistry. Regarding ion concentration changes downstream, on both dates the values of the major ions were very similar at the flats site and the site immediately below the confluence of the north and south forks. This result suggests that exchange of water between the stream and the adjacent hyporheic zone does not result in a downstream increase in solute concentrations as has been observed in Von Guerard Stream in Taylor Valley.
In summary, the geomorphological features of the Garwood River are similar to those of streams throughout the McMurdo Dry Valleys. The same types of algal mats appear to be common in Garwood Valley. The main differences of this system, compared to those in the Taylor and Wright Valley, may be a warmer, sunnier climate and a longer period of streamflow during some colder summers.
This research was supported by National Science Foundation grant OPP 92-11773.
Alger, A.S., D.M. McKnight, S.A. Spaulding, C.M. Tate, G.H. Shupe, K.A. Welch, R. Edwards, E.D. Andrews, and H.R. House. 1997. Ecological processes in a cold desert ecosystem: The abundance and species distribution of algal mats in glacial meltwater streams in Taylor Valley, Antarctica (occasional paper no. 51). Boulder: Institute of Arctic and Alpine Research.
Chinn, T.H. 1993. Physical hydrology of the dry valley lakes. In W.J. Green and E.I. Friedmann (Eds.), Physical and biogeochemical processes in antarctic lakes (Antarctic Research Series, Vol. 59). Washington, D.C.: American Geophysical Union.
Vincent, W.F., C. Howard-Williams, and P.A. Broady. 1993. Microbial communities and processes in antarctic flowing waters. In Antarctic Microbiology . New York: Wiley-Liss.
Von Guerard, P., D.M. McKnight, R.A. Harnish, J.W. Gartner, and E.D. Andrews. 1994. Streamflow, water-temperature, and specific-conductance data for selected streams draining into Lake Fryxell, Lower Taylor Valley, Victoria Land, Antarctica, 1990-92. U.S. Geological Survey Open-File Report 94-545. Denver: U.S. Geological Survey.