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Colorado's Four Major Aquifers

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Click the Fluent Water Facts above to learn more about Colorado’s groundwater and the challenges it poses.

Colorado has aquifers, which contain groundwater, throughout the state. Groundwater can be found in many rock formations, and major streams and rivers have accompanying aquifers, although their size and accessibility varies. Below are Colorado’s four major aquifers:

South Platte Aquifer

The South Platte Aquifer follows the South Platte River through northeastern Colorado. This region is heavily populated (70% of Colorado’s population lives there, according to the 2000 Census) and heavily farmed. But the aquifer’s connection to the river means that pumping groundwater affects surface rights.

Type: Alluvial  

Depth: 20 feet under Denver, but depths of 200 feet some 160 miles further downstream; average depth is 36 feet

Used for: Mostly irrigation, but some water for local communities

Challenges:Since the late 1960s, drilling of new wells has been restricted, due to the effects on surface rights. Colorado must abide by the terms of the South Platte River Compact, which requires certain amounts of surface water to be sent downstream. Recharge is heavily influenced by return flows, which mostly come from agricultural users. As the water is used and reused for irrigation, it has higher concentrations of dissolved solids and nutrients, which affect water quality. Discharges of municipal wastewater also affect water quality.

San Luis Valley Aquifers

San Luis Valley aquifers are connected to the Rio Grande and its tributaries, so pumping groundwater affects surface water and its rights holders. The Valley faces critical surface and groundwater shortages.

Type: Alluvial

Depth: 100 feet in the northern part of the valley, 40 feet in the southern portion

Used for: Vast majority for irrigation for farms, but some drinking water for local communities

Challenges: Aquifer levels have been steadily dropping. Natural recharge is difficult, as the San Luis Valley receives 7-8 inches of precipitation a year, half of Colorado’s state average. Pumping groundwater affects surface water, and Colorado must meet the terms of the Rio Grande Compact, which requires certain amounts of water to be sent downstream.

Read more about the San Luis Valley's challenges in the Summer 2013 issue of Headwaters magazine.

Denver Basin Aquifer

The Denver Basin aquifer system is a major source of water for South Metro Denver. The system extends from Colorado Springs in the south to Greeley in the north, from the foothills near Golden in the west to the eastern plains near Limon, a surface are of about 6,700 square miles. It includes four aquifers: the Dawson, Denver, Arapahoe, and Laramie-Fox Hills. Each aquifer has different water quality, depths, and water availability.

Type: Sedimentary bedrock

Depth: Shallow wells can reach the Dawson aquifer, the shallowest in the system, but other aquifers are a half-mile below the surface.

Used for: Mostly homes and businesses

Challenges: Extensive development in the South Metro area of Denver has resulted in steady and substantial aquifer declines, especially in the Arapahoe aquifer. Some of the water stored in these deep aquifers may be more than 50 million years old. Studies show that it took tens of thousands of years or more for nature to fill this resource. Since much of this groundwater is considered non-renewable, these rates of decline are not considered sustainable.

Read more about the Denver Basin aquifer, its geology and its challenges, in the Citizen's Guide to Denver Basin Groundwater.

High Plains Aquifer

The High Plains aquifer currently supplies water to about 20% of irrigated farm ground in the United States. It is found in parts of eastern Colorado, but the total aquifer underlies about 174,000 square miles from South Dakota to Texas and New Mexico. In eastern Colorado and eastern New Mexico, this is called the Ogallala formation.

Type: Sedimentary bedrock

Depth: Average of 300 feet below the surface

Used for: Mostly irrigation, with relatively little domestic use

Challenges: Since the 1960s, people have been extracting more water from this aquifer than has been returned. Aquifer recharge comes mostly from local precipitation – but the region receives relatively little precipitation and has high rates of evaporation. New conservation strategies have reduced the amount of water pumped, and withdrawal levels appear to have stabilized.

What is an Aquifer?

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Click the Fluent Water Facts above to learn more about Colorado’s groundwater and the challenges it poses.

Nature’s Underground Water Storage

Aquifers store groundwater. An aquifer is a layer of saturated rock through which water can easily move. Aquifers are everywhere – under the plains, mesas, and mountains. But aquifers vary significantly in the amount of water they hold, their depth underground, and their availability for use by humans. Geology often defines how this water moves and can be accessed.

Aquifers in Colorado are usually one of three types:

Alluvial Aquifers

Alluvial aquifers are generally shallow sand and gravel deposits laid down over time in a river channel or floodplain. The name “alluvial” refers to the loose, unlayered nature of the material – often silt, clay, sand, and gravel, deposited by running water in and around rivers.

Alluvial aquifers are often referred to as “tributary aquifers,” meaning that they exchange water back and forth with surface streams. Major alluvial aquifers surround every large river in the state, and smaller alluvial aquifers surround all the creeks and streams.

Colorado’s alluvial aquifers can supply water to cities and farms, but unless they are managed carefully, they can be over-pumped and/or polluted. If wells pump more water than is returned to an alluvial aquifer, this may mean less water is available in nearby lakes and rivers.

Sedimentary Bedrock Aquifers

Sedimentary aquifers exist deep under ground, primarily in sandstones and limestones. Multiple geological layers and aquifers exist at different depths. Examples of sedimentary aquifers in Colorado include the Denver Basin, High Plains, and Piceance Basin in northwestern Colorado.

These deep aquifers are often confined – rock layers above them have low permeability, which limit the amount of water that can move back and forth to the aquifer. These aquifers are typically not connected to nearby rivers, as alluvial aquifers are, so their groundwater is usually considered non-tributary. Deep aquifers still have recharge areas, but these may be many miles from the aquifer itself. Recharge for deep aquifers requires very long time periods – potentially thousands of years.

Fractured Rock Aquifers

Fractured rock aquifers are common in the mountains. Underneath a layer of soil and loose rocky material, aquifers exist in bedrock full of cracks and fractures created by the natural folding and faulting of the rock over millions of years. These cracks can fill with water supplied by infiltrating snow and rain. Not all fractures contain water, however. Springs can arise where fractures intersect the land’s surface.

Groundwater Basics

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What is groundwater?

Groundwater does not take the form of underground lakes and streams. It is, essentially, saturated rock, and so knowing the geology of the area is crucial to understanding its water. Most groundwater is located in very small water-filled pore spaces between rock grains in sedimentary rocks, between sand and gravel particles in alluvial deposits, or narrow crevices such as fractures and faults in crystalline rocks. Some of these cracks are as small as a human hair.

Groundwater is stored in aquifers. An aquifer is a layer of saturated rock through which water can easily move. Aquifers are everywhere – under the plains, mesas, and mountains. But aquifers vary significantly in the amount of water they hold, their depth underground, and their availability for use by humans.

How does groundwater accumulate?

It all starts with water on the surface. Precipitation can work its way through the soil quickly, but it takes longer to infiltrate the rocks below. Groundwater in the Denver Basin has taken thousands of years to accumulate. Rain and snowmelt continue to recharge aquifers today, but the depth of the aquifer and the type of rock affect these recharge rates. In some cases, this process takes so long that precipitation does not impact the amount of water stored in aquifers, and the groundwater is considered essentially non-renewable.

Types of Groundwater

Tributary groundwater is hydraulically connected to a surface stream and can influence the amount or direction of flow of water in that stream. Water in sand and gravel alluvial aquifers adjacent to major rivers is an excellent example of tributary groundwater.

Non-tributary groundwater is typically produced from aquifers geologically confined such that they have little physical connection to surface waters.

Designated groundwater under natural conditions does not recharge or supplement to any significant degree continuously flowing surface streams. This legal definition refers to eight basins created by the Colorado Ground Water Commission. All eight are located on Colorado’s eastern plains. Designated groundwater is a type of non-tributary water.

SWCD Member Profile

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Why does the Southwestern Water Conservation District support Water Education Colorado?

Southwestern Water is a long time supporter of Water Education Colorado.  SWCD’s former office manager, Lynn Herkenhoff was a founding member of Water Education Colorado and continued consulting with WEco after her retirement. SWCD is a proud member and sponsor of Water Education Colorado, and share the water education goals of WEco.

Why is water resources education important?

SWCD has a long established reputation for valuing water information and education.  The District provides an annual water seminar each spring that offers national, state, and regional speakers to a well attended event.  Education is a primary focus of the District’s Water Information Program (WIP).  Each year a Children’s Water Festival is open to all 5th graders in the region---they get to experience a case in a “water court”, learn about watersheds, water conservation, and water quality.  The book, “My Water Comes from the San Juan Mountains” was made available to all 3rd grade schools in the District.  An adult learning experience, Water 101, is presented each year in different areas in the District and provides useful information about water law, water agencies,  administration, regulations, and the multiple beneficial uses of water in Colorado.

What does SWCD do?

Southwestern Water Conservation District is one of 4 water conservation districts in the state.  Sitting in the southwest corner of Colorado it encompasses all or parts of nine counties.  The Directors are appointed by the Board of County Commissioners from each county including; Archuleta, Dolores, Montezuma, La Plata, San Miguel, San Juan, Mineral, Montrose and Hinsdale counties.  SWCD is responsible for the conservation, protection, use and development of the water resources in the San Juan, Dolores, and San Miguel River Basins.  The SWCD provides approximately $300,000 in funding each year to assist in meeting both consumptive and non-consumptive needs in the basin.  These funds are used for a wide variety of purposes for diverse groups including; feasibility studies, water quality monitoring, watershed restoration, formation of water districts, education, and for projects sponsored by other water entities.

Contact info

841 E. 2nd Avenue
Durango, CO  81301
Fax 970-259-8423

Water Quality and Environmental Laws

Click the Fluent Water Facts below to learn more about Colorado's water history.

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Water for People – and a Healthy Environment, Too

Colorado’s water law initially focused on access and quantity, not quality. Laws ensured that people could fairly obtain the amount of water needed for drinking water, irrigation, mining, and other uses, and government regulation was intended to promote economic development. As the twentieth century progressed, however, citizens became increasingly concerned about the effects of pollution on the quality of water and natural environment. This concern prompted the adoption of new environmental laws.

Coloradans traditionally preferred to exercise local control, entrusting regulations and enforcement to the level of government closest to citizens performing the work – usually at the city or county level. New environmental laws, however, were enacted at the federal level. These laws have prompted new methods of designing, approving, constructing, and enforcing water projects and state laws. They also reveal Coloradans’ changing water values.

Early Laws Protect Water Quality

Prior to the late nineteenth century, cities rarely regulated land use. The major exception were regulations to protect drinking water. People increasingly understood the need for clean water sources to prevent disease, and realized the potential hazards of pollution from sewage, mining, and agriculture. State law allowed municipalities to regulate land use in watersheds where they had water supply reservoirs, even if those watersheds were far away from the cities.

Protecting Communities Near Water Projects

Since Colorado is so dry, many communities are far away from their water sources. When cities plan to build new reservoirs or other water infrastructure projects, they are often building on land in other counties. Questions began to arise about the decision-making role that communities near the water sources – and water projects – should have in planning and implementing the projects.

This local concern is addressed in the Colorado Land Use Act, adopted in 1970, which established the Areas and Activities of State Interest Act, commonly known as House Bill 1041. The statute authorizes local governments to regulate the development of “areas and activities of state interest” within their jurisdiction, including

  • Major domestic water and sewer treatment systems
  • Municipal and industrial water projects
  • New communities
  • Natural hazard areas, including floodplains

Aurora and Colorado Springs planned to exert their right to divert water from the Eagle River in Eagle County. The 1041 statute required the cities to obtain a permit from Eagle County to build their diversion structures in that county. Eagle County denied the request, and the case went to court. The Court of Appeals ruled that the cities had a valid property right to obtain the water, but they did not have an absolute right to build and operate particular water diversion projects.

Counties along the Lower Arkansas River have adopted 1041 regulations requiring a permit for activities such as removing irrigation water from land which has historically been irrigated. These regulations help address the environmental impacts of drying up agricultural land, including topsoil loss, noxious weed invasion, and loss of wildlife habitat. Permit conditions may require revegetation or wildlife mitigation plans. Read more about modern solutions to drying up agricultural land in the Headwaters magazine story 'Keeping Water on the Farm'.

Environmental Concern Prompts New Laws

In the late 1960s, citizens across the United States began to demand a cleaner environment and reduced air and water pollution. In response, Congress expanded its previously limited federal role in protection of water quality, adopting a range of environmental laws. These laws established incentives for states to create their own clean air and water laws and to take over enforcement of the federal laws.

The Clean Water Act of 1972 seeks to eliminate releases of high amounts of toxic substances into water and ensure that surface waters meet standards necessary for human sports and recreation.

The Safe Drinking Water Act of 1974 established the Environmental Protection Agency (EPA) to set standards for drinking water quality and to oversee all states, localities, and water suppliers who implement these standards.

The National Environmental Policy Act (NEPA) of 1970 requires any project funded with federal funds, or any project located on federal lands, to conduct an environmental assessment to see what impacts the project will have on the environment. Where significant impacts occur, a full environmental protection study must be undertaken, and alternatives to the project must be explored.

The Endangered Species Act of 1973 requires analysis of threatened or endangered species and mandates that future land management must be consistent with maintaining the viability of the endangered species population. This act permits the destruction of a species and its habitat in particular development locations where an alternative habitat and relocation of the species can be arranged.

The Resource Conservation and Recovery Act of 1976 requires generators and transporters of certain listed hazardous materials and chemicals to obtain permits and file plans for emergency containment and disposal.

The Comprehensive Environmental Response Compensation and Liability Act of 1980 established that landowners and other potentially responsible parties may be held liable for toxic and other hazardous materials that can migrate offsite, generally as water pollution.

Changing Attitudes

Click the Fluent Water Facts below to learn more about Colorado's water history.

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Changing Attitudes to Colorado’s Water

Water has always been the lifeblood of Colorado. But Coloradans are increasingly supportive of other uses besides drinking water, irrigation water, and industrial water. Some also advocate for different ways of collecting the water necessary for these human purposes.

Leaving Water in Streams

Colorado law is a system built upon “beneficial use.” “Beneficial use” originally meant taking water from the stream for some human purpose. Leaving the water in the stream was not considered beneficial. In the twentieth century, however, Coloradans began to challenge this idea. They proposed that leaving water in streams created beautiful landscapes and important habitat for fish and other wildlife. Leaving water in streams was not only “beneficial,” it was crucial to Colorado’s economy – for tourists, fishermen, and rafters.

In 1973, the State Legislature created the Instream Flow and Natural Lake Level Program, overseen by the Colorado Water Conservation Board (CWCB). This program allows CWCB to acquire water rights in streams and lakes. These rights are non-consumptive, so the water stays in its original body. These rights are intended to preserve the natural environment by maintaining minimum flows in streams and minimum levels in lakes. This help preserve fish, which often need a certain depth and temperature of water to survive, as well as waterfowl habitat, riparian vegetation and wildlife, and critical habitat for threatened or endangered species.

Challenging Reservoirs

From the time of the Ancestral Puebloans, Coloradans have seen reservoirs, which can collect and store water for later use, as crucial for survival in our dry state. Throughout the nineteenth and early twentieth centuries, the creation of new dams and reservoirs was considered essential to the growth of Colorado’s population and economy. But new construction has increasingly met resistance from environmentalists and other concerned citizens.

In the 1950s, the U.S. Bureau of Reclamation proposed construction of two dams within Dinosaur National Monument. Citizens protested the loss of beautiful scenery within a National Park unit. In response, Congress authorized a single dam in Glen Canyon in Arizona, which created Lake Powell. In the 1970s, passage of environmental laws made the permitting and planning process for new dams and reservoirs more difficult and comprehensive, especially those built on federal lands or with federal funds. These regulations also made construction more costly, as years could be spent evaluating environmental impacts.

Tangle Over Two Forks

In 1990, protest again halted the construction of a Colorado dam. Denver Water had proposed building a new reservoir in Waterton Canyon on the South Platte, about one mile downstream from the confluence with the North Fork. This water project was known as Two Forks. This reservoir would have inundated 21 miles of the South Platte and 9 miles of the North Fork, and would have stored approximately 1 million acre-feet of water. Denver Water claimed that this reservoir was necessary for the growth of Denver.

The Environmental Caucus – a coalition of national and Colorado environmental groups – challenged the construction of Two Forks. They argued that Denver did not need a new dam to obtain water for the city’s growth. Denver Water could instead obtain this water by metering houses to encourage less water use, by sharing water across governmental boundaries, and by building smaller reservoirs. These alternatives would cost less and have less impact. The group thus argued against the dam on economic grounds, as well as by highlighting the beauty of the Two Forks site and the ecological impact from the dam. On November 23, 1990, President George H.W. Bush vetoed the Two Forks Project, and it has not been built.

Colorado’s Water Future?

Will Coloradans choose to build fewer reservoirs? Only the future will tell. If climate change reduces the amount of Colorado water available, building more reservoirs may be one of many possible solutions – but the construction of new reservoirs will have to be weighed against other Colorado values, including impacts on the environment.

CCWCD Member Profile

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Why does the Central Colorado Water Conservancy District support Water Education Colorado?

Water Education Colorado does an outstanding job of working with diverse entities from around the state to present impartial, non-partisan water resource education. The Citizen Guides and Headwaters magazines are invaluable. Central also appreciates the work Water Education Colorado does to develop and educate existing and future water policy leaders.

Why is water resources education important?

In order to make wise decisions about water policy in the state and water use in people’s daily lives, organizations like Central and Water Education Colorado must provide the citizens of Colorado with as much background information as possible. Only when you understand an issue can you appreciate it or have the desire to do something about it.

What does Central do?

Central Colorado Water Conservancy District was formed to develop, manage and protect water resources in northeast Colorado. We currently provide water augmentation and decree administration for over 1100 irrigation wells within our district. District boundaries cover land from Brighton north to Greeley, and east to Fort Morgan, encompassing parts of three different counties.

In addition to the augmentation plans, Central conducts an extensive water quality-testing program, provides strong water education outreach and active legislative efforts to protect water sources and water rights.

 Contact info

Central Colorado Water Conservancy District
3209 W 28 Street
Greeley, CO 80634



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WEco Climate Resources

Guide to Colorado Climate Change presents a range of contemporary climate change information written by experts. Take a look.

Water 101 Sheets are one-page references available for download and distribution. Explore the basics of drought, and wildfire or read various water conservation tips through a series of fact sheets. Interested in additional resources? Find them herefact_sheetsClimate Workshop
Participants tour the National Ice Core Lab, hear how researchers study climate and what that means locally. Learn more.

Connecting the Drops Radio

Listen to a radio feature on climate change's effects on Colorado farmers, spring runoff, and irrigation.

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