Elevated water temperatures have been implicated as a factor limiting the recovery of anadromous salmonids in the Klamath River basin. This article reviews evidence of a multi-decade trend of increasing temperatures in the lower main-stem Klamath River above the ocean and, based on model simulations, finds a high probability that water temperature has been increasing by approximately 0.5 degrees C/decade (95% confidence interval [CI] = 0.42-0.60 degrees C/decade) since the early 1960s. The season of high temperatures that are potentially stressful to salmonids has lengthened by about I month over the period studied, and the average length of main-stem river with cool summer temperatures has declined by about 8.2 km/ decade. Water temperature trends seem unrelated to any change in main-stem water availability but are consistent with measured basinwide air temperature increases. Main-stem warming may be related to the cyclic Pacific Decadal Oscillation, but if this trend continues it might jeopardize the recovery of anadromous salmonids in the Klamath River basin.

Freshwater resources, because of a host of human assaults, but especially because of dams, are the most degraded of the Earth's major ecosystems. Now the future of every dam on Earth is threatened-- not by environmental protests or economic constraints-- but by the Greenhouse Effect and the world's changing climate. Historical and geological evidence over past millennia indicate that even small changes in climate can cause major changes in the size of floods. Insurers increasingly are convinced that global warming is to blame for the greater frequency and severity of violent storms, floods and droughts since the late 1980s.
Hydrologists cannot predict exactly how much water will flow into a planned reservoir. To make a "best guess," they project past streamflow data into the future. Overestimates of average flows mean that many dams fail to yield as much power and water as predicted, the Buendia-Entrepenas reservoir in Spain is an example.
Sedimentation, despite over 60 years of research, still may be the most serious technical problem faced by the dam industry. In the US, large reservoirs lose storage capacity at an average rate of 0.2% per year, in China the rate is closer to 2.3%. Despite all the uncertainties surrounding reservoir sedimentation, authorities very rarely stop planned projects due to a lack of adequate sediment data.

Major rivers worldwide have experienced dramatic changes in flow, reducing their natural ability to adjust to and absorb disturbances. Given expected changes in global climate and water needs, this may create serious problems,including loss of native biodiversity and risks to ecosystems and humans from increased flooding or water shortages.Here, we project river discharge under different climate and water withdrawal scenarios and combine this with data on the impact of dams on large river basins to create global maps illustrating potential changes in dischargeand water stress for dam-impacted and free-flowing basins. The projections indicate that every populatedbasin in the world will experience changes in river discharge and many will experience water stress. The magnitude of these impacts is used to identify basins likely and almost certain to require proactive or reactive managementintervention. Our analysis indicates that the area in need of management action to mitigate the impacts ofclimate change is much greater for basins impacted by dams than for basins with free-flowing rivers. Nearly one billion people live in areas likely to require action and approximately 365 million people live in basins almost certainto require action. Proactive management efforts will minimize risks to ecosystems and people and may be lesscostly than reactive efforts taken only once problems have arisen.

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climatechange detection and attribution study, using a high-resolution hydrologic model forced by globalclimate models, focusing on the changes that have already affected this primarily arid regionwith a large and growing population. The results show that up to 60% of the climate-related trendsof river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced.These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.

The overall goal of the National Assessment is to analyze and evaluate what is known about the potential consequences of climate variability and change for the Nation in thecontext of other pressures on the public, the environment, and the Nation's resources. Itis also addressing the question about why we should care about, and how we might effectively prepare for, climate variability and change.

Climate change is likely to affect the generation of energy from California's high-elevation hydropower systems. To investigate these impacts, this study formulates alinear programming model of an 11-reservoir hydroelectric system operated by theSacramento Municipal Utility District in the Upper American River basin.

The Pacific Northwest (PNW) hydropower resource, central to the region's electricity supply, is vulnerable to the impacts of climate change. The Northwest Power andConservation Council (NWPCC), an interstate compact agency, has conducted long termplanning for the PNW electricity supply for its 2005 Power Plan. In formulating its power portfolio recommendation, the NWPCC explored uncertainty in variables that affect theavailability and cost of electricity over the next 20 years. The NWPCC conducted an initialassessment of potential impacts of climate change on the hydropower system, but these results are not incorporated in the riskmodel upon which the 2005 Plan recommendations are based. To assist in bringing climate information into the planning process, we present an assessment of uncertainty in future PNW hydropower generation potential based on a comprehensive set ofclimate models and greenhouse gas emissions pathways. We find that the prognosis for PNW hydropower supply under climate change is worse than anticipated by the NWPCC's assessment. Differences between the predictions of individual climate models are found to contribute more to overall uncertainty than do divergent emissions pathways. Uncertainty in predictions of precipitation change appears to bemore important with respect to impact on PNW hydropower than uncertainty in predictions of temperature change. We also find that a simple regression model captures nearly all of the response of a sequence of complex numerical models to large scale changes in climate. This result offers the possibility of streamlining both top-down impact assessment and bottom-up adaptation planning for PNW water and energy resources.

Systems for management of water throughout the developed world have been designed and operated under the assumption of stationarity. Stationarity-the idea that natural systems fluctuate within an unchanging envelope of variability-is a foundational concept that permeates trainingand practice in water-resource engineering. It implies that any variable (e.g., annual streamflow or annual flood peak) has a time-invariant(or 1-year-periodic) probability density function (pdf), whose properties can be estimated from the instrument record. Under stationarity, pdf estimation errors are acknowledged, but have been assumed to be reducible by additional observations, more efficient estimators, or regional or paleohydrologic data. The pdfs, in turn, are used to evaluateand manage risks to water supplies, waterworks, and floodplains; annual global investment in water infrastructure exceeds U.S.$500 billion.

The potential effects of climate change on the hydrology and water resources of the Columbia River Basin (CRB) were evaluated using simulations from the U.S. Department of Energy and National Center for Atmospheric Research Parallel Climate Model (DOE/NCAR PCM).

This study focuses on three climate projections for the 21st century based on a ‘business as usual' (BAU) global emissions scenario, evaluated with respect to a control climate scenario based on static 1995 emissions. Time-varying monthly PCM temperature and precipitation changes werestatistically downscaled and temporally disaggregated to produce daily forcings that drove a macroscale hydrologic simulation model of the Columbia River basin at 1/4-degree spatial resolution.

This report presents a review of economic studies for the United States and relates them to predicted impacts of climate change. The summary findings are organized by region and identify the key sectors likely affected by climate change, the main impacts to be expected, as well as estimates of costs. The report builds on the 2000 Global Change Research Program National Assessment, using additional regional and local studies, as well as new calculations derived from federal, state and industry data sources. From this review and quantification, five key lessons emerge:

1. Economic impacts of climate change will occurthroughout the country
2. Economic impacts will be unevenly distributedacross regions and within the economy andsociety.
3. Negative climate impacts will outweighbenefits for most sectors that provide essentialgoods and services to society.
4. Climate change impacts will place immensestrains on public sector budgets.
5. Secondary effects of climate impacts caninclude higher prices, reduced income and joblosses.