that allow the integration of these diverse data into mathematical models.

The desire to restore landscapes and ecosystems to their “predisturbance” states has led to an emerging field of restoration geomorphology. A key question is whether it is possible to help a dynamic landscape persist through human-induced changes and retain its most important and desirable attributes. A good example is stream restoration (Bernhardt et al., 2005), which presents a surprisingly complicated set of objectives. In a typical situation the desired state might be a laterally migrating, self-maintaining stream channel that passes the sediment it receives, rather than allowing it to accumulate in undesirable places; maintains habitat for plants and animals; and maintains its dissolved load and nutrient content at appropriate levels. Although we are learning how to address some of these objectives, we lack mechanistic models for river channels that represent their morphology, sediment load, and interaction with vegetation. And even a good design for current conditions might not be useful through flood-drought cycles and longer term climate changes. Another example of landscape change is dam building. While this kind of change is completely human caused and initially local, it is now recognized to have effects that are global in scale (Syvitski et al., 2003, 2005; see Box 4.4).

Given the inevitability of environmental change, whether natural or human induced, stream systems need to be managed for the desirable ecosystem characteristics even if, for example, sea level rises, precipitation changes, or mountain glaciers disappear. For example, global warming brings permafrost melting in polar regions, along with a range of hydrological, ecological, and geochemical changes (Chapin et al., 2006). Because warming will continue well into the future no matter how we attempt to manage greenhouse gases today, human societies need the capability to predict the consequences and take actions that preserve functions and resources (e.g., see Box 4.3).

Hazards from surface processes include landsliding, flooding, and coastal retreat. Hazard mitigation has traditionally relied on the use of maps that delineate some aspect of risk, but such maps tend to rely on the intuitive skill of the mapmaker and are typically based on a fixed environmental state. This means that the maps rapidly become inaccurate. With advances in weather and climate forecasting, the availability of digital topography, and improved understanding of processes, hazard prediction is becoming spatially explicit, up to date, and much more useful for mitigation efforts. With today’s 10-day forecasts of weather, flood forecasts are becoming commonplace as well, although not yet achieving good spatial extent and accuracy. Scientists are also beginning to forecast landslides in response to predicted rainfalls, but they still lack the ability to predict landslide size, location, travel distance, or speed. Sea-level rise, changes in storminess, and reductions in sediment due to dams may influence the effects of large storms on lowland river, delta, and coastal systems. However, we cannot yet predict how sea-level rise will affect levees or the flood heights on lowland rivers or determine whether artificial levees could be removed while still retaining flood protection. Answering these and many other such questions will require a body of field studies, experiments, theory, and numerical modeling sufficient to build the predictive science of watershed resiliency and hazard mitigation.

Summary

Our ability to manage natural resources, safely dispose of wastes, and sustain the environment depends on our understanding of fluids, both at the surface and below ground. In particular, we need a better grasp of how fluids flow, how they transport materials and heat, and how they interact with and modify their surroundings. The list of significant fluids begins with water, the most abundant and important Earth fluid, and includes steam, hydrocarbons, liquid and gaseous carbon dioxide, other organic liquids, and multiphase fluids (gas plus liquid, immiscible liquids, and gas plus immiscible liquids). For subsurface processes we need to understand how these fluids are distributed in heterogeneous rock and soil formations, how fast they flow, and how they are affected by chemical and thermal exchange with the host formations. At Earth’s surface we are concerned with the flow of water in rivers and streams, how stream erosion and sediment transport change landscapes, and how human activities and climate change affect the evolution of streams and landscapes.

Decades of research have brought substantial knowledge about the flow and transport of fluids, but application of this knowledge is strained by increased



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