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How Are We Changing the Physical Environment of Earth’s Surface?

Accelerated human modification of the landscape and human-driven climate changes are fundamentally altering Earth’s surface processes and creating ecological challenges that scientists and policy makers are struggling to address. The environmental impacts of human activity are expected to increase as the climate continues to warm and as the world becomes progressively more populated, industrialized, and urbanized. Scientific research has generally succeeded in documenting the magnitude of these biophysical changes, including habitat loss and fragmentation, soil erosion, biodiversity loss, and water depletion and degradation. Yet the exact processes leading to these changes are still not adequately understood and quantified, and we still lack the best methods and techniques for detecting, measuring, and analyzing global change.

Soil erosion provides a prime example to understand what is at stake. Although a natural process, soil erosion has greatly accelerated globally due to cultivation, deforestation, and a host of other land-use practices (Montgomery, 2007a,b; Figure 1.1). Increased soil erosion generates sediment supply that often exceeds the transport capacity of stream systems, leading to vast sediment storage on channel beds, on hillslopes, and in floodplains. This historical sedimentation has already had significant impacts on channel processes, aquatic systems, and fisheries (Waters, 1995; NRC, 2004). Moreover, these legacy sediments represent a future risk because they can be remobilized and introduced into aquatic systems even following landscape amelioration (Walter and Merrits, 2008).

Anticipated climate change will heighten the human impact on the physical environment in many places. Predicting the magnitude and timing of these future impacts remains uncertain, but measurable changes have already occurred climatically (Elsner et al., 2008) and hydrologically over the past few decades, with earlier ice-out dates, reduced magnitudes of spring runoff and summer low flows, and changes in the timing of peak streamflows (Hodgkins et al., 2002, 2003; Huntington et al., 2003, 2004). Future climate change will likely bring greater hydrological and ecological shifts nationally and globally, with potentially profound impacts on water availability (Arnell, 2004; Milly et al., 2005; IPCC, 2007).

Earth surface changes, then, frequently raise resource management challenges, prompting efforts at ecological restoration, and environmental legislation often requires communities or other stakeholders to restore stream channels or wetlands. Yet it is uncertain how, and under what circumstances, most disturbed natural systems can recover, and even less is known about the baseline conditions that may potentially guide restoration efforts. Despite the development of a billion-dollar-a-year restoration industry, the science of watershed restoration is still in its infancy (Wohl et al., 2005; Walter and Merrits, 2008). Large uncertainties remain in other aspects of wetland and river restoration as well, including the ecological and economic tradeoffs of structural (“hard”) vs. nonstructural (“soft”) approaches and, more importantly, the metrics, goals, and time frames for guiding and achieving watershed restoration. These are just a few examples of the



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1 How Are We Changing the Physical Environment of Earth’s Surface? A ccelerated human modification of the land- Anticipated climate change will heighten the scape and human-driven climate changes human impact on the physical environment in many are fundamentally altering Earth’s surface places. Predicting the magnitude and timing of these processes and creating ecological challenges that scien- future impacts remains uncertain, but measurable tists and policy makers are struggling to address. The changes have already occurred climatically (Elsner et environmental impacts of human activity are expected al., 2008) and hydrologically over the past few decades, to increase as the climate continues to warm and as with earlier ice-out dates, reduced magnitudes of spring the world becomes progressively more populated, runoff and summer low flows, and changes in the tim- industrialized, and urbanized. Scientific research has ing of peak streamflows (Hodgkins et al., 2002, 2003; generally succeeded in documenting the magnitude of Huntington et al., 2003, 2004). Future climate change these biophysical changes, including habitat loss and will likely bring greater hydrological and ecological fragmentation, soil erosion, biodiversity loss, and water shifts nationally and globally, with potentially profound depletion and degradation. Yet the exact processes lead- impacts on water availability (Arnell, 2004; Milly et al., ing to these changes are still not adequately understood 2005; IPCC, 2007). and quantified, and we still lack the best methods and Earth surface changes, then, frequently raise re- techniques for detecting, measuring, and analyzing source management challenges, prompting efforts at global change. ecological restoration, and environmental legislation Soil erosion provides a prime example to under- often requires communities or other stakeholders to stand what is at stake. Although a natural process, soil restore stream channels or wetlands. Yet it is uncertain erosion has greatly accelerated globally due to cultiva- how, and under what circumstances, most disturbed tion, deforestation, and a host of other land-use prac- natural systems can recover, and even less is known tices (Montgomery, 2007a,b; Figure 1.1). Increased soil about the baseline conditions that may potentially erosion generates sediment supply that often exceeds guide restoration efforts. Despite the development of a the transport capacity of stream systems, leading to vast billion-dollar-a-year restoration industry, the science of sediment storage on channel beds, on hillslopes, and in watershed restoration is still in its infancy (Wohl et al., floodplains. This historical sedimentation has already 2005; Walter and Merrits, 2008). Large uncertainties had significant impacts on channel processes, aquatic remain in other aspects of wetland and river restoration systems, and fisheries (Waters, 1995; NRC, 2004). as well, including the ecological and economic trade- Moreover, these legacy sediments represent a future risk offs of structural (“hard”) vs. nonstructural (“soft”) because they can be remobilized and introduced into approaches and, more importantly, the metrics, goals, aquatic systems even following landscape amelioration and time frames for guiding and achieving watershed (Walter and Merrits, 2008). restoration. These are just a few examples of the 

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 UNDERSTANDING THE CHANGING PLANET changes. For example, because river systems respond to the integrative effects of climate and watershed processes, changes in streamflow, channel properties, and fluvial deposits provide information on the tim- ing, direction, and magnitude of postglacial climate changes, suggesting that even modest climate shifts can generate significant changes in streamflow (Knox, 1993). Analyses of fluvial stratigraphic records have proved to be important because the identification of paleoflood occurrence extends the researchable time frame of these low-frequency events well beyond the stream gauge record, thus improving flood forecasting (Enzel et al., 1993; Baker, 1998) and capturing the periodicity of highly variable climatic episodes such as El Niño events (Gomez et al., 2004; Magilligan et al., 2008). These paleorecords suggest that climatic stationarity (the mean and variance of a time series) has not remained constant over time (Milly et al., 2008), which raises questions about existing water allocation arrangements because the stationarity assumption is the cornerstone of dam design and water allocation FIGURE 1.1 Comparison of natural erosion rates (over geo­ logical time) to agricultural soil erosion rates in relation to rates strategies. Higher resolution and longer-term datasets, of soil production. The graph line comprising squares shows such as those that can come from dendrochronology, the rates of natural soil production, the circles show natural can help capture these statistical shifts. geological erosion, and the top line of diamonds shows agri­ The geographical sciences have contributed to cultural erosion far exceeding the other two rates. SOURCE: Montgomery (2007a). our understanding of floods as well, especially in rela- tion to land-use changes. The massive construction of dams over the past several hundred years has had a profound impact on the hydrological regime (Figure practical and scientific reasons why we need to better 1.2), often leading to hydrological modifications ex- understand the impacts of humans on Earth’s physical ceeding the impacts of climate change (Magilligan et environment. al., 2003; Magilligan and Nislow, 2005). Using archi- val national data, Graf (1999) identified more than 80,000 dams that have been constructed in the United role oF The geograPhical scieNces States—essentially 1 dam per day on average since the Because natural processes vary spatially and across signing of the Declaration of Independence. Graf ’s scales, a geographical perspective is essential to under- examination of the geographical location and context standing their nature and character. The perspec- of these dams showed marked regional variations in tives and tools of the geographical sciences used by dam number and type; most of the dams in the United geographers, geologists, ecologists, and others provide States are in the eastern half of the country, although insights into soil erosion, flood magnitude and fre- dams with the greatest impact on storage are found in quency, and ecological adjustments to climate change the West (Graf, 1999, 2001). This pattern suggests that, on both contemporary and paleotimescales. One although watershed fragmentation may be consider- significant area of investigation focuses on watershed able in the eastern United States, ecological impacts response to and recovery from environmental changes, due to flow reductions may be more significant in the including Quaternary (past 2-3 million years) climatic western part of the country. Other field-based studies changes and historical human-induced landscape have provided fundamental insights into the profound

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 BIOPHYSICAL FIGURE 1.2 Number of dams constructed in the United States over the past 200 years (by decade) categorized by dam height. SOURCE: Doyle et al. (2003). and sustained changes resulting from flow regulation, These records provide the only means of identifying the including changes in channel properties, sediment processes creating climatic variability and determining transport, and reduced ecological habitat (Chin et al. when anthropogenic climate changes have exceeded 2002; Phillips et al., 2005). natural variability (Diffenbaugh et al., 2006; Herweijer Because of their concern with spatiotemporal et al., 2006; MacDonald et al., 2008b). dynamics, geographical scientists have been at the The integrated and synthetic research that is a hall- forefront of efforts to use paleoenvironmental data mark of the geographical sciences is essential to address to provide long proxy records of climatic and environ- one of the major challenges in climate-change research: mental change. Through techniques such as fossil pol- determining the natural (as opposed to human) contri- len analysis, fossil charcoal analysis, tree-ring analysis, bution to climatic variability. Many paleoclimatic re- diatom analysis, chironomid analysis, and various sedi- cords and long instrumental data series provide evidence mentological and geochemical techniques, geographi- of variations in temperature that persist for decades to cal scientists have been able to reconstruct changes centuries. This natural variability in the climate system in terrestrial and aquatic environments on timescales has two important implications for anticipating the ranging from decades to millennia. Such reconstruc- impacts of global warming from increased greenhouse tions can identify the specific nature of human impacts gases. First, if we do not understand their causes and in the past, provide insight into the natural variability properties, natural variations in climate make it dif- in environmental systems prior to human alteration, ficult to detect or attribute current and future changes and show how environments have responded to past in climate to anthropogenic factors such as increased episodes of climate change. They can also be used greenhouse gases. Second, such natural variations to validate climate models used for estimating future are likely to persist even in the face of greenhouse climate change scenarios (Figure 1.3). In addition to gas–induced climate changes and should be taken into providing qualitative and quantitative information account when planning for climate change. Often the on past environments, paleontological approaches relationships between the ultimate climatic forcing are increasingly being refined and used to provide factors are mediated by complex relationships between quantitative records of past temperature, precipitation, the atmosphere, oceans, and land surface that play out drought severity, and river flow (Cook et al., 2007). differently from place to place (Feddema et al., 2005).

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 UNDERSTANDING THE CHANGING PLANET FIGURE 1.3 A mapped summary of changes in plant taxa distributions and biome distributions over the past 10,000 years based on sites in the North American Pollen Data Base. NOTES: CCON = cool conifer forest, CDEC = cold deciduous forest, CLMX = cool mixed forest, CWOD = conifer woodland, DESE = desert, MXPA = mixed parkland, SPPA = spruce parkland, STEP = steppe, TAIG = taiga, TDEC = temperate deciduous forest, TUND = tundra, WMMX = warm mixed forest, XERO = xerophytic scrub. SOURCE: Williams et al. (2004). The synthesis of different measures of climate change on the biophysical environment. Examples from the over long temporal scales and across space is required fluvial sciences are used to illustrate the importance of to link particular forcing factors to climatic variations. the research, largely because watershed processes are The following questions are examples of research that major landscape-forming agents. However, applications would be particularly productive to pursue as part of the in the coastal, aeolian, hillslope, weathering, and glacial effort to refine understanding of the impacts of humans sciences also represent important avenues for research.

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5 BIOPHYSICAL research suBQuesTioNs Equally important, more refined geographical analysis of hydrological responses to climate and envi- ronmental change can provide insight into the variable What are the natural rates of earth’s surface processes contributions of nature and humans to earth-surface and how has human activity affected them? changes. Climate models generally agree that wet areas Human activity has altered terrestrial, aquatic, and will become progressively wetter (IPCC, 2007), but marine ecosystems, and these effects exceed natural it remains uncertain how that atmospheric shift will baseline conditions. Although progress has been made translate hydrologically. Fluvial theory suggests that in determining natural rates of earth-surface processes the magnitude and frequency of floods will increase as relative to anthropogenic effects, more research is the climate gets wetter, but the impacts may be more needed across a suite of processes and regions, with complex, including changes in the timing of floods and greater attention paid to theoretically informed, the relative shift in the relationship between sediment empirically grounded assessments of the causes and peaks and streamflow. Previous research on paleofloods consequences of anthropogenic disturbance. Anthro- provides important insights into fluvial responses to pogenic impacts have been profound across a suite of climate change. Research in this vein is spatially in- earth-surface processes. The significance and scope complete, however, with most of the work restricted of these impacts is evident in fluvial systems, for ex- to the United States and Western Europe (Baker, ample. Yet erosion and transport rates of sediments 2008). More studies are needed globally, and past flood stored within watersheds are still poorly understood, chronologies need greater temporal calibration and as is the residence time of these sediments. More- resolution. Recent advances in dating techniques, in- over, sediments stored in floodplains represent a vast cluding optically stimulated luminescence (OSL), can but currently unknown reservoir of material. These help in this task, because they allow for more accurate sediments are often contaminated with agricultural dating of paleofloods, especially in regions where 14C pesticides and herbicides. If stored for an adequate dating is limited, such as in deserts or in situations ex- time, the toxicity of these contaminants attenuates, ceeding the temporal bounds of 14C dating (~50 ka). but if released by subsequent channel erosion, they may lead to progressive degradation of biotic habitats how can we best plan for and implement landscape and contribute to degraded water quality, especially restorations when disturbed areas are constantly if resulting sediment concentrations exceed Environ- influenced by human activity? mental Protection Agency water quality standards for turbidity. Few places on Earth remain unaffected by human The renewed focus on landscape evolution requires activity. As anthropogenic disturbances have increased in accurate measures of erosion and sediment yield and magnitude and areal coverage during the past century, necessitates studies to determine what component of there has been a corresponding increase in efforts to the total contemporary sediment yield can be attributed mitigate their impacts. Hydrological systems, for ex- to the human imprint. The application of contem- ample, have been especially affected by human activity, porary measured sediment yields to these long-term ultimately leading to demands for remediation; however, studies may lead to unknown errors in calculating the science of watershed restoration lags far behind the long-term (geological time) landscape erosion rates. On need for, and application of, mitigation strategies (Wohl more contemporary timescales, the recent application et al., 2005). Management strategies range from com- of fallout radionuclides, such as 7Be, 137Cs, and 210Pb, plete preservation and removal of direct human impacts has led to greater understanding of erosion and sedi- to attempts to restore and rehabilitate some element of mentation rates and their spatial variability (Walling biophysical functioning and ecological integrity. Poten- et al., 1999; Kaste et al., 2006), but more studies are tial approaches may be constrained by legal and socio- needed over larger spatial scales to more accurately link economic limitations, but also by not having an accurate processes of erosion to sedimentation and contaminant understanding of biophysical processes, relaxation times, sequestration. and the scientific metrics of successful restoration.

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6 UNDERSTANDING THE CHANGING PLANET Because of extensive channelization, damming, and impacts. As dam removal is increasingly recommended other structural modifications, most rivers—both in the as a panacea for habitat restoration, it is important to United States and globally—are ecologically impaired, consider whether the release of stored sediment, in resulting in a wide range of impacts including habitat some instances, may be harmful to ecosystem func- loss and fragmentation, interruptions in the hydrologi- tioning (Stanley and Doyle, 2002; Sethi et al., 2004; cal regime, and changes in water quality and tempera- Snyder et al., 2004). The focus on sediment dynamics ture (Stanford and Ward, 1993, 2001; Poff et al., 1997; will require more sophisticated approaches, includ- Magilligan et al., 2003). To combat this degradation, ing numerical modeling, parameter estimation, flume efforts are under way to restore everything from small studies, and field-based empirical approaches. Novel tributaries to rivers as large as the Rhine and ecosystems tracing studies are emerging to document sediment as vast as the Everglades. Management agencies such fluxes and residence times, including single-grain OSL, geochemical tracers such as 7Be and 210Pb, and active as the U.S. Forest Service and the Nature Conservancy (radio) and passive (iron, magnetic) tracers (Schmidt are demanding the implementation of “environmental and Ergenzinger, 1992; DeLong and Arnold, 2007; flows” that capture predisturbance conditions, but these Salant et al., 2007). These approaches need refine- goals may not be attainable given existing stakeholder ment and broader application across a continuum of demands and sociopolitical realities. Moreover, un- depositional environments. And studies that focus on known complexities exist where, for example, estab- the role of geographical context in producing detected lishing a flow regime to meet hydrological connectivity stream-channel adjustments offer tremendous potential may have repercussions on sedimentation and aquatic across the geosciences. habitat (Kondolf and Wilcock, 1996). More research needs to be directed at determining the correct mag- nitude and timing of flows to accommodate manage- What tools offer particular promise in the effort ment goals within a context of humanized landscapes to detect and measure changes in earth-surface where complete restoration is impractical and where it processes, and how might those tools be deployed is difficult or impossible to assess the precise character to enhance understanding of the impacts of humans of a system not disturbed by humans. There needs to on earth’s physical environment? be better development of process-based restoration ef- Advances in remote sensing and geographic infor- forts (Kondolf et al., 2006; Doyle et al., 2007; Simon et mation systems have radically transformed the physi- al., 2007). In some instances, though, river restoration cal sciences, providing innovative opportunities to cannot be fully achieved because of social and technical measure, analyze, and visualize geographical data and limitations; hence there is a growing focus on river re- to raise and answer important new research questions habilitation aimed at reestablishing fundamental river- (see Chapter 10). The magnitude and scale of envi- ine processes (Wohl et al., 2005). Successful restoration ronmental change makes it imperative that we utilize and rehabilitation efforts typically require collaborative these new technologies to document global change, research teams of geographers, ecologists, and other and to develop appropriate mitigation and adaptation scientists conducting long-term field experiments and strategies. Remote sensing technologies have enormous manipulations to assess the best possible restoration potential to facilitate the identification of regions at risk outcomes. and to assess the magnitude and types of environmental The coming decades will require greater attention changes that are occurring. They are also critical to the to sediment impacts associated with changing envi- development of early warning systems. Moreover, some ronmental conditions. For example, with more than of the most far-reaching future environmental changes 500 dams removed thus far in the United States and are likely to occur in regions lacking adequate data or many more targeted for removal in the relatively near long-term databases (e.g., eastern Africa, southwestern future, there is a pressing need to advance understand- Asia, and the polar regions) (IPCC, 2007)—making ing of the impacts of sediment fluxes as stream chan- the application of remote sensing in these areas all the nels reestablish new equilibrium profiles. Moreover, more important (Box 1.1). Remote sensing analysis has the release of stored sediment has unknown ecological

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 BIOPHYSICAL BOX 1.1 Monitoring Changing Hydrological Conditions in Polar Regions Using Remote Sensing With global climate change models indicating that high-latitude regions will experience the greatest impact of global warming, there has been a concentrated effort to understand the complex environmental shifts occurring in polar regions. Because these regions are vast and often lack on-the- ground observation and data collection, remote sensing offers an important opportunity to monitor and track hydrological, ecological, and geomorphic adjustments. Using a historical archive of satellite images in Siberia, Smith et al. (2005) monitored the changing surface area of more than 10,000 large lakes and showed a widespread decline in lake abundance and area since 1973 (see Figure). The rapid warming occurring in this vast region over the ~50 years generates major permafrost thawing, leading to significant subsurface lake drainage. The total number of large lakes (those >40 ha) decreased by ~11 percent between 1973 and 1997-1998. In general, most lakes shrank to sizes below 40 ha rather than disappearing completely, with total regional lake surface area decreasing by ~6 percent. Satellite imagery revealed that 125 lakes vanished completely. Their subsequent monitoring further confirms that none of these lakes have refilled since 1997-1998 and are thus considered to be permanently drained. (A) Locations of Siberian lake inventories, permafrost distribution, and vanished lakes. (B) Total lake abundance and inundation area have declined since 1973, including (C) permanent drainage and revegetation of former lakebeds (the arrow and oval show representative areas). (D) Net increases in lake abundance and area have occurred in continuous permafrost, suggesting an initial but transitory increase in surface ponding. SOURCE: Smith et al. (2005).

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 UNDERSTANDING THE CHANGING PLANET already proved beneficial in documenting the recent lated. Because the costs are relatively high, most lidar shrinking of subtropical highland glaciers (Coudrain missions cover only relatively small areas, and many et al., 2005), but more detailed, longer-term remote regions have not yet been mapped. Nonetheless, the sensing can expand our understanding further. Simi- centimeter-scale resolution of lidar offers enormous larly, remote sensing is capturing the widespread frag- opportunities in the physical sciences, especially in mentation of tropical forests (Morton et al., 2006), documenting global sea-level change, erosion and uplift yet longer term geographical records are needed, as of mountain ranges, agricultural soil erosion, glacial are remote sensing studies and on-the-ground surveys retreat, and postflood stream-channel changes. When that can synthesize and analyze the integrated physical multiple lidar returns are recorded for each location, and biological effects of deforestation on human and the data can be used both to map the topography of the biological communities. ground surface and to infer characteristics of vegeta- One of the most significant recent advances in tion, such as tree height (Andersen et al., 2006). Other remote sensing sources of topographic infor- remote sensing is lidar (light detection and ranging), mation also exist. Interferometric radar has been used which provides very high resolution topographic data to map broader areas at coarser spatial resolution than (Figure 1.4). Lidar systems transmit pulses of visible lidar systems, as demonstrated in the February 2000 or near-infrared laser light from an aircraft to the sur- Shuttle Radar Topography Mission, which mapped face. By measuring the time it takes for the pulses to 80 percent of the world’s land surface during an 11-day be reflected, the elevation of the surface can be calcu- FIGURE 1.4 A high­resolution lidar image of a coastal bluff south of San Francisco, California, showing erosion over a 6­month period. The majority of bluff loss occurred in the fall (blue), with smaller sections of the bluff eroding in the spring (orange and pink) for a total horizontal loss at the top of the bluff of 7­10 meters (23­33 feet). Lidar also allowed the total volume of the eroded bluff material to be calculated, a key component in the development of an accurate sediment budget for this section of coastline. Such lidar­derived information is valuable to coastal managers interested both in the timing and magnitude of coastal erosion events and understanding where material lost from eroding bluffs is likely to accumulate. This information can guide development decisions adjacent to the bluffs, but was not commonly available prior to the development of lidar. SOURCE: Brian Collins, U.S. Geological Survey, 2004.

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 BIOPHYSICAL period. When interferometric radar data are collected Experiment ) satellite has been the documentation of at multiple points in time, differential interferometric groundwater levels from space (Strassberg, et al. 2009). analysis can be used to measure centimeter-scale changes Destined to be launched in 2013 (NRC, 2007a), the in topography over broad areas, such as those resulting SWOT (Surface Water Ocean Topography) satellite from seismic activity along faults, ground subsidence mission offers enormous potential to monitor global- due to groundwater or oil extraction, and changes in scale hydrological changes and map surface-water el- glaciers and ice sheets (Kwok and Fahnestock, 1996; evations. These rapid and remote techniques have great Bürgmann et al., 2000). Although interferometric potential for the geographical study of fluvial systems. radar provides coarser resolution than lidar systems, The utilization and incorporation of remote sensing its global coverage from satellite platforms is currently into a range of investigations of hydrological and eco- more temporally and spatially extensive, and it shows logical phenomena offer researchers opportunities for exceptional promise for many applications in the physi- collaborative and interdisciplinary studies (Walsh et al., cal sciences. 2003) that can lead to more spatially explicit, and there- There are several areas in watershed science that fore more useful, models of biophysical processes. could benefit from remote sensing applications. The U.S. Geological Survey operates a dense network of summarY stream gauges in the United States, yet there is a pau- As the foregoing examples make clear, spatial analysis, city of gauges globally and thus large parts of the world field-based research, geographical visualization, and lack adequate data on streamflow. With the application fine-grained contextual studies are critical to assessing of SAR (synthetic aperture radar) and MODIS (Mod- erate Resolution Imaging Spectroradiometer)—two the magnitude and types of global biophysical adjust- satellites gathering remote sensing data—it is becom- ments that are presently occurring. The approaches ing increasingly possible to measure streamflow (Smith, and techniques of the geographical sciences can help 1997; Brakenridge et al., 1998, 2007) and sediment load identify and quantify the biophysical changes unfold- (Gomez et al., 1995) from satellites. Other promising ing on Earth’s surface, and they can offer insights into approaches include mapping of stream-channel habitat the processes shaping those changes at different scales. using hyperspectral imagery (Marcus et al., 2003; Mar- Geographical science approaches and techniques thus cus and Fonstad, 2008) and integrating meteorologi- have an important role to play in advancing scientific cal data and watershed response (Smith et al., 2007). understanding of biophysical changes and facilitating Although launched for mapping gravity anomalies and the efforts of resource managers and policy makers to crustal characteristics, one of the important extensions confront Earth’s changing environment. from the GRACE (Gravity Recovery and Climate

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