Colorado River Basin Development

The Colorado River flows 2300 km (about 1400 mi) from the high mountain regions of Colorado through seven basin states to the Sea of Cortez in Mexico (Figure 1). The river supplies much of the water needs of seven U.S. states, two Mexican states, and 34 Native American tribes. These represent a population of 25 million inhabitants, with a projection of 38 million by the year 2020. Approximately 2% of the basin is in Mexico. The Colorado does not discharge a large volume of water. Because of the scale of impoundments and withdrawals relative to its flow, the Colorado has been called the most legislated and managed river in the world. It has also been called the most "cussed" and "discussed" river in the United States. About 86% of the Colorado's annual runoff originates within only 15% of the area, in the high mountains of Colorado and the Wind River Range in Wyoming. In the semiarid Southwest, even relatively small changes in precipitation can have

Figure 1 The Colorado River basin. (From the U.S. Department of the Interior, Bureau of Reclamation.)

large impacts on water supplies. The coefficient of variation for the Colorado is about 33%.

Climate and weather events form a variable background on which water agreements and conflicts are played out. Indeed, Powell's comment above, as dire as it might seem, was not made in the context of potentially large swings in the climate system. The specter of long-term climate variations overlays a series of other issues, including growth in municipal and industrial water demands, groundwater depletion, unmet ecosystem needs, and water quality requirements. Decadal-scale climatic factors influencing present water alloca-

tions, discussed in greater detail elsewhere (Dracup, 1977; Stockton and Boggess, 1979), are of increasing significance in the management of the Colorado. In addition, it is likely that climatic changes may already be affecting the snowpack and runoff conditions in the Colorado watershed. This introduces a new set of forcings on regional climate factors that affect water supply.

As has been well documented, the most important management agreement (the Colorado River Compact of 1922) was based on overestimation of the reliable average annual supply of water due to a short observational record. Briefly, the period 1905-25 was the wettest such period in 400 years of record, with 16.4 million acre-feet (maf1) reconstructed annual average flow at Lees Ferry. The 1922 compact signatories used this average number as the base minimum for fixed allocation between upper and lower basins. As a nod to interannual variability in water supply, the signatories assumed that flow would average out over 10 years and made the downstream requirement 75 maf over the said 10-year period. Colorado River streamflow, however, exhibits strong decadal and longer variations (Figure 2). Since the signing of the compact, the reliable estimated annual virgin flow has been about 14.3 maf, with a historic low flow of 5.6 maf in 1934.

Emphases on water demand management, meeting obligations to Native American tribes, maintaining water quality, and environmental concerns have also altered the traditional roles of federal, state, and local agencies. The impacts of recent events such as the continuing regional-scale droughts since 1999, including the extreme drought of 2002, and recent enforcements restricting California to its compact allotment are only just beginning to be understood in terms of system criticality and requirements for noncrisis or proactive mitigation of drought impacts.

1 1 maf = 1.24 million liters (325, 851 million gallons). Million acre-feet (maf) is used as the unit of water volume throughout this chapter. All entities on the Colorado River use maf as the unit of measure.

Annual Colorado River Flow at Lees Ferry, Arizona

Annual Colorado River Flow at Lees Ferry, Arizona


Figure 2 Decadal-scale variability of Colorado River streamflow at Lees Ferry, 1896-2003. Smoothed using a 9-year moving average. (Data from the U.S. Department of the Interior, Bureau of Reclamation.)


Figure 2 Decadal-scale variability of Colorado River streamflow at Lees Ferry, 1896-2003. Smoothed using a 9-year moving average. (Data from the U.S. Department of the Interior, Bureau of Reclamation.)

This chapter uses climate-sensitive decision environments along the Colorado River to illustrate the breadth and complexity of the water management issues and the role of climate in these contexts. The four examples are in: (1) the border region: international issues; (2) Arizona and California: interstate issues in the Lower Basin; (3) Native American water rights; and (4) conjunctive use and management: groundwater and surface water in Arizona.

Recent drought impacts on the Colorado River reservoirs have raised significant concern about the reliability of deliveries in the event of a decadal or multi-decadal drought. Until recently, the expectation of Colorado River managers was that significant shortages in the Lower Basin would not occur until after 2030. With reservoir levels at historic lows, newspaper headlines and politicians are focused directly on the drought/water supply issue. Generally, focusing events like this expose critically vulnerable conditions and, although they warn of potential crisis, are also opportunities for innovation. Potential water resource-related focusing events across the western United States include: (1) extreme climatic conditions (e.g., drought and floods); (2) large-scale inter-basin transfers; (3) quantification of tribal water rights; (4) an energy crisis; (5) changing transboundary responsibilities; and (6) regulatory mandates such as the Endangered Species and Clean Water Acts. Crisis conditions can be said to be reached when focusing events occur concurrently with public awareness of a finite time necessary for effective response. In this context, institutional conditions that limit flexibility tend to exacerbate the underlying resource issues.

We begin with a broad overview of the history of Colorado River basin development and the scales of decision making (governance and operational requirements) involved. The decision-making environments are discussed in terms of drought-sensitive issues at international, inter-state, Native American, and state levels. The development of the Colorado River Compact (and its use of a limited record of streamflow) mentioned above is discussed in great detail in numerous books and articles (see Weatherford and Brown, 1986) and will be referred to here only when it introduces a criticality to the management problem being considered. Two issues that were not in the original compact but have since become more important will be addressed in some detail: conjunctive use (i.e., joint use of surface and groundwater) and water quality.

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