From its headwaters in the Rocky Mountains to its terminus at the Gulf of California, the Colorado River defines life in the American Southwest.

Water 101

• Surface water: Water flowing or stored above ground in the Colorado River.
• Groundwater: Water located beneath the Earth’s surface within aquifers, interacting with and supplying the Colorado River and its tributaries.
• Return flows: Water that has been diverted from the Colorado River system for use (e.g., irrigation, municipal) and then flows back into the river or its tributaries.
• Precipitation and water cycle: Snow and rain falling within the Colorado River Basin, the primary source of its water, driving snowpack accumulation, melt, runoff, and evaporation that replenish the river system.
• Headwaters: The upper reaches and smaller streams in the Rocky Mountains of the Upper Basin states (Colorado, Wyoming, Utah, New Mexico) that form the initial sources of the Colorado River and its tributaries.
• Tributaries: Smaller rivers and streams (e.g., Gunnison, San Juan, Green) that flow into and contribute water to the main stem of the Colorado River.
• Mainstem: The primary channel of the Colorado River as it flows from its headwaters in the Upper Basin southwestward towards the Gulf of California.

Overview of the Colorado River

The Colorado River Basin encompasses a vast and geographically diverse area of approximately 250,000 square miles in the southwestern United States. Originating in the high elevations of the Rocky Mountains in Colorado, the Colorado River flows for about 1,450 miles southwestward, eventually reaching the Gulf of California in Mexico.

The basin is divided into the Upper Basin, which includes Colorado, New Mexico, Utah, and Wyoming, and the Lower Basin, which includes Arizona, California, and Nevada. The 1  river and its tributaries are a vital water source for these seven states and also serve parts of Mexico.

Within the basin reside numerous Federally Recognized Tribes, each with distinct histories and cultural ties to the region and its waters. These include the Navajo Nation, the Hopi Tribe, the Ute Mountain Ute Tribe, the Colorado River Indian Tribes, and many others – totaling around 30 sovereign tribal nations. These Tribes hold significant water rights within the basin, some of which are the most senior.

Two key reservoirs on the Colorado River, Lake Powell and Lake Mead, are crucial for water storage and management in the basin.

Lake Powell, created by the Glen Canyon Dam on the Utah-Arizona border, is the second-largest artificial reservoir by maximum water capacity in the United States. It boasts nearly 2,000 miles of shoreline and numerous canyons, making it a popular recreational area. Lake Powell plays a critical role in regulating water flow from the Upper Basin to the Lower Basin.

Lake Mead, formed by the Hoover Dam on the Arizona-Nevada border, is the largest reservoir in the United States by maximum water capacity. It serves as a major water storage facility for the Lower Basin states and Mexico, supplying water for municipal, industrial, and agricultural uses, including drinking water for millions of people. Lake Mead also offers extensive recreational opportunities along its approximately 750 miles of shoreline.

The Colorado River Basin is a complex system governed by a collection of laws, agreements, and decrees known as the Law of the River. Balancing the water needs of the various states, Tribes, and Mexico, especially in the face of increasing demands and climate change, presents ongoing challenges for the region.

Laws that Shape the River

What is a Compact?
An interstate compact is a legally binding agreement established between two or more states. This mechanism is authorized by Article I, Section 10 of the United States Constitution.
For an interstate compact to take effect, it must be ratified by the legislatures of the participating states and, in many cases, receive the consent of the U.S. Congress. Upon ratification and congressional consent (if required), the compact becomes both state law within the signatory states and federal law.
Interstate compacts hold unique significance. When Congress authorizes a compact, the federal government effectively cedes a portion of its sovereign authority to the participating states, allowing them to govern specific matters through mutual agreement. This authority is derived from the Constitution, enabling states, as sovereign entities, to collaborate on shared concerns and establish legally enforceable agreements among themselves.

1922 Colorado River Compact
In 1922, following a decade of unusually high precipitation, the seven basin states of the Colorado River drafted the Colorado River Compact. This agreement allocated 7.5 million acre-feet of water annually to the Upper Basin states and 7.5 million acre-feet annually to the Lower Basin states. Subsequently, a treaty allocated an additional 1.5 million acre-feet per year to Mexico.

The Colorado River Compact holds the distinction of being the first interstate water compact in the United States.
A key provision of the Colorado River Compact is Article III(d), which stipulates that the Upper Division states shall not cause the flow of the Colorado River at Lee Ferry to fall below an aggregate of 75 million acre-feet for any ten consecutive-year period.

It is important to note that this is not an annual delivery obligation. This distinction was a critical point for Delph Carpenter, a negotiator of the compact. Recognizing the significant variability in the Colorado River Basin’s hydrology, he was concerned that a fixed annual delivery requirement of 7.5 million acre-feet could lead to violations for the Upper Division states in the initial years following the compact’s ratification, particularly during periods of low natural flow.

Carpenter advocated for the inclusion of a flexible ten-year average, framing the obligation as a commitment not to cause the flows to be depleted below the specified aggregate. This provision aimed to protect the Upper Division states from being forced to curtail water use due to drought conditions or other natural variations in river flow over which they had no control.

Therefore, any determination of whether the Upper Division states have failed to meet their non-depletion obligation to the Lower Basin would necessitate an inquiry into the factors causing the flows to drop below the 75 million acre-feet threshold over a ten-year period.

In essence, if reduced water availability in the Colorado River system is attributable to factors such as drought or climate change, and the Upper Division states have not exceeded their water allocations as defined by the compact, then the Upper Division states are not considered the cause of the depleted flows at Lee Ferry.

Mexico Treaty

This international agreement guarantees Mexico an annual allocation of 1.5 million acre-feet of water from the Colorado River system.

The treaty includes provisions for reduced water allotments to Mexico during periods of extraordinary drought, although the specific criteria defining such conditions are not explicitly outlined within the document.

To facilitate the implementation and management of this treaty, the International Boundary and Water Commission (IBWC) was established. This binational commission is responsible for applying the terms of the agreement.

Subsequent “Minutes” issued by the IBWC serve to clarify and provide further detail regarding the implementation of the treaty. These Minutes address specific operational aspects and interpretations but do not alter the fundamental terms established in the original treaty.

1948 Upper Colorado River Basin Compact 

The 1948 Upper Colorado River Basin Compact was developed to facilitate congressional authorization and funding for water storage projects within the Upper Basin states.

This compact further apportioned the water allocated to the Upper Division states under the 1922 Colorado River Compact. The apportionment is defined as percentages of the total water supply available in a given year. The allocations are as follows: Colorado is allocated 51.75% of the available supply, New Mexico 11.25%, Utah 23%, and Wyoming 14%. For example, under a full supply of 7.5 million acre-feet for the Upper Basin, Colorado’s share would be approximately 3,881,250 acre-feet. If the available supply is reduced to 6 million acre-feet, Colorado’s share would be approximately 3.1 million acre-feet, with the other states’ allocations adjusted proportionally.

The Upper Colorado River Commission (UCRC) is an interstate administrative agency established to administer the 1948 Upper Basin Compact among the four Upper Division states. This agency is unique among compacts to which Colorado is a signatory because the obligations outlined in both the Colorado River Compact and the Upper Basin Compact are shared among these states, rather than a direct delivery obligation to a specific state line.

The UCRC undertakes various activities to facilitate the administration of the interstate compact, including:

• Establishing rules and regulations for compact implementation.
• Forecasting water runoff in the Colorado River and its tributaries.
• Conducting cooperative studies of the Colorado River System’s water supplies.
• Collecting and reporting data on streamflows, storage, diversions, and water use within the tributaries of the Upper Basin.
• Making findings regarding flows at Lee Ferry.
• Making findings regarding the necessity and extent of curtailment, if required.

Additionally, the UCRC is authorized to undertake all actions deemed necessary, proper, and convenient for the performance of its duties, independently or in cooperation with any state or federal agency.

It is important to note that the UCRC does not control the in-state administration of water rights, which remains the responsibility of the individual states.

The “Big Three” Today 

These three foundational documents, along with subsequent laws, agreements, and judicial decrees, collectively form what is known as the Law of the River (LoR). This body of legal instruments seeks to balance the need for certainty in water resource planning with the flexibility required to adapt to evolving conditions within the Colorado River Basin.

Over the past century, this framework has facilitated significant growth and development in the Southwestern United States, contributing to a regional economy now valued at $1.4 trillion annually.

Each generation of water managers and users in the Colorado River Basin has faced unique challenges. Throughout this history, the stability provided by the Law of the River has been recognized, leading to collaborative efforts to operate within its provisions and adapt to changing circumstances without fundamentally altering the established legal structure.

Opportunities continue to exist within the current legal framework to find flexible solutions for addressing contemporary challenges, such as climate change and increasing water demands, for present and future generations.

Other Foundational Documents 

Beyond the “Big Three” compacts and treaty, several federal laws and a significant equitable apportionment decree have played a crucial role in shaping the management of the Colorado River. These include:

Where the Water Comes From

The Colorado River’s lifeblood originates high within the rugged peaks of the Rocky Mountains. This majestic mountain range acts as a vast natural reservoir, accumulating significant amounts of snowpack throughout the winter months. As spring arrives and temperatures begin to rise, this accumulated snow begins to melt.

The melt and runoff from the high-elevation snowfields and glaciers cascade down the steep mountain slopes, forming countless small streams and rivulets. These nascent waterways converge, gradually increasing in volume and forming larger streams and rivers.

As these waters flow downhill, they are augmented by contributions from numerous tributaries. These smaller rivers and streams, fed by their own snowmelt and precipitation within the broader Upper Basin, join the main stem of the Colorado River. Prominent tributaries such as the Gunnison, Dolores, Green, and San Juan Rivers in the Upper Basin significantly contribute to the overall flow.

This continuous process of snow accumulation, spring melt, and the confluence of tributaries sustains the Colorado River as it carves its way through the landscape. The Rocky Mountain snowpack is, therefore, the fundamental source of the river’s water, driving its flow and making it a vital resource for the arid and semi-arid regions it traverses. The natural cycle of winter snowfall and spring runoff is the primary engine that has shaped the Colorado River and continues to provide its water supply.

Where the Water Goes

The Colorado River serves as a critical water source for seven states in the southwestern United States: Colorado, New Mexico, Utah, Wyoming (Upper Basin), and Arizona, California, and Nevada (Lower Basin). The allocation and use of this water are governed by a complex framework of laws, agreements, and decrees known as the Law of the River. Due to the intricate nature of water rights, environmental factors like drought, and the variability of the river’s flow, precise, real-time breakdowns of use by sector, geography, location, and exact quantity are challenging to provide in a static format.

Colorado

Geography/Location: As a headwater state in the Upper Basin, Colorado’s use is primarily concentrated in the western and southwestern portions of the state, where the Colorado River and its tributaries originate. However, significant diversions also occur across the Continental Divide to serve the more populous Front Range urban corridor (including Denver, Colorado Springs, and Fort Collins) and agricultural areas on the eastern plains.

Sector: A significant portion is used for agriculture, irrigating fields in the Western Slope. Municipal and industrial uses are substantial, especially with transmountain diversions supplying major cities. The environment and recreation (including maintaining river flows for ecological health and supporting activities like rafting and fishing) also constitute important uses.

New Mexico

Geography/Location: Located in the Upper Basin, New Mexico’s use is concentrated in the northwestern and southwestern parts of the state, primarily relying on the San Juan River (a major tributary) and the Colorado River itself.

Sector: Agriculture is a significant user, particularly in the Four Corners region. Municipal and industrial uses support communities and energy development. Similar to Colorado, environmental and recreational needs are also considered.

Utah

Geography/Location: As an Upper Basin state, Utah’s use is focused in the eastern and southern parts of the state, drawing from the Colorado and its tributaries like the Green River.

Sector: Agriculture is a major consumer, supporting farming and ranching. Growing municipal and industrial demands, particularly in the Salt Lake City metropolitan area (which receives some Colorado River water via diversions), are also significant. Recreational uses, especially related to Lake Powell and the numerous rivers, are also important.

Wyoming

Geography/Location: Located in the Upper Basin, Wyoming’s use is concentrated in the southwestern and south-central parts of the state, where the headwaters of some Colorado River tributaries are found.

Sector: Agriculture, primarily livestock and hay production, is a dominant user. Energy development (coal, oil, and gas) also requires water. Recreational and environmental flows are important in its mountainous regions.

Arizona

Geography/Location: A Lower Basin state, Arizona’s primary use is in the central and southern parts of the state, relying on diversions from the Colorado River via canals like the Central Arizona Project (CAP).

Sector: Agriculture in the south-central region is a major consumer. Significant amounts are used for municipal and industrial purposes, supplying the Phoenix and Tucson metropolitan areas. Tribal water rights also constitute a significant portion of Arizona’s allocation.

California

Geography/Location: As the largest user in the Lower Basin, California’s use is primarily in the southern part of the state, including major urban centers like Los Angeles and San Diego, and the vast agricultural regions of the Imperial and Coachella Valleys.

Sector: Agriculture in the south is a massive consumer, supporting a significant portion of the nation’s winter vegetables and other crops. Large quantities are also used for municipal and industrial purposes, serving the large urban populations.

Nevada

Geography/Location: A Lower Basin state, Nevada’s primary use is concentrated in the southern part of the state, particularly to support the Las Vegas metropolitan area.

Sector: Predominantly municipal and industrial use to serve the large population and tourism industry of Las Vegas. Some water is also used for smaller-scale agriculture and recreation.

Managing Water in the Upper Basin

The Upper Colorado River Basin states – Colorado, New Mexico, Utah, and Wyoming – face unique challenges in managing their share of the Colorado River, stemming from the river’s origins in their mountainous regions and the legal framework governing its allocation.

Prior Appropriation:

This fundamental principle of water law in the Western United States dictates that the right to use water is acquired by the first person to divert it and put it to a beneficial use. This creates a hierarchy of water rights based on the date of appropriation. In times of limited water supply, those with the oldest (“senior”) water rights have a higher priority and are entitled to their full allocation before those with more recent (“junior”) rights receive any water. This system contrasts with riparian water rights, common in the eastern U.S., which tie water rights to land ownership adjacent to a water body.

The administration of the prior appropriation system in the Upper Basin is the responsibility of each state’s State Engineer. The State Engineers administer and enforce water rights, water development, streamflow measurement, and water data collection. During periods of water scarcity, the offices of the State Engineers regulate diversions based on the priority dates of water rights. Junior water rights may be curtailed or shut off entirely to ensure that senior water rights receive their full decreed amount.

The Fragmented Nature of the Upper Basin Water Supply

Conceptually, the Colorado River begins as snowpack accumulating in the high elevations of the Rocky Mountains. As this snow melts, the water forms small seasonal creeks that eventually coalesce into streams and then rivers. However, what many outside the region might consider the singular “Colorado River” in its upper reaches is, in reality, a network of distinct sub-basins and tributaries, each with its own hydrological characteristics. These include significant rivers like the White, Yampa, San Juan, Gunnison, Dolores, and the mainstem of the Colorado as it forms on the Western Slope.

This fragmented nature of the water supply has significant implications for water management in Colorado and the broader Upper Basin. Colorado does not benefit from a single, large river flowing consistently from its high peaks. Instead, water availability is determined by the specific snowpack and runoff conditions within each of these sub-basins.

Challenges of Water Management in the Upper Basin

Managing water in the Upper Basin states presents several key challenges:

1. Hydrologic Variability and the Reliance on Snowpack: Unlike the Lower Basin states which can rely on stored water in Lake Powell and Lake Mead, the Upper Basin’s primary “reservoir” is the annual snowpack. This natural storage is highly variable from year to year. A dry winter can lead to significantly reduced streamflows, directly impacting water availability for all users within a sub-basin. Upper Basin users cannot draw upon stored water from the large downstream reservoirs to compensate for a lack of local snowmelt.
2. Sub-Basin Specific Hydrology: Each sub-basin within the Upper Basin can experience vastly different hydrologic conditions in a given year. One area might have average or even above-average snowpack and runoff, while a neighboring sub-basin could be facing severe drought. This creates a situation where water availability and the need for management actions can vary significantly across relatively short distances within a single state. There is no “blanket fairness” in water availability across the entire state in any given year.
3. Prior Appropriation in Action: The prior appropriation system, while providing a legal framework, means that in dry years, junior water rights holders may receive little to no water, even if senior rights holders in a different sub-basin are experiencing relatively good supplies. This can lead to localized economic hardship and necessitates careful administration and communication.
4. Limited Storage Capacity: Compared to the massive reservoirs in the Lower Basin, individual water users and districts in the Upper Basin generally have smaller storage facilities. This limits their ability to buffer against short-term fluctuations in water supply within a given year. When streamflows decline in late summer, diversions often cease.

Growing Demands and Climate Change: Like the Lower Basin, the Upper Basin states are facing increasing demands for water due to population growth, energy development, and recreation. Climate change is exacerbating the challenge by contributing to decreased snowpack, earlier runoff, and more prolonged drought conditions, further straining the existing water management systems.

Managing Water in the Upper Basin

Water management in the Lower Colorado River Basin states – Arizona, California, and Nevada – differs significantly from the Upper Basin due to the region’s reliance on large storage reservoirs and the greater role of the federal government. Unlike the direct diversions common in the Upper Basin, the Lower Basin states primarily receive their Colorado River water through a system of orders and deliveries.

Colorado River Orders and Delivery

Each Lower Basin state submits annual orders to the Bureau of Reclamation (part of the U.S. Department of the Interior) specifying the amount of water they require. These orders are then fulfilled by releases from Lake Mead, which is managed by the Bureau. The water is delivered through a network of canals and pipelines, often traversing long distances to reach agricultural fields and urban centers. This system allows for more centralized control and distribution of water compared to the Upper Basin’s decentralized approach.

Role of the Federal Government in Managing the Colorado River in the Lower Basin

The federal government, particularly through the Bureau of Reclamation and the Secretary of the Interior, plays a substantial role in managing the Colorado River in the Lower Basin.  The federal government’s authority is exercised through:

• Construction and Operation of Dams and Reservoirs: The Bureau of Reclamation constructed and operates major infrastructure like Hoover Dam (Lake Mead) and Imperial Dam, which regulate the flow of the river and provide storage for the Lower Basin states.
• Administration of Water Allocations: The federal government, through the Secretary of the Interior, is responsible for administering the allocations of Colorado River water among the Lower Basin states as defined by the Law of the River and court decrees. This includes approving water delivery contracts and managing releases from Lake Mead.
• Implementation of Environmental Regulations: The federal government enforces environmental regulations, such as the Endangered Species Act, which can impact water management decisions in the Lower Basin.
• International Treaty Obligations: The federal government is responsible for meeting the water delivery obligations to Mexico as specified in the 1944 treaty.

The Secretary of the Interior plays a crucial role in managing the Colorado River in the Lower Basin, often referred to as the “Water Master.” The Secretary’s responsibilities include:

• Administering the allocation of Colorado River water among the Lower Basin states.
• Approving and overseeing water delivery contracts.
• Managing releases from Lake Mead to meet water orders and treaty obligations.
• Enforcing the provisions of the Law of the River.
• Making critical decisions during times of shortage, including implementing shortage guidelines and demand management programs.
• Acting as a mediator between the states, and other water users.

In essence, the Secretary of the Interior acts as the federal steward of the Colorado River in the Lower Basin, ensuring that the complex system of allocations and deliveries operates in accordance with federal law and the Law of the River. This role is particularly important in managing the river during periods of drought and shortage, requiring difficult decisions that balance competing water demands.

Climate Change in the Colorado River Basin

Climate change is significantly altering the hydrological dynamics of the entire Colorado River Basin, impacting both water supply and demand across the Upper and Lower Basin states. This phenomenon is characterized by long-term shifts in temperature and precipitation patterns, driven primarily by increased greenhouse gas emissions from human activities.

Throughout the Colorado River Basin, rising temperatures are leading to several key changes:

• Reduced Snowpack: Warmer winters result in more precipitation falling as rain rather than snow at higher elevations. Additionally, the snowpack that does accumulate melts earlier in the spring. Since snowpack is the primary natural reservoir for the Colorado River, this decline and earlier runoff diminish the amount of water available later in the crucial summer and fall months when demand is highest.
• Increased Evapotranspiration: Higher temperatures lead to increased evaporation from surface waters (rivers, lakes, reservoirs) and increased transpiration from plants, drying out soils and further reducing the amount of water available for human and ecological uses.
• Altered Precipitation Patterns: While the overall trend in precipitation across the entire basin is less certain than warming, many climate models suggest a shift towards a drier future in the Southwest. Even without a significant decrease in total precipitation, changes in the timing and intensity of storms can negatively impact water availability and increase flood risks.

Impacts of the Mega-Drought Over the Last 20 Years

The Colorado River Basin has been experiencing a severe mega-drought for the last two decades, starting around the year 2000. This prolonged dry period, considered one of the worst in the last 1,200 years, has been significantly exacerbated by rising temperatures driven by climate change. The impacts of this mega-drought are evident throughout the basin:

• Declining Reservoir Levels: Lake Powell and Lake Mead, the two largest reservoirs on the Colorado River, have seen dramatic declines in their water levels, reaching historically low elevations. This has threatened water supplies, hydropower generation, and recreational opportunities.
• Reduced River Flows: The overall flow of the Colorado River has significantly decreased, impacting water availability for all downstream users, including the Lower Basin states and Mexico. Some studies indicate that climate change has already reduced the river’s flow by a substantial percentage.
• Water Shortages and Restrictions: The persistent drought has triggered the implementation of water shortage declarations and mandatory water use restrictions in Arizona, Nevada, and California, as outlined in the established shortage guidelines.
• Ecological Stress: Reduced river flows and warmer water temperatures have negatively impacted aquatic ecosystems, threatening native fish species and riparian habitats.
• Increased Wildfire Risk: Drier conditions have contributed to a significant increase in the frequency and intensity of wildfires across the basin, further stressing water resources and ecosystems.

Agricultural Impacts: Farmers and ranchers have faced reduced water allocations, leading to crop losses, changes in agricultural practices, and economic hardship.

Warming in Colorado

Within the Upper Basin, Colorado has experienced significant warming. Over the last 120 years, the state’s average temperature has increased by approximately 2.5 degrees Fahrenheit. This warming trend has accelerated in recent decades, with the five hottest summers on record occurring since the year 2000. This pronounced warming in a headwaters state directly contributes to the challenges faced throughout the entire Colorado River Basin by reducing snowpack, increasing evaporation, and altering the timing of runoff. The hotter summers also increase demand for water for irrigation and municipal uses.

In conclusion, climate change is a fundamental driver of the ongoing water crisis in the Colorado River Basin. Rising temperatures are altering the natural hydrology, exacerbating the impacts of the prolonged mega-drought, and creating significant challenges for water management in both the Upper and Lower Basin states. The observed warming trend in Colorado, particularly the accelerated warming in recent summers, underscores the direct impact of climate change on the very source of the Colorado River’s water supply.