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A Report to the Consortium of Universities
for the Advancement of Hydrologic Sciences, Inc.
the Hydrologic Science Standing Committee
Jim Smith (Chairman), Princeton University
Claire Welty (Vice Chairman), University of Maryland Baltimore County
Ken Potter, University of Wisconsin
Larry Band, University of North Carolina Chapel Hill
Stu Schwartz, now at Cleveland State University
Greg Pasternak, University California Davis
Steve Burges, University of Washington
David Freyberg, Stanford University
David Furbish, Florida State University (now at Vanderbilt)
Jim Gosz, University New Mexico
Patricia Maurice, University Notre Dame
Leal Mertes, University California Santa Barbara
Andy Miller, University Maryland Baltimore County
Jorge Ramirez, Colorado State University
Guido Salvucci, Boston University
Laura Toran, Temple University
Introduction ........................................................................................................ 1
1. Storage, Fluxes, and Transformations in the Hydrologic Cycle ....................... 2
1.1 Land-Atmosphere Interactions ................................................................. 3
1.2 Groundwater Recharge and Discharge ..................................................... 5
1.3 Sediment Storage and Transport ............................................................... 7
2. Scaling of Hydrologic Processes ...................................................................... 9
3. Linkages Between Ecosystems and the Hydrologic Cycle ............................. 11
4. Hydrologic Prediction.................................................................................... 13
5. Water Resources Management ...................................................................... 15
References ......................................................................................................... 18
Hydrologic science deals with the “occurrence, distribution, circulation and properties of water on
the Earth. It is clearly a multidisciplinary science as water is important to and affected by physical,
chemical and biological processes within all the components of the Earth system: the atmosphere,
glaciers and ice sheets, solid Earth, rivers, lakes and oceans (NRC [1991])”. Hydrologic science also
deals in the most fundamental ways with human needs and activities. The scientific community
has produced convincing evidence that, because of the effects of human civilization, Earth is expe-
riencing environmental changes of an unprecedented magnitude. In the next hundred years, most
areas will likely undergo major changes in temperature, and significant changes in regional precipi-
tation are possible. Shifts in the water cycle will be perhaps the single most significant aspect of
these changes. These changes will have enormous impacts on human populations. Are we in a po-
sition to predict the nature of these shifts and their effect on the hydrologic cycle? At the present
time, the answer clearly is “no” (Phillips [2002]).
If we are to successfully adapt to changes and uncertainty in our freshwater resources, we need to
improve our understanding of, and ability to characterize and predict, the storage, movement, and
transformations of water in natural and impacted conditions. The principal science objective underly-
ing CUAHSI infrastructure proposals is to develop a predictive understanding of storage, fluxes, and
transformation of water, sediment, and associated chemical and microbiological constituents.
Some of the most challenging problems associated with developing a predictive understanding of
hydrologic processes concern interfaces and transition zones, such as the land-atmosphere inter-
face, the water table, the interface between groundwater and surface water flow systems, and the
transition zone between hillslopes and stream channels. These interfaces challenge our process un-
derstanding, our measurement capabilities, and our ability to model coupled hydrologic processes.
This theme is described in Section 1 with special emphasis on the connections between interfaces
in the hydrologic cycle and their links to human impact on water resource systems.
Three additional themes closely intertwined with the core science objective are: (1) the role of scale
in hydrologic storage, fluxes, and transformation, (2) the linkage between ecosystems and hydro-
logic cycle, and (3) hydrologic prediction. These themes are elaborated in Sections 2-4. In Section
5, we describe the linkages between water management and hydrologic science.
As water cycles at Earth’s surface, it undergoes numerous where the characteristic physical processes change and pa-
phase changes between solid, liquid, and gas, while it flows rameterizations are required to express their coupling. But
through and is stored in a wide variety of media. In the at- it is generally at these interfaces that we are most interested
mosphere it mixes as a gas, and coalesces in clouds. At the in, and in many cases define, hydrologic fluxes. For some
land surface it perches above the land in lakes, rivers, wet- fluxes, our heuristic understanding (in a sense, our progress)
lands, and glaciers. In the near surface it hangs in the void is expressed in terms of which “side” of the interface holds
space of soil and rocks, and fills the cells, xylem, and sto- the processes that rate-limits the magnitude of flux across
mata of vegetation. Deeper from the surface it completely the interface. For other fluxes understanding is expressed in
fills the pore spaces and cracks of rocks and sediments. The terms of which sub-processes on the rate-limiting side are
physical, dynamical, and chemical processes that control the controlling. And for others it is expressed in terms of pro-
movement and storage of water throughout these media are cesses at the interface itself. In general, the rate-limiting pro-
incredibly varied. Water is subject to forces of gravity, pres- cess, where there is one, changes with ambient conditions
sure, surface tension, and osmosis, and it is heated by radia- (geographically, seasonally, and even diurnally). Taking soil
tion and conduction. It responds with viscous and turbulent evaporation at the atmosphere-land surface interface as an
flows, diffusion, and numerous phase changes. example, the limiting factor could be the net flux through the
soil (vs. turbulent transport in the atmosphere), and within
To develop a predictive understanding of these processes, it the controlling sub-process could be vapor or liquid diffu-
is necessary to develop the analytical and measurement tools sion. Identifying such rate-limiting processes lends itself not
to quantify all components of the hydrologic cycle. Quan- only to a basic process understanding , but also leads to pa-
tification of hydrologic budgets and transport of sediment, rameterizations required to couple relevant processes in pre-
microbes, and chemicals across hydrologic boundaries re- dictive and diagnostic models.
quires knowledge of water fluxes across these boundaries. The
boundaries of interest are the: Identifying rate-limiting processes can lead not only to a
basic understanding of process, but also to improved mod-
1. atmosphere-land surface interface, els for prediction and methods for estimation. For example,
2. land surface-groundwater interface, knowing under which precipitation/soil conditions runoff
3. groundwater-surface water interface, and generation becomes infiltration-limited allows us to predict
4. land surface-surface water interface. the potential impact of higher or lower intensity rains under
a changed climate. If the runoff generation is not infiltra-
Our process understanding and ability to quantitatively pre- tion-limited, it is likely that changing rainfall intensity will
dict water fluxes in isolated media, away from boundaries, is have little to no impact. Likewise it points to the critical soil
generally better than at the interfaces of these boundaries, parameters that need to be measured for estimation and pre-
diction. For example, porosity and depth to water table mat- and connected to water resource management problems. The
ter more for storage-limited runoff, while hydraulic conduc- land-atmosphere interface is examined in Section 1.1 in con-
tivity matters more for infiltration-excess runoff. nection with problems related to land-atmosphere interac-
tions. The land surface-groundwater and groundwater-surface
As scale increases from the few centimeters over which Dar- water interfaces are discussed jointly in Section 1.2 in con-
cy fluxes are defined to the hundreds of kilometers of large nection with groundwater recharge and discharge. Problems
watersheds and aquifers, more and more of these interfaces of sediment storage and transport are discussed in Section 1.3
and processes are included, and it becomes more difficult to in connection with the land surface-surface water interface.
identify controlling processes. Large-scale observation net-
works covering groundwater, vadose zone, and atmosphere 1.1 LAND-ATMOSPHERE INTERACTIONS
processes, in conjunction with coupled process models, could
lead to new insights into what really controls water balance The land surface exerts a profound influence on weather and
at basin scales. It is entirely possible that what we assume to climate. Some of the effects of the land surface on weath-
control the balance, based on point-scale/stand-alone process er and climate are associated with the natural heterogene-
understanding, may be incorrect. Identifying the controlling, ity of Earth’s surface, especially contrasts in topography and
or rate-limiting factors is necessary for understanding, pre- land-ocean boundaries. Other effects are associated with
diction (including sensitivity analysis of land-cover land-use anthropogenic modifications of the land surface, especially
changes and climate change), and estimation techniques. urbanization and conversion of forest land, desert, and wet-
lands to agriculture. In addition to these relatively static pro-
The linkage between storage, fluxes, and transformations in cesses, the rapidly varying moisture state of the land surface
the hydrologic cycle and water resource management prob- can have a significant impact on weather through its control
lems is complex. Human activities may immediately affect of evaporative fluxes to the atmosphere. Spatial contrasts in
one component of the hydrologic cycle, and additionally have the land surface are translated into heterogeneities in the
long-term impacts on other components of the hydrologic precipitation forcing that drives the surface and subsurface
cycle. An example is sewage leaking into the ground from hydrologic cycle. For many of the water resources manage-
a septic tank (an anthropogenic activity), which then slow- ment problems and environmental change problems faced by
ly moves through an aquifer under natural flow conditions society, characterization and quantification of these hetero-
into the surface water system, contaminating a stream and geneities is either the crux of the problem or a pre-condition
potentially having an impact on drinking water quality and to solving the problem. Research in land-atmosphere inter-
stream biota. In order to adequately assess the water supply actions is needed to examine the impact of heterogeneous
and the movement of chemicals, microorganisms, and sedi- land surface properties on the atmospheric branch of the hy-
ment through the complete hydrologic cycle, these processes drologic cycle.
must be evaluated on a watershed-scale basis. Our ability to
predict hydrologic change and associated water quality over The effects of mountains on weather and climate range from
large distances and times (kilometers, years) within the com- modulations of the global-scale Rossby waves that determine
plete hydrologic cycle has been limited by the scope of the the continental-scale structure of weather systems to the lo-
typical university-based research grant, in that the processes cal amplification of precipitation through orographic precipi-
cannot be evaluated for long periods of time over large scales tation mechanisms. The role of mountains in determining
due to the prohibitive costs of such studies. the precipitation-rich and the precipitation-poor parts of the
globe is of fundamental importance for water resources man-
In the following three sub-sections, examples of scientific agement. Equally important is the role of mountains in de-
problem areas linked to hydrologic boundaries are presented termining the regions subject to hazards associated with ex-
tremes of precipitation. The global extremes of precipitation There is growing recognition that anthropogenic changes to
on both the high end (the Khasi Hills of India, La Reunion, the land surface can result in changes to regional climate. Of
and Mt. Waialeale on the island of Kauai, for example) and particular importance to water resources management are
the low end (Atacama Desert of Chile) are tied to orograph- changes in precipitation distribution. Experimental and nu-
ic precipitation mechanisms. Mountains are one of the ma- merical model studies of the effects of deforestation in the
jor sources of heterogeneities of the land surface hydrologic Amazon basin on regional climate have pointed to a number
cycle and these heterogeneities are propagated through the of difficult scientific problems that have important conse-
hydrologic cycle. The physical mechanisms by which moun- quences for coupled land and water resource management.
tains determine the spatial and temporal distribution of pre- Impacts of deforestation on regional climate are tied not
cipitation are not understood well enough to make informed only to the extent of deforestation, but also to the pattern of
decisions on efficient management of water resource systems. deforestation. The effects of a 50 km by 50 km region of de-
Understanding is hampered by the absence of an experimen- forestation would be different from the effects of a 250 km
tal base for scientific advances. The Mesocale Alpine Pro- by 10 km region of deforestation or 100 deforested patches
gram, which was carried out in northern Italy in 1999, has of 25 km2 embedded in a 10,000 km2 region. Accurate char-
provided a successful example of the experimental frame- acterization of land surface composition over a broad range
work that can address this problem. of scales is crucial for assessing and managing the impacts of
land transformation on regional climate. The impacts of land
Coastal regions are among the most densely populated ar- surface change are closely linked to the changing fluxes of
eas of the world. They are subject to chronic problems of both latent and sensible heat. The coupling of water and en-
freshwater availability and environmental hazards that are ergy budgets is central to assessments of changing regional
characterized by sharp land-ocean gradients. These gradients climate. The difficulty measuring evaporation is one of the
are closely linked to contrasts in precipitation distribution key problems of addressing the impacts of anthropogenic
strongly influenced by the land-sea boundary. changes to the land surface on regional climate.
Land breeze and sea breeze circulation systems are an im- The Metromex experiment in St. Louis during the early
portant determinant of the spatial, seasonal, and diurnal dis- 1970s demonstrated the influence of the “urban heat island”
tribution of precipitation in coastal regions. Land-falling on regional precipitation distribution. At the time some of
tropical cyclones are a major flood hazard and an important the conclusions were quite surprising. One surprise was that
element of the precipitation distribution in many coastal re- urbanization exerted a significant influence on regional pre-
gions. Tropical storms over open ocean can persist for days cipitation distribution. The effects were most pronounced
in near steady state condition. Once tropical storms interact during the warm season and had the greatest impact on sys-
with land, they typically weaken (as reflected in decreased tems of thunderstorms. Rainfall amplification did not oc-
wind speeds and pressure gradients) due to frictional effects cur in St. Louis proper but in rural areas “downwind” of the
and diminished latent heat supply to the storm. The behavior city. Urbanization has become an increasingly important is-
of tropical storms over open ocean still contains many mys- sue both in the United States and globally, but Metromex
teries, but it is far better understood than their behavior over continues to provide much of the scientific base for assessing
land. Management of water resources in coastal regions is impacts of the urban heat island on regional weather and cli-
hampered by the difficulties in characterizing the influence mate. Urbanization in the corridor from Washington D.C. to
of the land-ocean boundary on precipitation distribution. Boston, Massachusetts has created one of the most complex
precipitation regimes in the United States. The combination
of urban heat island effects, land-sea breeze circulation sys- and natural processes playing a significant role. At the ex-
tems, and orographic precipitation mechanisms is recognized treme end of the hydrologic spectrum, the processes respon-
by every weather forecaster in the region as part of the mix sible for desertification may include feedback effects associ-
that affects precipitation distribution. How this mix works ated with human activities.
is poorly understood relative to the water resource manage-
ment and hazard assessment problems at stake. Metromex Evaporation is one of the principal means of communica-
provides a clear guide to the experimental approach need- tion between the land surface and atmosphere. It is also one
ed for addressing the impacts of urbanization on regional of the most difficult elements of the hydrologic cycle to mea-
weather and climate. sure. Advances in assessing the impacts of land surface het-
erogeneities on regional weather and climate require major
The time-varying moisture state of the surface provides a strides in measurement of evaporation.
dynamic control of regional weather and climate. Soil mois-
ture anomalies have been examined as one of the ingredients 1.2 GROUNDWATER RECHARGE AND
of the period of heavy rainfall resulting in the Mississippi DISCHARGE
flood of 1993. A direct link between soil moisture anomalies
and precipitation distribution was established by the Illinois Because of the high quality, constant temperature, and rela-
State Water Survey for a storm that produced record rain- tively low variability of groundwater, its recharge and dis-
fall (425 mm in less than 24 hours) in the upper Midwest. charge near the surface is critical to both humans and eco-
Heavy rainfall on 17-18 July 1996 in Illinois can be linked to systems. Quantitative information concerning the spatial and
soil moisture anomalies in Iowa produced by heavy rainfall temporal distribution of these fluxes, under existing condi-
on the preceding day. An important lesson from these analy- tions or conditions of changed climate and/or land use and
ses is that the impact of soil moisture on weather and climate cover, is important to a number of issues including water
is tied to the coupled fluxes of water and energy to the at- supply development, regulation of point and non-point con-
mosphere. A principal influence of soil moisture anomalies tamination of surface waters, aquatic habitat assessment, es-
on weather and climate is through thermodynamic modifi- tablishment of instream flow requirements, and assessment
cations of the atmosphere that promote storm development of pumping impacts on aquatic ecosystem. The need for this
and intensification. As with problems related to deforesta- information can only increase with increases in population
tion, the pattern of soil moisture anomalies plays an impor- and its associated impacts on aquatic systems. Particular at-
tant role in assessing impacts of these anomalies on regional tention in problems of groundwater recharge and discharge
weather and climate. Characterization and monitoring of soil is focused on the unsaturated zone and the hyporheic zone.
moisture and vegetation over a wide range of scales is a cen-
tral problem in assessing the role of the changing moisture The hyporheic zone is the region of sediment immediately
state of the surface on regional weather and climate. surrounding and beneath a stream where the groundwater
and surface water environments come into contact. The size
Climate variability over decadal to millennial time scales of the hyporheic zone can vary with the types of streambed
represents one of the greatest challenges for water resources and bank sediments, the slope of the streambed, and the hy-
management. Water systems must be robust to climate vari- draulic gradients in the surrounding groundwater environ-
ability over these time scales, yet the basic scientific under- ment. Hyporheic exchange affects the chemical properties of
standing of climate variability over these time scales is poor. both surface water and shallow groundwater, and has been
Emerging evidence points to the importance of land surface shown to have pronounced ecological consequences. Stor-
processes for climate variability, with both anthropogenic
age within the hyporheic zone can potentially dampen the A formidable challenge for quantifying fluxes across the land
hydrologic and biogeochemical (including pollutant) conse- surface-groundwater (recharge) interface and groundwater-
quences of floods. Groundwater exchange into streams can surface water (discharge) interface is characterizing the spa-
lessen the impact of droughts, at least in the short term. The tial heterogeneity of the subsurface properties controlling
hyporheic zone can serve as a source of nutrients and micro- these fluxes.
organisms to the stream and to the shallow subsurface, can
play an important role in mitigation of surface-water pollu- A combination of large-scale remote sensing coupled with
tion, and is a region of active hydrobiogeochemical processes, land-based and subsurface instrumentation and appropriate
with gradients in temperature, oxygen content, nutrient con- large-scale tracer tests is required to quantify these compo-
centrations, and microbial ecology. Because the hyporheic nents of the hydrologic budget on the watershed scale. An
zone is by definition an interface between surface and sub- example of a promising technology for locating discharge
surface water environments, it is one of the least understood areas from the subsurface to surface water bodies is infrared
portions of the hydrologic cycle, and requires new approach- imaging. However, of infrared radar flyovers are normally
es at the boundaries of surface and subsurface hydrology. prohibitively expensive and not accessible to the average uni-
versity researcher.
The problem of understanding and predicting the spatial and
temporal distribution of groundwater recharge and discharge Few studies have had sufficient data to develop methodolo-
is an important challenge to the science of hydrology. These gies to incorporate spatial and temporal variation of recharge.
fluxes are the integrated result of virtually all other hydro- Key elements currently limiting the development of recharge
logic processes, and their variation in space and time appears estimation are: (1) limited field comparison and testing of
to depend on numerous local and regional conditions. Up to available and emerging recharge estimation methodologies;
now, interest in the latter has dominated, and predictive ef- (2) limited ability to scale point measurements of recharge
forts have largely been empirically driven. The challenge to up to the modeling scale, and lack of validation/testing data;
the science is to show that significant improvement in pre- and (3) limited testing of combining water balance com-
dictive capacity can result from scientific efforts focused on ponents measured at differing scales, i.e., rain gauges, radar
both individual processes related to recharge and discharge estimation of precipitation, satellite-derived estimates of
and their couplings. evapotranspiration, and watershed scale measures of runoff
into accurate measures of aquifer recharge.
The key processes and factors that potentially control the
space-time pattern of groundwater recharge and discharge The impacts of the proposed watershed scale research go di-
are generally well known. The processes include precipita- rectly to the heart of water resource issues. Currently, few
tion, infiltration, evapotranspiration, percolation and capil- tools exist to balance the often-conflicting objectives of water
lary rise through the unsaturated zone, and groundwater flow resource extraction with protection of water resources for oth-
divergence. The factors affecting these processes include to- er uses such as in-stream flows or endangered species. With
pography, soils, vegetation, land use, subsurface geology, and the development of efficient recharge estimation tools for
climate. In spite of this general understanding, however, we climatic regions ranging from humid to arid, public debate
have very limited ability to predict groundwater recharge and over the allocation and use of water resources will have a solid
discharge in specific watersheds. Reliable tools and method- foundation of rigorously tested tools and will have the meth-
ologies simply do not exist to assess aquifer recharge at the odologies with which to make water management decisions.
typical watershed scale (10-1000 km2) or at the watershed Water resource planners and public policy makers will be able,
modeling grid scale (1-10 km2). in quantitative terms, to assess the impacts of future climate
change on groundwater availability, to quantify the impacts increased land-surface impervious area can give rise to in-
of groundwater extraction on stream flows and to better un- stream erosion, thereby contributing to the sediment
derstand the linkages between groundwater and surface water export load from a watershed.
across a wide spectrum of hydroclimatological regions.
Soil erosion is well understood and can be reasonably well
Problems of groundwater recharge and discharge are particu- quantified. But because the transport of eroded soil is high-
larly difficult in karst regions. Karst covers approximately a ly intermittent, it is much less well understood and poorly
quarter of Earth’s land surface and provides potable water to quantified. Of particular concern is the transport of fine-
a quarter of the world’s population. In these regions, the dis- grained sediments, which accounts for most of the contami-
tinction between ground and surface water is blurred because nation. Restoration of water quality and ecosystem function
of rapid recharge through sinkholes and high transmissivity in the Chesapeake Bay, for example, has focused on reduc-
of the conduit systems. This link between surface and ground tion of agricultural contaminants mobilized by soil erosion.
water makes land use changes critical for both surface and Development of remediation strategies, however, has been
ground water quality. In addition to providing water for hu- plagued by uncertainties about transport and storage of sedi-
man consumption, karst aquifers also support numerous en- ment-associated contaminants, especially those associated
demic and endangered species (snails to manatee) that rely with fine-grained sediment. It remains difficult to deter-
on discharge from karst springs. These aquifers also support mine whether a soil particle eroded from a farm in Pennsyl-
chemosynthetic microbial communities that may be the base vania will arrive in the Chesapeake Bay within two weeks,
of the food web and might also provide habitats for novel two years, two centuries, or two millenia. The answer to this
microbial species. In spite of rapid flow through karst aqui- problem has major implications for development of strate-
fers, some preliminary water quality monitoring indicates gies for efficient management of freshwater resources.
chemical contamination and ecological disturbance has been
increasing on decadal time scales. Water quality and physical A challenge facing researchers today is quantification of
hydrogeology are thus critical issues for karst aquifers. Re- sediment budgets on a watershed scale, like the drainage of
gardless of their significance, little is known of contaminant Chesapeake Bay, so that the impact of sediment transport
pathways through the aquifers, flow paths to springs, chemi- can be assessed. This type of activity is needed to document
cal behavior of contaminants, or the ultimate effect of con- the effects of anthropogenic perturbations giving rise to sedi-
tamination on endemic flora and fauna. On a regional scale, ment transport versus natural effects that would be expected
karst aquifers can be approached by traditional hydrogeologic to contribute to the process. An example type of study that
methods (as equivalent to porous media), but on a local scale, could be used to distinguish anthropogenic effects versus
the complex nature of karst that results from the range of natural effects would be a paired watershed study of various
porosities and permeabilities requires alternate approaches. types of disturbed versus undisturbed watersheds in similar
geologic and climatic environments, where the transport of
1.3 SEDIMENT STORAGE AND TRANSPORT sediment and sediment budget would be quantified over the
long term.
A significant effect of the interaction between humans
and the environment involves the movement of sediment There are numerous examples of deleterious effects of sedi-
from land into streams and within the stream system. mentation on surface water systems in addition to those fur-
Elevated transport of sediment into surface water bodies nished by the Chesapeake Bay. One example is fish spawn-
arises from agricultural practices, deforestation, and con- ing areas composed of gravel where sediment clogging the
struction activities. In addition, flash flooding caused by gravel can kill embryos in fish eggs by depriving them of
oxygen and reduce the flux exchange between groundwater for human uses, widespread efforts are underway to make the
and surface water systems. A second example is the grow- 21st century an era of returning rivers back to natural func-
ing problem in the United States of sedimentation of many tioning. The need for such efforts stems from the well-docu-
multi-purpose reservoirs that are part of our aging water mented loss of in-channel and streamside habitat, extensive
management infrastructure. While sedimentation is reduc- river regulation, increasing flood risks and flood damages,
ing the useful life and water storage capability of these struc- changing societal attitudes, and the poor water quality of
tures, management decisions must be made as to whether to American rivers with ~50% non-fishable and non-swimma-
restore the reservoirs to their former water volume, and if so, ble. However, limited theory and data are available to guide
what to do with the dredged sediment. the current efforts, which results in risky experiments yield-
ing little or no basis for scientific advancement. In many cas-
Another issue with sediment transport is the co-transport of es, these experiments result in trade-offs in which hypotheti-
chemicals (e.g., PCBs, phosphorous) sorbed to moving sedi- cal gains offset the destruction of well-functioning natural
ment particles that would not otherwise be present in the areas. Furthermore, larger scale integrated watershed man-
water column due to their chemical properties. This type of agement is rarely considered in localized project designs. If
coupled process has significant implications for water quality the science of river restoration and rehabilitation is not pur-
management, because moving sediment can introduce chem- sued aggressively, it is highly likely that the era of well-inten-
icals into the water system that would otherwise be bound to tioned projects will merely serve as an efficient mechanism
the land. Water quality is affected by a combination of hy- for further destruction of the environment.
drologic, geologic, chemical, and biological processes. Inte-
grated research accounts for the coupling of these factors and
thus can accurately describe the chemical transformations
that determine water quality.
Storage and transport of sediment in stream channels also
plays a fundamental role in problems related to the structur-
al properties of channel floodplain systems. Although river
channel/floodplain systems are frequently examined from
the perspective of equilibrium theory, many river systems
throughout the world are in a disturbed or disequilibrium
condition owing to altered flow regimes and sediment supply
resulting from human activities. Implications for society are
often profound but patterns of response in many cases are
poorly understood. Consequently there is a need for more
comprehensive understanding of characteristic spatial and
temporal patterns of channel/floodplain system response to
anthropogenic disturbance.
Public agencies and private groups presently are carrying out
a second generation of river projects in the United States at-
tempting to improve riverine conditions by returning them
to a more natural condition. While the 20th century was re-
plete with river projects seeking to tame and convert rivers

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