Urban transformation of river landscapes in a global context

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.

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 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, Gregory, 1987a, Gregory, 1987b, Gregory, this volume).

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 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.