Barry, R.G. and M.C. Serreze (1999) Atmospheric components of the Arctic Ocean freshwater balance and their interannual variability, in The Freshwater Budget of the Arctic Ocean, Kluwer Academic Press, Dordrecht, pp 45-65.

Fekete, B.M., C.J. Vörösmarty and R.B. Lammers (2001) Scaling gridded river networks for macroscale hydrology: Development, analysis, and control of error, Water Resources Research, 37(7):1955-1967.

Fekete, B.M., C.J. Vörösmarty and W. Grabs (1999) Global runoff fields determined by observational discharge and water balance modeling, for Global Runoff Data Centre Report No. 22, 133 pages.

Forman, S.L., W. Maslowski, J.T. Andrews, D. Lubinski, M. Steele, J. Zhang, R.B. Lammers and B.J. Peterson (2000) Researchers Explore Arctic Freshwater's Role in Ocean Circulation, EOS, Transactions, American Geophysical Union, v.81, no.16 (April 18, 2000), pages 169 and 174.

Frolking, S., K. McDonald, J. Kimball, J. Way, R. Zimmerman, and S. Running, Using the space-borne NASA Scatterometer (NSCAT) to determine the frozen and thawed seasons of a boreal landscape. Arctic and Alpine Research, in preparation.

Frolking, S., K. McDonald, J. Kimball, R. Zimmerman, J.B. Way, and S. Running (1999), Using the space-borne NASA Scatterometer (NSCAT) to determine the frozen and thawed seasons of a boreal landscape, J. Geophys. Res. 104: 27,895-27,908.

Gordeev, V.V., A.V. Zhulidov, R.M. Holmes, and B.J. Peterson (2001) River input of nutrients to the Russian Arctic: achievements and problems, in Ecology of Siberian Arctic Rivers, pp. 108-113, Tomsk, Russia (in Russian).

Holden, J.B. (2000) A large scale water balance model for the pan-Arctic drainage basin, Masters Thesis, Department of Earth Sciences, University of New Hampshire, Durham.

Holden, J.B., C.J. Vörösmarty, S. Frolking, and R.B. Lammers, A large-scale water balance model for permafrost terrain, in prep.

Holmes, R. M., J. W. McClelland, B. J. Peterson, I. A. Shiklomanov, A. I. Shiklomanov, A. V. Zhulidov, V. V. Gordeev, and N. N. Bobrovitskaya (2001) A circumpolar perspective on fluvial sediment flux to the Arctic Ocean, Global Biogeochemical Cycles (submitted).

Holmes, R.M., B.J. Peterson, A.V. Zhulidov, V.V. Gordeev, P.N. Makkaveev, P.A. Stunzas, L.S. Kosmenko, G.H. Kohler, and A.I. Shiklomanov (2001) Nutrient chemistry of the Ob' and Yenisey rivers, Siberia: results from June 2000 expedition and evaluation of long-term data sets, Marine Chemistry, 75(3):219-227.

Holmes, R.M., B.J. Peterson, V.V. Gordeev, A.V. Zhulidov, M. Meybeck, R.B. Lammers, and C.J. Vörösmarty (2000) Flux of nutrients from Russian rivers to the Arctic Ocean: Can we establish a baseline against which to judge future changes? Water Resources Research, 36:2309-2320

Holmes, R.M., A. Aminot, R. Kérouel, B.A. Hooker, and B.J. Peterson (1999) A simple and precise method for measuring ammonium in marine and freshwater ecosystems, Canadian Journal of Fisheries and Aquatic Sciences, 56:1801-1808.

Hurst, M.P. and M.C. Serreze, Representation of mean Arctic precipitation from NCEP/NCAR and ERA reanalyses, J. Climate (in press).

Kimball, J.S., K.C. McDonald, S. Frolking, and S.W. Running (2001), Radar remote sensing of the spring thaw transition across a boreal landscape, Remote Sensing of the Environment, Boreas Special Issue.

Kimball, J.S., K.C. McDonald, A.R. Keyser, S. Frolking, and S.W. Running (2001), Application of the NASA Scatterometer (NSCAT) for determining the daily frozen and non-frozen landscape of Alaska, Remote Sensing of the Environment 75: 113-126.

Lammers, R.B., A.I. Shiklomanov, C.J. Vörösmarty, B. M. Fekete and B.J. Peterson (2001) Assessment of Contemporary Arctic River Runoff Based on Observational Discharge Records, JGR-Atmospheres, 106(D4):3321-3334.

Lammers, R.B., B.M. Fekete, and C.J. Vörösmarty, A pan-Arctic river network for simulating regional water balance, in prep.

Makkaveev, P.N., P.A. Stunzhas, P.V. Khlebopashev, V.V. Gordeev, R.M. Holmes, B.J. Peterson, and A.I. Shiklomanov (2001) Flux of nutrients from Ob' and Yenisey rivers to the Arctic Ocean: results from June 2000 expeditions, in Proceedings of the Arctic Regional Centre, pp. 97-106.

Makkaveev, P.N., P.A. Stunzas, R.M. Holmes, B.J. Peterson, and P.V. Klebopashev (2001) The experience of measuring nutrients in the lower reaches of the Ob' and Yenisey rivers during June 2000 expedition, Oceanology, in prep.

Makkaveev, P.N., P.A. Stunzas, R.M. Holmes, B.J. Peterson, P.V. Klebopashev, V.V. Gordeev, L.S. Kosmenko, and A.V. Zhulidov (2001) Nitrogen transport by Siberian rivers, Priroda, in prep, (in Russian).

McGuire, A.D., J.S. Clein, J.M. Melillo, D.W. Kicklighter, R.A. Meier, C.J. Vörösmarty, and M.C. Serreze (2000) Modeling carbon responses of tundra ecosystems to historical and projected climate: sensitivity of pan-Arctic carbon storage to temporal and spatial variation in climate. Global Change Biology, 6:141-159.

Meybeck, M.M. and C. J. Vörösmarty. 1999. Global transfer of carbon by rivers. IGBP Newsletter 37: 18-19.

Rawlins, M.A. (2000) Winter air temperature change over the terrestrial arctic, Master of Science, Department of Geography, University of Delaware.

Running, S., J.B. Way, K.C. McDonald, J. Kimball, S. Frolking, A.R. Keyser, R. Zimmerman (2000).  Radar remote sensing proposed for monitoring freeze-thaw transitions in boreal regions.  Earth in Space 12: 5-9.

Running, S.W. J.B. Way, K. McDonald, J. Kimball, S. Frolking, a.R. Keyser, and R. Zimmerman (1999).  Radar remote sensing proposed for monitoring freeze-thaw transitions in boreal regions.  AGU Eos Transactions 80(19): 213, 220-221.

Serreze, M.C. and R.G. Barry (2000) Atmospheric components of the Arctic Ocean hydrologic budget assessed from rawinsonde data, in E.L. Lewis et al. (eds.) The Freshwater Budget of the Arctic Ocean, Kluwer Academic Press, Dordrecht, pp141-162.

Serreze, M.C., J.E. Walsh, F.S. Chapin III, T. Osterkamp, M. Dyurgerov, V. Romanovsky, W.C. Oechel, J. Morison, T. Zhang and R.G. Barry (2000) Observational evidence of recent change in the northern high-latitude environment, Climatic Change 46:159-207.

Serreze, M.C., D.H. Bromwich, M.P. Clark, A.J. Etringer, T. Zhang and R. Lammers, 2002: The large-scale hydro-climatology of the terrestrial Arctic drainage system, Journal of Geophysical Research (in press).

Serreze, M.C., A.H. Lynch and M.P. Clark, 2001: The Arctic frontal zone as seen in the NCEP/NCAR Reanalysis, Journal of Climate, 14, 1550-1567.

Serreze, M.C., M.P. Clark, A.J. Etringer and D.H. Bromwich, 2001: Variability and trends in the hydro-climatology of the major Eurasian Arctic drainages, in: Extended Abstracts, Second Wadati Conference on Global Change and the Polar Climate, March 7-9 2001, Tsukuba Science City, Japan, pp. 83-86.

Shiklomanov A.I., R.B. Lammers, C.J. Vörösmarty (2002) Widespread Decline in Hydrological Monitoring Threatens Pan-Arctic Research, EOS Transactions, American Geophysical Union, v. 83, no. 2 (January 8, 2002), pages 13,16,17.

Shiklomanov A.I., R.B. Lammers, C.J. Vörösmarty and B.J. Peterson (2001) River runoff in the Arctic drainage basin and its changes for the last years, Vodnye Resursyu, in press (in Russian).

Shiklomanov, I.A., A.I. Shiklomanov and R.B. Lammers, B.J. Peterson and C.J. Vörösmarty (2000) The Dynamics of River Water Inflow to the Arctic Ocean, in E.L. Lewis, et al. (eds.) The Freshwater Budget of the Arctic Ocean, pp. 281-296, Kluwer Academic Press, Dordrecht, pp 281-296.

Vörösmarty, C.J. P. Green, J. Salisbury, R.B. Lammers (2000) Global Water Resources: Vulnerability from Climate Change and Population Growth, Science, 289: 284-288, July 14, 2000.

Vörösmarty, C.J. and B.J. Peterson (2000) Macro-scale models of water and nutrient flux to the coastal zone, in J. Hobbie (ed.) Estuarine Synthesis - A synthetic appraoch to research and practice. Island Press, Washington, pp. 43-80.

Vörösmarty, C.J., B.M. Fekete, M. Meybeck and R.B. Lammers (2000) Global system of rivers: Its role in organizing continental land mass and defining land-to-ocean linkages, Global Biogeochemical Cycles, 14(2):599-621.

Vörösmarty, C.J., B.M. Fekete, M. Meybeck, and R.B. Lammers (2000) Geomorphometric attributes of the global system of rivers at 30-minute spatial resolution, J. Hydrology, 237:17-39.

Zhulidov, A.V., V.V. Khlobystov, R.D. Robarts and D.F. Pavlov (2000) Critical analysis of water quality monitoring in the Russian Federation and former Soviet Union, Canadian Journal Of Fisheries And Aquatic Sciences, 57:1932-1939.

Zhulidov, A.V., R.D. Robarts, R.M. Holmes, and B.J. Peterson (2001) Water quality monitoring in the former Soviet Union and Russian Federation: assessment of analytical methods, in prep.

Gordeev, V.V., A.V. Zhulidov, R.M. Holmes, B.J. Peterson (2000) River input of nutrients, trace metals and persistent organic pollutants to the Arctic Ocean: the state of the problem, LOIRA Conference, Moscow.

Holden, J.B., Vörösmarty, C.J. and S. Frolking (1996) A Large-Scale Water Balance Model for Permafrost Terrain: Its Calibration and Validation Against Site Data, American Geophysical Union, Spring Meeting, April 5, 1996, Baltimore, MD.

Holmes, R.M. , B.J. Peterson, J.W. McClelland (2002) Biogeochemical Tracers in Arctic Rivers: Linking the Pan-Arctic Watershed to the Arctic Ocean (the PARTNERS project), Poster presented at American Geophysical Meeting, December 6-10, 2002

Holmes, R.M., B.J. Peterson, J.W. McClelland, A.V. Zhulidov, V.V. Gordeev, A.I. Shiklomanov, R.B. Lammers, C.J. Vörösmarty (2001) A Circumpolar Perspective on Fluvial Nutrient Flux to the Arctic Ocean, Poster at the LAII All Hands Meeting, Salt Lake City, UT, November 14-17, 2001.

Holmes, R.M., B.J. Peterson, A.V. Zhulidov, V.V. Gordeev, R.B. Lammers, C.V. Vorosmarty (2001) Suspended sediment flux to the Arctic Ocean and shelf seas from Eurasian rivers: analysis of long-term data sets, American Society of Limnology and Oceanography, Albuquerque, New Mexico.

Holmes, R.M, B.J. Peterson, A. Zhulidov, V.V. Gordeev, P. Makkaveev, P. Stunzas, L. Kosmenko, A. Shiklomanov, H. Kohler, P. Khlebopashev (2000) Nutrient chemistry of the Yenisey and Ob' Rivers, Siberia: results from June 2000 expedition and evaluation of long-term data sets, American Geophysical Union, San Francisco.

Holmes, R. et al (1999) Contemporary Water and Constituent Balances for the Pan-Arctic Drainage System: Continent to Coastal Ocean Fluxes, Water and Watersheds Review Meeting. Easton MD. April 1999.

Holmes, R. M., B. J. Peterson, V. V. Gordeev, A. Zhulidov, M. Meybeck, R. B. Lammers, C. J. Vörösmarty (1999) Delivery of nutrients from the pan-Arctic watershed to the Arctic Ocean: Can we establish a baseline against which to judge future changes? Poster presented at the American Geophysical Union, San Francisco, California.

Holmes, R. M., B. J. Peterson, C. Vörösmarty, R. Lammers, V. Gordeev, I. Shiklomanov, A. Shiklomanov, M. Meybeck, A. Zhulidov (1998) Riverine nutrient flux from Russia to the Arctic Ocean, Poster presented at the ARCUS 11th Annual Meeting, Arlington, VA.

Holmes, R.M. et al., (1998) Nutrient Fluxes from Eurasia to the Arctic Ocean, poster presented at the American Geophysical Union Fall Meeting, San Francisco, CA, December 6-10, 1998.

Lammers, R.B., A. Shiklomanov, M.A. Rawlins, M. Serreze, C. Oelke, C.J. Vorosmarty (2002) Characterizing the Pan-Arctic Hydrologic Cycle with Arctic-RIMS, Poster presented at American Geophysical Meeting, December 6-10, 2002

Lammers, R.B. and R.M. Holmes (2002) Water and Constituent Fluxes Across the Eurasian Arctic: Evolving Land-Ocean Connections Over the Past 20,000 Years, joint presentation given at the Russian American Initiative on Shelf-Land Environments in the Arctic (RAISE) Steering Committee Meeting, San Francisco, CA, Dec 4-5, 2002.

Lammers R.B. (2002) Experiences in Assembling Regional and Global Scale Hydrological Data Sets, presented at the Hydrological Data For Global Studies, Expert Meeting, Toronto, Canada, 18-20, November 2002, hosted by World Meteorological Organization.

Lammers, R.B., S.L. Forman, and C.J. Vorosmarty (2002) Assessing Hydrologic Impacts of Ice Sheet Extent in Northern Eurasia, Poster presented at the 32nd Annual International Arctic Workshop, Boulder, CO, March 14-16, 2002.

Lammers, R.B., S. Forman, and C.J. Vörösmarty (2001) Assessing Hydrologic Impacts of Ice Sheet Extent in Northern Eurasia, Poster presented at the OAII All Hands Meeting, Salt Lake City, UT, November 14-16, 2001.

Lammers, R.B., A. Shiklomanov, M. Rawlins, M. Serreze, and C.J. Vörösmarty (2001) Rapid Estimation of Runoff and River Discharge across the Pan-Arctic Region, Poster presented at the OAII All Hands Meeting, Salt Lake City, UT, November 14-16, 2001.

Lammers, R.B., S. Forman, and C.J. Vörösmarty (2001) Assessing Hydrologic Impacts of Ice Sheet Extent in Northern Eurasia, Poster P.2.04.13, Challenges of a Changing Earth, IGBP Open Science Conference, Amsterdam, Netherlands July 10-13, 2001.

Lammers, R.B., A. Shiklomanov, M. Rawlins, M. Serreze, and C.J. Vörösmarty (2001) Rapid Estimation of Runoff and River Discharge across the Pan-Arctic Region, Poster P.4.02.02, Challenges of a Changing Earth, IGBP Open Science Conference, Amsterdam, Netherlands July 10-13, 2001.

Lammers, R.B. (2001) Freshwater Resources at Continental and Global Scales, Keynote address to the Resource Modeling Association World Conference, Logan, Utah, June 27-30, 2001

Lammers, R.B., S. Forman, and C.J. Vörösmarty (2000) Assessing hydrologic impacts of ice sheet extent in northern Eurasia, American Geophysical Union, Fall Meeting, San Francisco, December 15-19, 2000.

Lammers, R.B. (2000) Continental Paleohydrology of Eurasia during the Last Glacial Maximum, paper presented at NSF-ARCSS Arctic Hydrology Workshop, NCEAS, Santa Barbara, CA, 18 Sept. 2000

Lammers, R.B. (2000) The Role of Arctic Drainage Basins in Climate Change. Invited paper presented at the Second World Water Forum and Ministerial Conference, 17-22 March, 2000, The Hague, Netherlands.

Lammers, R.B., C.J. Vörösmarty, B.J. Peterson, A.I. Shiklomanov, B.M. Fekete (1999) Historical Analysis of Discharge from Pan-Arctic River Systems. Paper presented at The American Society of Limnology and Oceanography (ASLO), Feb. 1-5, 1999, Santa Fe, NM.

Lammers, R.B. et al. (1999) Historical Analysis of Discharge from Pan-Arctic River Systems. Paper presented at the Global Precipitation Climatology Project (GPCP) Workshop, Washington, D.C., May, 1999

Lammers, R.B. et al. (1999) Historical Analysis of Discharge from Pan-Arctic River Systems. Paper presented at the Workshop on Assessing Impacts of Arctic Bathymetry Changes and Fresh Water Inputs on Shelf and Ocean Circulation for the Past 20,000 Years, Naval Postgraduate School, Monterey, CA, October 1-2, 1999

Lammers, R.B. et al. (1999) Historical Analysis of Discharge from Pan-Arctic River Systems. Paper presented at the International Workshop on Arctic Coastal Dynamics, Marine Biological Laboratory, Woods Hole, MA, November 2-4, 1999

Linder, E. (2000) Strategies for Correlating Spatial-Temporal Random Fields for Large-Scale Geophysical and Climate Studies, International Conference on Statistics in the 21st Century, Orono, Maine, June 29 - July 1, 2000

McClelland, J.W., R.M. Holmes, B. J. Peterson, I.A. Shiklomanov, A.I. Shiklomanov A.V. Zhulidov, and V.V. Gordeev(2001) A Circumpolar Perspective on Fluvial Sediment Flux to the Arctic Ocean, Poster at the LAII All Hands Meeting, Salt Lake City, UT, November 14-17, 2001.

Peterson, B.J. R.M. Holmes, J.W. McClelland, C.J. Vorosmarty, R.B. Lammers, A.I. Shiklomanov, I.A. Shiklomanov, S. Rahmstorf, (2002) Increasing Arctic River Discharge: Responses and Feedbacks to Global Climate Change, Poster presented at American Geophysical Meeting, December 6-10, 2002

Peterson, B.J. et al., (1998a) The Pan-Arctic Water Balance, paper presented at the NSF/EPA Water and Watersheds Program Review, Corvallis, Oregon, Jan. 28-30, 1998.

Peterson, B.J. et al., (1998b) Contemporary Water and Constituent Balances for the Pan-Arctic Drainage System: Continent to Coastal Ocean Fluxes, Poster presented at the Arcus Annual Meeting, Arlington, Virginia, March 22-24, 1998.

Peterson, B.J. et al., (1998c) Contemporary Water and Constituent Balances for the Pan-Arctic Drainage System: Continent to Coastal Ocean Fluxes, Poster presented at the Arctic Opportunities Workshop, Arlington Virginia, September 3-4, 1998

Peterson, B.J. et al., (1998d) Contemporary Water and Constituent Balances for the Pan-Arctic Drainage System: Continent to Coastal Ocean Fluxes, Poster presented at the AAAS 49^th Annual Arctic Science Meeting, Fairbanks, Alaska, 26-28 October, 1998

Rawlins, M.A. (2002) Estimating active-layer thickness and simulated runoff across the pan-Arctic drainage system. Circumpolar Active Layer Monitoring (CALM) Workshop, Lewes, DE, November 10-15, 2002

Rawlins, M.A. , R.B. Lammers, S. Frolking, B.M. Fekete, C.J. Vorosmarty (2002) Simulating Pan-Arctic Runoff With a Macro-Scale Terrestrial Water Balance Model, Poster presented at American Geophysical Meeting, December 6-10, 2002

Rawlins, M.A., R.B. Lammers, A.I. Shiklomanov, and C.J. Vörösmarty (2002) A Permafrost Water Balance Model for the Arctic: Model Runoff and Discharge Validation Sites, Poster presented at the 32nd Annual International Arctic Workshop, Boulder, CO, March 14-16, 2002.

Rawlins, M.A. (1998) Estimation of Arctic Temperature and Precipitation Fields. Seminar presented at the Atmospheric Chemistry and Dynamics Branch, Goddard Space Flight Center, Greenbelt, Maryland (October).

Rawlins, M.A. and C.J. Willmott (1997) Estimation of Arctic Climate Fields from Station Data. Paper presented at the annual meeting of the Association of American Geographers, Fort Worth, Texas (April).

Serreze M.C. , M.P. Clark, A.J. Etringer, T. Zhang, D.H. Bromwich, R. Lammers (2002) The Large-Scale Hydro-Climatology of the Terrestrial Arctic Drainage System, Paper presented at American Geophysical Meeting, December 6-10, 2002

Shiklomanov A.I., R.B. Lammers, C.J. Vorosmarty (2002) Contemporary Changes in Eurasian pan-Arctic River Discharge, Poster presented at American Geophysical Meeting, December 6-10, 2002

Shiklomanov A.I., Golovanov O.Ph. and Lammers R.B. (2002) Status of the Hydrometric Monitoring Network in the Arctic Drainage Basin. . Proc. Of The Conference for Hydrometeorology of the Commonwealth of Independent States 23-26 April 2002, pages 138-141, (in Russian).

Shiklomanov A.I., Golovanov O.Ph and Golovanova S.V. (2002) The Increase of River Discharge in the Arctic Ocean and its possible consequences. Proc. Of The Conference for Hydrometeorology of the Commonwealth of Independent States 23-26 April 2002, pages 147-149, (in Russian).

Shiklomanov, A.I., R.B. Lammers, C.J. Vörösmarty, B.J. Peterson, R.M. Holmes, and J.W. McClelland (2002) Long-term Variability of the Pan-Arctic Hydrological Budget and the Decline in Hydrological Monitoring Networks, Poster presented at the 32nd Annual International Arctic Workshop, Boulder, CO, March 14-16, 2002.

Shiklomanov A.I., R.B. Lammers and C.J. Vörösmarty (2001) Freshwater Flux to the Arctic Ocean: Contemporary Estimates, Problems and Future Outlook, Poster at the LAII All Hands Meeting, Salt Lake City, UT, November 14-17, 2001.

Shiklomanov A.I., R.B. Lammers and C.J. Vörösmarty (2001) Freshwater Flux to the Arctic Ocean: Contemporary Estimates, Problems and Future Outlook, Poster P.4.02.01, Challenges of a Changing Earth, IGBP Open Science Conference, Amsterdam, Netherlands July 10-13, 2001.

Shiklomanov A.I., R.B. Lammers, C.J. Vörösmarty, B.J. Peterson, I.A. Shiklomanov and B.M. Fekete (1998) Summary of Existing River Discharge Data for Pan-Arctic Hydrological Modeling. Joint Presentation by A.I. Shiklomanov and R.B. Lammers at the GEWEX/ACSYS Wokshop on Cold Regions Hydrological Modelling, Quebec City, Canada, August 25-27, 1998.

Vörösmarty, C.J. and L. Hinzman (2001) The Hydrology Initiative, presentation at the joint LAII/OAII/PARCS/RAISE Plenary Session, OAII All Hands Meeting, Salt Lake City, UT, November 14-16, 2001.

Vörösmarty, C.J. and L. Hinzman (2001) The Hydrologic Cycle and Its Role in Arctic and Global Environmental Change, Poster at the LAII All Hands Meeting, Salt Lake City, UT, November 14-17, 2001.

Vörösmarty, C.J., B. Fekete, S. Glidden, P. Green, R. Lammers, M. Routhier, and A. Schloss (2001) Global Hydrological Archive and Analysis System (GHAAS): Hydrographic Data Sets Available On-Line, Poster P.1.20.07, Challenges of a Changing Earth, IGBP Open Science Conference, Amsterdam, Netherlands July 10-13, 2001.

Vörösmarty, C.J. (1999) Contemporary Water and Constituent Balances for the Pan-Arctic Drainage System: Continent-to-Coastal Ocean Fluxes, NSF Arctic Systems Science, Land-Atmosphere-Ice Interactions Program 1999 Planning Meeting. Seattle WA. March 1999.

Willmott, C.J. (1997) Estimating Interannual Climate Variability from Weather-Station Observations. Invited poster and presentation given at the NASA Workshop on Interannual Biosphere-Atmosphere Variability, Tucson, Arizona (March).

Willmott, C.J. (1998) Spatially Interpolating and Mapping Climate Fields from Lousy Weather-Station Networks. Invited Lecture presented to the Department of Geography, University of North Carolina, Chapel Hill, North Carolina (February).

Willmott, C.J. (1998) Musings on Climate, Climatic Change and Clues Distilled from the Historical Weather-Station Record. Invited Lecture presented to the Department of Geography, University of Minnesota, Minneapolis, Minnesota (April).