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Water Supply

A-1 Click to enlarge
Click to enlarge


Ground Water Resources

The A-1 diagram illustrates the location and extent of the Denver Basin with respect to water administration. As you can see, the Denver Basin extends well outside the boundaries of Douglas County.  The Basin occupies approximately 6,700-square miles encompassing the Denver metro area, extending north-to-south from Greeley to Colorado Springs, and west-to-east from the foothills to the Town of Limon.  Approximately 485-square miles of the Denver Basin underlies Douglas County.  Additional information is available about the Denver Basin and Aquifer Storage and Recovery Programs.

The main source of water in Douglas County comes from groundwater pumped from one or more of the four aquifers of the Denver Basin.  The geologic formations of the Denver Basin include the Dawson, Denver, Arapahoe, and Laramie.

Recharge and Discharge of the Denver Basin Aquifers

The number of Douglas County homes on individual wells has increased along with the population.  The total number of domestic private wells in 1970 was 1,242, and by 2008 the number has increased to 8,264.

Average Front Range precipitation is approximately 15-16 inches per year.  The semi-arid climate leads to low recharge rates in the basin’s aquifer system.  The recharge rate is estimated to be less than 10 percent of the total precipitation.  Most of the precipitation is lost to evaporation, run-off, or is absorbed by vegetation.  The majority of aquifer recharge occurs in the western portions of the basin where the formations come above surface.  If you have driven on I-70 west of Denver, you have most likely seen a portion of this recharge zone.

The basin’s estimated annual recharge rate is thought to be around 40,000-acre-feet during an average water year.  In perspective, the basin aquifers are pumped an estimated 325,000-acre-foot per year, a near 10-fold increase to its recharge rates.  Due to the relatively low amounts of precipitation and infiltration, Denver Basin groundwater is being mined.hat is the Geology of the Denver Basin?

What is the Geology of the Denver Basin?

Beneath the ground in the Denver Basin, there is a view into the ancient past.  Each geologic layer or formation within this basin took millions of years to form.  Below is a cross-section image of these layers looking from east to west and north to south across the basin.  The deeper the formation, the older it is.  Notice the asymmetric bowl shape of the basin.

Below the four aquifers of the Denver Basin lies a thick deposit of shale, which was formed over 100 million years ago during the Cretaceous Period.  During this timeframe, a shallow sea invaded the middle portion of the North American Continent, including present-day Colorado.  Ancient rivers transported and deposited mud and clay onto the bottom of the seabed.  Through time (millions of years) and immense pressure due to burial, the mud and clay formed into a sedimentary rock known today as Pierre Shale.

As the sea receded, sand was deposited as beaches and offshore sandbars. Through burial and immense pressure, the sand formed into sandstone; this enormous deposit is now known as Fox Hills Sandstone.  As the land continued to rise, river deposited sands were interbedded with coal pockets (lenses), mudstone, silts, and clays to form the Laramie Formation.  The sands within the Laramie Formation and the Fox Hills Sandstone make up the oldest of the four Denver Basin aquifers, the Laramie-Fox Hills.

The Laramide Orogeny, which was a major mountain building event that created the present-day Rocky Mountains, began approximately 72 million years ago and continued for another 60 million years.  The Rocky Mountains were created through massive uplift caused by the collision of the westerly moving North American Plate with the easterly-moving Pacific Plate.   Through millions of years of uplift and erosion caused by precipitation and wind, particles from the rocks and soil were transported and deposited by rivers eastward into the subsiding Denver Basin where these silts, sands, and gravels accumulated.  With deep burial and compaction, these materials became compacted to form mudstones, siltstones, and sandstones.  The pore spaces between the rock grains became water-saturated through precipitation and infiltration.  This geologic process is what eventually formed the most water-rich aquifers of the Denver Basin; the Arapahoe, Denver and Dawson Formations.

Denver Basin’s most productive aquifer is the Arapahoe, which is largely composed of sandstones and conglomerates.  The conglomerates contain rounded chert and quartz pebbles, explaining why the Arapahoe Formation contains a large number of pore spaces, and thus ideal water storage capacities.  The Arapahoe is a major source of water for the users of Douglas County given its continually high well production rates.

Directly above the Arapahoe Formation are the Denver and Dawson Formations.  The Denver and Dawson aquifers outcrop at the surface in certain locations of Douglas County and can thus be either a confined or unconfined aquifer depending on location in the basin.  The Denver aquifer is primarily made up of volcanic rock while the Dawson is high in granitic fragments.  Aquifer production rates vary significantly in the Denver and Dawson aquifers, depending on the amount of sandstone encountered at your location.


Source: Colorado Geological Survey

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Major Surface Water Systems in Douglas County map
Major Surface Water Systems in Douglas County (click on map to enlarge)

Surface Water Systems

Colorado is hydraulically divided into nine major river basins, one of which is the South Platte River Basin.  Bounded by the Continental Divide to the west, the Basin includes all of Adams, Arapahoe, Boulder, Clear Creek, Denver, Douglas, Gilpin, Jefferson, Morgan and Weld Counties.  The South Platte River Basin drains nearly a 19,000-square-mile area in the northeastern quarter of Colorado.  Within Douglas County, three major surface streams – the South Platte River, Cherry Creek and Plum Creek – are an intricate part of the South Platte River Basin hydrology.  Douglas County enjoys considerably lower surface water amounts than other drainage systems in the Basin. For example, the total annual run-off within Douglas County from Plum Creek, Cherry Creek, and the South Platte averages approximately 150,000-acre-feet per year, whereas northern portions of the Basin see approximately 870,000-acre-feet per year.

Given the limited surface water within the county, the system was fully appropriated nearly a century ago, meaning no new water rights are available for acquisition.  Any change in place or use will be required to come from an existing water right rather than a new appropriation. While some Douglas County water providers have water rights to the South Platte, Cherry and Plum Creeks, their allocations do not provide enough water to satisfy the renewable supplies necessary to fulfill the existing water demands of the county.

Surface Water Reservoirs

Rueter-Hess Reservoir

In 1985, the Parker Water and Sanitation District, one of Douglas County’s largest water providers, projected a water shortfall and began implementing several water conservation measures. These measures helped the situation, but they continued to project water shortfalls.  It was determined that additional storage capacity would need to be created in order to capture the District’s reuse water, as well as any other additional water rights they could acquire.  After conducting a feasibility study and completing an Environmental Impact Statement, construction began on the Rueter-Hess Reservoir in 2004.  The reservoir generated a great deal of interest from nearby water providers, including the Town of Castle Rock, Castle Pines Metropolitan District, and Stonegate Metropolitan District.  Therefore, the project expanded from a 16,200-acre-feet reservoir to a 70,000-acre-feet one.  The reservoir is located about three miles southwest of downtown Parker on Newlin Gulch, tributary drainage of Cherry Creek.  For more information on Rueter-Hess Reservoir, please visit Parker Water and Sanitation District.

Chatfield Reservoir

The U.S. Army Corps of Engineers built the Chatfield Reservoir and Dam in 1975 at the confluence of the South Platte River and Plum Creek.  The primary purpose of Chatfield Reservoir is to serve as flood protection for the Denver metropolitan area.  Therefore, a majority of the reservoir’s storage capacity is reserved for flood control; however, the reservoir also provides water storage for industrial, municipal, agricultural and recreational uses.  The Colorado Water Conservation Board (CWCB), a division of the State of Colorado’s Department of Natural Resources, requested that the Army Corps of Engineers consider reallocating space within Chatfield Reservoir for water supply purposes.  Some water users rely on non-tributary groundwater from the Denver Basin aquifer. They have decreased their dependence on non-renewable aquifers through securing rights to surface water in the South Platte River and Plum Creek.

Water users requesting the reallocation consider Chatfield Reservoir as a potential solution to store diverted surface water from Plum Creek and the South Platte River.  They propose to use this stored water during low-flow periods, thereby providing reliable water supplies and further reducing their dependence on non-renewable groundwater.  This can be achieved by increasing the storage capacity of the reservoir.

Currently, water stored in the Chatfield Reservoir is designated into four storage pools for specific purposes.  In ascending order they are inactive/sediment, multipurpose-conservation, flood control, and surcharge water.  Each pool is delineated through elevations in feet above sea level, limiting the volume of water for the allowed purpose.  A feasibility study and an Environmental Impact Statement (EIS) considered four alternatives including an evaluation of the potential impacts and costs of each: 1) a proposed alternative for maximum reallocation; 2) an alternative for an intermediate level of reallocation; and 3 and 4) two alternative scenarios if no action is taken at Chatfield Reservoir.

South Platte Reservoir

The Centennial Water and Sanitation District partnered with the City of Littleton, South Suburban Parks and Recreation District and the Last Chance and Nevada Ditch Companies to plan the South Platte Reservoir.  In 2007, the South Platte Reservoir was completed and water began being stored in 2008.

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Water Infrastructure and Supply Efficiency (W.I.S.E.) is an infrastructure project involving 10 members of the South Metro Water Supply Authority (SMWSA), Aurora Water and Denver Water. The project works by treating reuse from Denver and Aurora water customers and providing it to South Metro WISE partners when available. This water reduces the use of nonrenewable groundwater; which is becoming less available in the underground aquifers. Douglas County has purchased a reservation of water to ensure providers within the county may have access to renewable water.

Water Alternatives Program

The purpose of the Water Alternatives Program is to assist homeowners and small domestic water providers in developing renewable water supply alternatives. The program works by the County partnering to provide a feasibility analysis evaluating various renewable water supply options, infrastructure predesign, and estimated costs. A project that utilized this program successfully was the Northwest Douglas County Water Project. The County assisted four subdivisions – Plum Valley Heights, Chatfield East, Chatfield Acres, and Titan Road Industrial Park – in obtaining renewable water service from Roxborough Water and Sanitation District (RWSD) to have the water needs of the community served.