A Colorado River Overview:
Education, Facts & Statistics

Today, the Colorado River Basin is a system. There are numerous reservoirs and infrastructure projects along the River, many built by the federal government in the previous century. The two largest reservoirs, Lake Powell and Lake Mead, are important for water storage and security, and are central to water management in the Basin.

Laws that Shape the River

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 changing hydrology and climate change, presents new challenges for the southwest region.

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.

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.

Where the Water Comes From

The Colorado River originates high in the Rocky Mountains, which act as a natural wintertime reservoir, accumulating significant amounts of snowpack. Once spring arrives and temperatures rise, the accumulated snow melts into countless small streams. These little waterbodies gradually converge into larger streams and rivers.

These smaller rivers and streams, fed by snowmelt and precipitation within the broader Upper Basin, eventually form the mainstem of the Colorado River. Prominent tributaries such as the Gunnison, Dolores, Green, and San Juan Rivers in the Upper Basin join the Colorado River mainstem to contribute more than 90% of the river’s overall flow.

The Rocky Mountain snowpack is, therefore, the fundamental source of the Colorado River’s water. The natural cycle of winter snowfall and spring runoff is the cycle that has shaped the Colorado River and continues to provide its water supply.

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. The Colorado River begins as snowpack, which “has challenges that don’t exist for constructed reservoirs” OR “follows a natural runoff pattern and can be unpredictable”. Additionally, the Upper Basin States have complex legal systems for administering how water can be used.

Prior Appropriation:

The Prior Appropriation Doctrine is a system for administering water, common in the drier Western United States. It dictates that the right to use water is acquired by the first person to put it to a beneficial use – often summarized as “first in time, first in right.” This creates a hierarchy of water rights based on the date of use, called 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. During periods of water scarcity, the State Engineers administer 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.

have enough water to give every water right holder the amount of water that they’re legally allowed to use. So when a senior user makes a “call,” they effectively tell the State water officials, “Shut off the junior water users upstream—I need my full amount first.”
Below is a map of active calls in Colorado. Calls for water are more common in the irrigation season (spring, summer, fall) and less common in the winter.

Challenges of Water Management in the Upper Basin

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

1. Hydrologic Variability and a 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 annual snowpack. This natural water supply 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 a large reservoir in years with poor snowpack.
2. Sub-Basin Hydrology Varies: Each sub-basin within the Upper Colorado River 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. Water availability can vary significantly across relatively short distances within a single state.
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. This means that Upper Basin water users must live within their means and get creative in times when they cannot divert their water rights.
   
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, if any at all. This limits their ability to buffer against short-term fluctuations in water supply within a given year. When streamflows decline in late summer, many water users cease diversions.
5. Growing Demands and Climate Change: Like the Lower Basin, the Upper Basin States are facing increasing demands for water due to population growth and new water projects. Climate change is exacerbating the challenge by contributing to decreased snowpack, earlier runoff, reduced runoff, and prolonged drought conditions.

Managing Water in the Lower Basin

Water management in the Lower Colorado River Basin States – Arizona, California, and Nevada – differs significantly from the Upper Basin due to the Lower Basin’s ability to order water from Lake Mead. Unlike the direct diversions from the river that are common in the Upper Basin, the Lower Basin States primarily receive their Colorado River water through a system of orders and deliveries via contracts with the federal government.

Colorado River Orders and Delivery

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

Climate Change in the Colorado River Basin

Climate change is significantly altering the hydrological realities of the entire Colorado River Basin, impacting both water supply and demand across the Upper and Lower Basin States.

Throughout the Colorado River Basin, climate change is resulting in reduced snowpack, increased evapotranspiration, and altered precipitation patterns.

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 exacerbated by rising temperatures driven by climate change. The impacts of this mega-drought are evident throughout the Basin, including:

• 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.
• Reduced River Flows: The overall flow of the Colorado River has significantly decreased, impacting water availability for all 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.
• Ecological Stress: Reduced river flows and warmer water temperatures have negatively impacted aquatic ecosystems, threatening native fish species and riparian habitats, and benefiting invasive species.
• Increased Wildfire Risk: Drier conditions have contributed to a significant increase in the frequency and intensity of wildfires across the Basin, stressing natural water systems and affecting runoff.

Satellite images of Lake Powell showing water level decline since 2017

Warming in the Upper Basin

The Upper Basin has been significantly affected by climate change.  For example, over the last 120 years, records indicate that Colorado’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 the headwaters of the Colorado River directly contributes to the challenges faced throughout the entire 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.