MODEL ARTICLE IN PRESS Geomorphology xx 2006 xxx xxx www elsevier com locate geomorph Urban transformation of river landscapes in a global context Anne Chin Department of Geography Texas A and M University College Station TX 77843 USA Received 11 November 2005 received in revised form 6 June 2006 accepted 6 June 2006 Abstract Over the past 50 years considerable progress has been made in understanding the impacts of urban development on river processes and forms Such advances have occurred as urban population growth has accelerated around the world Using a compilation of research results from more than 100 studies conducted in a range of areas 58 addressing morphological change this paper describes how urbanization has transformed river landscapes across Earth s surface emphasizing the distribution of impacts in a global comparative context Urban development induces an initial phase of sediment mobilization characterized by increased sediment production on the order of 2 10 times and deposition within channels followed by eventual decline that couples with erosion from increased runoff to enlarge channels Data from humid and temperate environments around the world indicate that channels generally enlarge to 2 3 times and as much as 15 times the original size Although research has emphasized temperate environments recent studies of tropical areas indicate a tendency for channel reduction resulting from strong sediment erosion and deposition responses because of intense precipitation and highly weathered soils Embryonic research in arid environments further suggests variable river responses to urbanization that are characterized by rapid morphological change over short distances Regardless of location the persistence of the sediment production phase varies from months to several years whereas several decades are likely needed for enlarging channels to stabilize and potentially reach a new equilibrium Urbanizing streams pose particular challenges for management given an inherent changing nature Successful management requires a clear understanding of the temporal and spatial variations in adjustment processes 2006 Elsevier B V All rights reserved Keywords Urbanization Channel adjustments Stream equilibrium River management 1 Introduction A half century has passed since scholars convened at the International Symposium on Man s Role in Changing the Face of the Earth and cast sharp focus on the reality of human impacts on Earth systems Thomas 1956 The question explored at the symposium was What has been and is happening to the earth s surface as Present address National Science Foundation Division of Behavioral and Cognitive Sciences Geography and Regional Science Program Arlington VA 22230 USA E mail address chin tamu edu a result of humans having been on it for a long time increasing in numbers and skills unevenly at different places and times Fejos 1956 In relation to river landscapes Strahler 1956 outlined erosion and aggradation as system responses when steady state is upset by human activity and Leopold 1956 connected changes in sediment yield driven by land use changes to adjustments in river channels Significant advances have been made along those lines in the years since with intensified research efforts producing a voluminous literature that documents a range of human impacts on fluvial geomorphology in general and on river channels in particular Gregory 1977a 1987a b 2006 this volume 0169 555X see front matter 2006 Elsevier B V All rights reserved doi 10 1016 j geomorph 2006 06 033 GEOMOR 02051 No of Pages 28 ARTICLE IN PRESS 2 A Chin Geomorphology xx 2006 xxx xxx This paper assesses the progress made on understanding the impacts of urban development on river landscapes with emphasis on the distribution of such impacts in a global comparative context Urban development has been a major driver of change across Earth s surface accelerating in recent decades in response to population growth Fig 1 In the third quarter of the twentieth century alone urban population increased over 100 worldwide and nearly 200 in less developed regions Gupta 1984 These rates represent an enormous number of people increasingly living in urban areas Whereas in 1952 the largest city in the world New York City had a population of less than eight million by 2001 17 cities had eight million inhabitants with the largest urban area Shanghai exceeding a population of 14 million United Nations 2004 As might be expected the development of infrastructure to accommodate expanding populations Fig 1 Worldwide urban population change from 1952 to 2001 approximately covering the time period discussed in this paper produced from original data United Nations 1952 2004 ARTICLE IN PRESS A Chin Geomorphology xx 2006 xxx xxx would pose formidable demands on river systems Eyles 1997 Douglas 2005 a situation that will likely continue into the future This paper synthesizes research results published since 1956 in a range of world areas to answer three questions First what have been the impacts of urban development on river systems across Earth s surface Second how do these impacts vary with locale and hydroclimatic environment to the extent indicated by empirical data Third how persistent are the impacts at different locales and environments and what does that persistence indicate about whether rivers can truly adjust to the impacts of urban development Lastly the paper concludes by highlighting some challenges for managing urban river systems 2 A half century of investigations 2 1 The database Developing from the work of Gregory 1977b 1987a b 1995 and Brookes and Gregory 1988 the data for this paper include results from more than 100 published studies that document urbanization impacts on river systems over the past five decades The investigations selected report hydrologic and sedimentologic process alterations but emphasize those quantifying morphological change within channels and watersheds 58 studies The publications are primarily 3 journal articles and book chapters although much of the early work in the 1960s on hydrologic effects of urbanization were reported by the U S Geological Survey in the Professional Paper series Water Supply Papers Open File Reports and Circulars and some of these have been included The studies comprise work that appeared in mainstream English language publications since 1956 Equivalent analysis of journals in other languages as well as review of unpublished materials could expand the