Chapter A. Introduction, Conceptual Model, Hydrogeologic 

Framework, and Predevelopment Groundwater Availability 

of the Denver Basin Aquifer System, Colorado

By Suzanne S. Paschke, Edward R. Banta, Jean A. Dupree, and Joseph P. Capesius

Municipal and domestic water supplies use Denver 

Basin groundwater resources to support population and housing 

growth in the Front Range urban corridor (Hillier and others, 

1978; Moore and others, 2007). Municipal utilities manage public water supplies in the basin and rely on a combination of surface water from the mountains of the Front Range, groundwater 

from alluvial aquifers, and groundwater from the Denver Basin 

bedrock aquifers. Because the prior appropriation doctrine 

governs water rights in Colorado, older established municipalities such as Denver hold water rights senior to more recently 

established development. Growing and recently established 

municipalities, such as those in Douglas and El Paso Counties, 

typically rely on Denver Basin groundwater for water supply 

because they are in areas with limited surface water, or limited 

available surface-water rights, or both (Hillier and others, 1978). 

Denver Basin bedrock groundwater provides more than 70 percent of the water supply in the south Denver metropolitan area, 

and some water providers consider groundwater availability in 

this area insufficient for long-term demand (South Metro Water 

Supply Study Board, written commun., 2003; Colorado Water 

Conservation Board, 2004a). Domestic groundwater use is less 

than municipal use but is widespread across the basin, and rural 

areas on the semiarid plains depend solely on self-supplied 

groundwater for water supply.

The Denver Basin aquifer system is administratively 

recognized as nonrenewable because the aquifers are primarily confined and receive little precipitation recharge (Graham 

and VanSlyke, 2004). As population and groundwater use 

increased in the 1970s and 1980s, Colorado Senate Bill 85–5 

(SB85–5) was implemented, which required the Colorado 

State Engineer to promulgate rules and regulations governing 

the withdrawal of groundwater from the Denver Basin aquifer 

system. SB85–5 adopted a 100-year aquifer life to determine 

decreed pumping rates for a given parcel of land and accepted 

storage depletion by not protecting artesian head or aquifer 

water levels (Graham and VanSlyke, 2004). Population, housing development, and associated groundwater pumping in 

the Denver Basin have continued to increase since the 1980s, 

causing renewed concern for groundwater availability and the 

usable life of the Denver Basin aquifer system (Moore and 

others, 2007).

Introduction

The Denver Basin aquifer system is a critical water 

resource for growing municipal, industrial and domestic uses 

along the semiarid Front Range urban corridor of Colorado. 

The confined aquifer system consists of Upper Cretaceousto Tertiary-age bedrock sandstones, which underlie about 

7,000 square miles (mi2) of the Great Plains along the eastern front of the Rocky Mountain Front Range (fig. A1) 

(Fenneman, 1931; Robson, 1987, 1989). Unconsolidated 

Quaternary alluvial and eolian deposits overlie the bedrock 

sandstone aquifers and claystone confining units, and the 

Quaternary deposits form a productive unconfined alluvial 

aquifer where saturated, primarily along present-day stream 

channels (Hunt, 1954; Scott, 1960; Robson, 1989, 1996). 

Streams draining eastward into the Denver Basin are generally 

perennial and originate as snowmelt runoff from the Rocky 

Mountain Front Range. Streams that originate on the semiarid 

plains within the Denver Basin are generally ephemeral and 

intermittent because they receive water primarily from local 

precipitation runoff and groundwater discharge. Groundwater 

in alluvial and bedrock aquifer interacts with surface water as 

streams cross the basin, and there is groundwater movement 

between the alluvial and bedrock aquifers (Robson, 1996).

Mining, transportation, agricultural, and urban activities 

developed along the Front Range beginning in the 1800s, with 

Denver serving as a local and regional transportation hub for 

agricultural products from the South Platte River Valley and 

mining products from the Front Range. Today, urban and suburban land use extends north from Denver to Fort Collins and 

south to Colorado Springs in an area known as the Front Range 

urban corridor (Robson, 1989). Population of the metropolitan 

areas overlying the Denver Basin aquifers has approximately 

doubled from 1.3 million in 1970 to about 2.7 million in 2005 

(T.L. Arnold, U.S. Geological Survey, written commun., 2009). 

Development has been especially rapid and extensive on the 

west side of the basin between Denver and Colorado Springs. 

This area experienced the fastest housing growth of any county 

in the United States in 2002 (Perry and Mackun, 2002) with a 

population increase in Douglas County of approximately thirtyfold from 1970 to 2005.
2 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Groundwater availability studies provide information 

and tools for water administrators, providers, consumers, 

and interested citizens to make informed decisions with 

respect to regional water-management planning (Reilly and 

others, 2008). In the Denver Basin, there is an ongoing need 

for monitoring and modeling to improve understanding of 

groundwater availability and to assess water use, water-level 

decline, and storage depletion on a recurring basis. Adequate 

monitoring of the hydrologic system is important to evaluate 

the effects of continued pumping (Taylor and Alley, 2001), 

and monitoring data are used to build, calibrate, and refine 

groundwater flow models. In turn, groundwater flow models 

serve as tools for predicting future aquifer conditions and 

guiding water-management decisions for complex aquifer 

systems such as the Denver Basin (Taylor and Alley, 2001).

Assessing groundwater availability is an ongoing process, 

which requires continual or periodic renewed evaluation 

of monitoring and modeling as available data, technology, 

water use, and other issues of concern change over time 

(Reilly, 2005). To understand and simulate a groundwater 

system, basic data regarding the hydrogeologic framework, 

water-budget components, hydraulic-head distribution, 

and aquifer properties of hydraulic conductivity and storage are needed (Reilly, 2005). Computer simulations of 

groundwater flow use this basic information to quantify flow 

and hydraulic heads for the system of interest and provide 

useful tools for quantifying and evaluating system changes 

over time.

!

!

!

!

!

Yampa River

White Riv er

Colorado River

Rio Grande

San Juan R

Arkansas River

South Platte River

Gunnison River

Denver 

Basin

Arkansas River 

Basin

South Platte 

River Basin Great Plains

Rocky Mountain Front Range

Pueblo

Denver

Fort

Collins

Grand

Junction

Colorado

Springs

Palmer Div ide

108° 107° 106° 105° 104° 103°

40°

39°

38°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W 0 50 100 KILOMETERS

0 50 100 MILES

C O L O R A D O

Figure A1. Location of the Denver Basin aquifer system, Colorado.
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 3

Two broad themes categorize groundwater issues in the 

Denver Basin: (1) hydrologic effects of continued pumping 

on future groundwater availability and (2) political, legal, 

and social concerns related to usable aquifer life and effective management of regional water resources. In 2004, the 

U.S. Geological Survey (USGS) initiated large-scale regional 

studies to provide updated assessments of groundwater availability in important principal aquifers across the United States, 

including the Denver Basin. This study of the Denver Basin 

aquifer system evaluates the hydrologic effects of ongoing 

pumping and documents an updated groundwater flow model 

used for assessing hydrologic conditions. The calibrated 

groundwater flow model provides quantitative estimates of 

groundwater flow conditions and provides a useful tool for 

managers to analyze temporal changes to the hydrologic 

system in response to changing climatic conditions and future 

groundwater development.

Purpose and Scope

The purpose of this Professional Paper is to describe 

groundwater availability in the Denver Basin aquifer system. 

A basin-scale evaluation of groundwater resources in the 

Denver Basin aquifer system is provided that advances the 

understanding of system response to development. Specific 

objectives of this Denver Basin groundwater availability analysis were to (1) advance the understanding of the three-dimensional nature of the hydrogeologic framework including the 

extent and thickness of aquifers and confining units; (2) advance 

the understanding of regional groundwater budgets and components including recharge, discharge, and aquifer storage for 

the entire system and separately for the bedrock and alluvial 

aquifers; (3) evaluate recent and future system response to 

development at a scale relevant to basinwide water-management 

decisions; and (4) explore the use of modeling to evaluate 

hydrologic monitoring of system changes.

The document consists of three chapters describing 

groundwater availability in the Denver Basin. This chapter, 

Chapter A, presents the introduction, previous work, study 

methods, and a conceptual model of the hydrogeologic setting 

including the hydrogeologic framework and predevelopment 

groundwater availability. Chapter B describes the land and 

groundwater-use changes in response to population growth 

in the Denver Basin and the effects of that development on 

groundwater availability. The changes in land and groundwater use are used as input to the groundwater flow model as 

described in Chapter C, and the interpretations of the effects 

of development on the groundwater flow system are based 

largely on modeling results from this study. Effects included 

in the discussion are changes to recharge and discharge 

components of the groundwater budget (recharge, evapotranspiration, pumping, groundwater/surface-water interaction, interaquifer flow, and storage) and the effects of those 

changes on groundwater levels and flow directions from 1880 

through 2003. Chapter B also presents predictions of future 

hypothetical conditions and a discussion of monitoring data 

needs. Chapter C presents details of the groundwater flow 

model used for the groundwater availability analysis including model setup, calibration results, parameter sensitivity, and 

future work.

Previous Work

Artesian groundwater conditions and the underlying 

structural geology of the Denver Basin were first recognized 

and described by Cross and others (1884), Emmons and 

others (1896), Darton (1905), and Richardson (1915). Since 

this early work, numerous geologic and hydrologic publications have described the Denver Basin tectonics, structural 

geology, stratigraphy, geologic history, energy resources, hazards, and groundwater. The Rocky Mountain Association of 

Geologists published compilations of papers related to Denver 

Basin geology (Katich and Bolyard, 1963; Rocky Mountain 

Association of Geologists, 1978) and Colorado Geology (Kent 

and Porter, 1980), which contain several papers cited herein. 

Other publications consist of indexes and bibliographies of 

early reports addressing the geology and hydrology of the 

Denver Basin (Chronic and Chronic, 1974; Hampton and 

others, 1974; Hampton, 1975; Anna, 1975; Norris and others, 

1983; and Wireman and Romero, 1989). In addition, a few 

documents provide overviews of groundwater resources for 

the State of Colorado, including Pearl (1974), Aikin and others 

(2000), and Topper and others (2003). The following discussion of previous work focuses on those studies most pertinent 

to this groundwater availability report.

Available geologic maps for the eastern plains encompassing the Denver Basin include the Geologic Map of Colorado 

(Tweto, 1979), geologic maps for 1° × 2° quadrangle sheets 

(Scott and others, 1978; Sharps, 1976, 1980; Bryant and others, 

1981), and maps for individual 1:24,000-scale quadrangles 

(for example Scott, 1963a, 1963b). The Colorado Geological 

Survey (CGS) currently (2010) is updating geologic maps of 

the Front Range Urban Corridor as part of the STATEMAP 

program of the National Geologic Mapping Act of 1997 (Carroll 

and Crawford, 2000), and several quadrangle maps have been 

completed for the southwest part of the Denver Basin (Carroll 

and Crawford, 2000; Thorson and others, 2002; Madole, 2003; 

Madole and Thorson, 2003; Rowley and others, 2003; Thorson 

and Madole, 2003; Thorson, 2003, 2004a, 2004b, 2005a, 2005b, 

2006, 2008; Thorson and Himmelreich, 2004; Morgan and 

others, 2005a, 2005b).

Systematic hydrogeologic characterization of the 

Denver Basin aquifers began in the 1970s as part of developing nontributary groundwater rules established by Colorado 

Senate Bill 213 (Graham and VanSlyke, 2004). The Laramie–

Fox Hills was the first aquifer mapped in 1971 (Romero 

and Hampton, 1972), and collaborative work between the 

Colorado Division of Water Resources (CDWR) and the 

USGS continued through the 1970s and 1980s, differentiating, mapping, and characterizing the four recognized aquifer 
4 Groundwater Availability of the Denver Basin Aquifer System, Colorado

units (Romero, 1976). Results of these efforts included 

hydrologic atlases by Robson and Romero (1981a, 1981b), 

Robson, Romero, and Zawistowski (1981), and Robson, 

Wacinski, and others (1981), which presented the geologic 

structure, hydrology, and water quality of the Dawson, 

Denver, Arapahoe, and Laramie–Fox Hills aquifers. Robson 

(1983) tabulated hydraulic-conductivity, transmissivity, and 

storage-coefficient data for individual bedrock aquifers. The 

hydrologic atlases and data compiled through the 1980s later 

supported construction of a groundwater flow model of the 

Denver Basin aquifers (Robson, 1987). The CDWR updated 

hydrologic atlases of the Denver Basin in 1988 using additional available data to create maps of geologic structure, 

silt-plus-sand thickness, and areas of nontributary groundwater 

for each of the bedrock aquifers (VanSlyke and others, 1988a, 

1988b, 1988c, 1988d). The updated hydrologic atlases formed 

the basis for defining aquifer boundaries since 1988 (Graham 

and VanSlyke, 2004).

Other water-resources studies have examined specific 

parts of the Denver Basin aquifer system. For example, 

Livingston and others (1975, 1976) evaluated surface-water 

and groundwater resources in El Paso County. Hillier and 

others (1978) examined the hydrology and water quality of 

the Arapahoe aquifer in the Englewood–Castle Rock area 

south of Denver, and Schneider (1980) examined the hydrology and water quality of the Laramie–Fox Hills aquifer in 

parts of Adams, Boulder, Jefferson, and Weld Counties. The 

hydrogeology of Cretaceous and older formations below the 

Denver Basin aquifers was investigated as part of the USGS 

Central Midwest Regional Aquifer-System Analysis (RASA), 

and results were reported by Robson and Banta (1987). The 

Central Midwest RASA report (Robson and Banta, 1987) 

presented geologic structure, hydraulic characteristics, and 

water-quality data for the Paleozoic and Mesozoic rocks of 

the Denver Basin up through the Laramie Formation and Fox 

Hills Sandstone.

The Laramie and Denver Formations contain coal and 

lignite beds, which, along with associated coal-bed methane, 

are potential energy resources in the Denver Basin (Colton 

and Lowrie, 1973; Soister, 1978a; Gaggiani and others, 1987; 

Britton and Gaggiani, 1989; Wray and Koenig, 2001; Roberts, 

2005). More than 130 million tons of coal and lignite were 

mined from the Denver region from 1883 to 1979, mostly 

from the Laramie Formation in the Boulder-Weld coal field 

near Boulder, Colorado, and more than 99 percent of the 

mined coal came from underground mines (Britton and 

Gaggiani, 1989). Several previous investigations provide 

geologic and coal resource information for the Denver Basin 

including Soister (1972, 1974, 1978a), Kirkham (1978), 

Kirkham and Ladwig (1979, 1980), Brand (1980), Brand 

and Caine (1980), Murray (1980), Eakins and Ellis (1987), 

Nichols (1999), and Carroll (2004). The potential for coal 

mining to affect Denver Basin surface-water and groundwater 

resources was addressed by Kirkham and Ladwig (1977) and 

Gaggiani and others (1987). Wray and Koenig (2001) discuss 

the coal-bed methane potential of the Laramie and Denver 

Formations based on previous work and provide electronic 

images and geographic information system (GIS) data files of 

selected maps and plates from previous studies by Kirkham 

and Ladwig (1979, 1980).

Two field studies examined stratigraphy and hydrologic 

properties of the Denver Basin bedrock aquifers by coring and 

geophysical logging deep boreholes through the units. The 

Castle Pines core hole, located about 20 miles south of Denver, 

was drilled and logged in 1987 to a depth of 3,110 ft below 

land surface and fully penetrated the Arapahoe aquifer (Raforth 

and Jehn, 1990). Laboratory analyses of selected core samples 

collected during drilling yielded mineralogy, grain size, bulk 

and grain density, porosity, specific yield, and specific retention 

data for the Denver Basin bedrock aquifers (Robson and Banta, 

1993; Robson, 1995). The study also collected water-level 

data, atmospheric-pressure measurements, aquifer-compression 

measurements, and borehole geophysical logs for a 2,400-ftdeep test hole constructed adjacent to the Castle Pines core hole 

(Robson and Banta, 1990; Robson and Banta, 1993).

The Kiowa core hole was drilled in the town of Kiowa, 

Colorado, in 1999 to a depth of 2,256 ft below land surface 

(Raynolds and others, 2001). The Kiowa core hole penetrated 

the full stratigraphic section of the Denver Basin aquifers in 

the central part of the basin where fine-grained strata were 

anticipated (Raynolds and others, 2001). Core samples from 

the Kiowa core hole were analyzed to determine the porosity, 

permeability, hydraulic conductivity, palynology, paleomagnetism, mineralogy, petrography, and fission-track properties 

of the Denver Basin bedrock aquifers (Raynolds and others, 

2001). Geophysical logging of the Kiowa core hole included 

caliper, gamma ray, spontaneous potential, resistivity, compensated density, and full-waveform sonic; data for a short seismic 

line were obtained adjacent to the core hole by using Vibroseis 

techniques, and the core hole was completed as a groundwater 

monitoring well (Raynolds and others, 2001). A series of 

publications from 2001 to 2004 present results from the Kiowa 

core hole and related Denver Basin field studies (Lapey, 

2001; Carpenter and Young, 2002; Farnham and Kraus, 2002; 

Kelly, 2002; Kelly and Blackwell, 2002; Lapey-Woodard and 

others, 2002; Nichols and Fleming, 2002; Obradovich, 2002; 

Raynolds, 2002, 2004; Raynolds and Johnson, 2002, 2003; 

Wilson, 2002; Barclay and others, 2003; Eberle, 2003; Ellis and 

others, 2003; Hicks and others, 2003; Hutchinson and Holroyd, 

2003; Johnson and others, 2003; Wheeler and Michalski, 2003; 

Barkmann, 2004; Kittleson, 2004).

Historical water-level data collection for Denver Basin 

bedrock aquifers has been irregular, and water-level monitoring efforts have decreased since the 1980s (Colorado Water 

Conservation Board, 2004b, 2006a). The first effort at bedrock 

water-level monitoring in the Denver Basin was the work of 

Emmons and others (1896), which included an inventory of 

wells, available water levels (or artesian pressures), and estimated artesian flow for wells completed in downtown Denver 

in the 1880s. Model calibration by this study used these waterlevel estimates to represent hydraulic head in the late 1880s as 

discussed in the “Model Calibration” section of Chapter C. The 
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 5

USGS measured the first set of basinwide water levels from 

1956 to 1963, and McConaghy and others (1964) published the 

results. Major and others (1983) published a second comprehensive set of water-level data for the bedrock and alluvial 

aquifers through 1981. The Robson (1987) groundwater flow 

model incorporated routine water-level measurements made 

by the USGS through the 1980s, and the data are available 

through the USGS National Water Information System (NWIS) 

database (http://waterdata.usgs.gov/co/nwis/). Model calibration by this study used the USGS NWIS data as discussed in 

the “Model Calibration” section of Chapter C. The CDWR 

established a water-level monitoring network of approximately 

278 wells in the 1980s, and data from that network are published in annual data reports (for example, Colorado Division 

of Water Resources, 2003, 2004). Model calibration by this 

study did not use the CDWR water-level data primarily because 

of uncertainty related to well-completion intervals and pumping status. However, this study does compare selected data 

from the CDWR water-level network to simulation results in 

the section “Changes in Hydraulic Heads and Groundwater 

Flow Directions” in Chapter B. The South Platte Decision 

Support System (SPDSS) published a compilation and bibliography of all available water-level data for bedrock and alluvial 

aquifers through 2004 (Colorado Water Conservation Board, 

2004b, 2006a).

Numerous studies have examined the alluvial aquifers 

of the South Platte River and its tributaries. Stratigraphy 

of the alluvial deposits was originally described by Hunt 

(1954) and Scott (1960) and later by Scott (1963a). Several 

workers developed maps of the South Platte alluvial-aquifer 

extent, thickness, and depth to water beginning in the 1950s 

(Bjorklund and Brown, 1957; Smith and others, 1964; Duke 

and Longenbaugh, 1966; Nelson and others, 1967; Hurr, 

Schneider, and others, 1972a, 1972b, 1972c; Hurr and others, 1975; Konikow, 1975; Nadler and Schumm, 1981; Robson, 1996; and Robson, Arnold, and Heiny, 2000a, 2000b; 

Robson, Heiny, and Arnold, 2000a, 2000b), and the SPDSS 

compiled selected maps into GIS data sets (Colorado Water 

Conservation Board, 2006b). Other workers developed data 

and maps for the major alluvial aquifers in the southern Denver Basin draining toward the Arkansas River as follows: 

(1) Black Squirrel Creek (Jenkins, 1961; Erker and Romero, 

1967; Bingham and Klein, 1973; and Buckles and Watts, 

1988); (2) Big Sandy Creek (Coffin, 1962; Willard Owens 

Associates, 1971); and (3) the Nussbaum Alluvium (Scott, 

1963c; Soister, 1967; Paschke, 1992). Many of the alluvial 

aquifer reports cited above and Robson (1989) describe the 

interconnection between bedrock and alluvial aquifers.

The aquifer system was previously evaluated using 

groundwater flow models by the USGS (Robson, 1987) and 

the CDWR (Colorado Division of Water Resources, 1998). 

Previous Denver Basin groundwater flow models used the 

USGS computer program MODFLOW (McDonald and 

Harbaugh, 1988) and its precursor simulation codes in quasithree-dimensional configurations. A quasi-three-dimensional 

groundwater flow model explicitly represents the geologic 

layers of aquifers, but confining units between aquifers are 

represented implicitly using vertical conductance variables 

rather than being explicitly represented as model layers.

The first numerical model of the Denver Basin aquifers 

was developed as a four-layer groundwater flow model (Robson, 

1987) using the USGS groundwater simulation code developed 

and described by Trescott (1975), Trescott and Larson (1976), 

and Trescott and others (1976), which predated MODFLOW. 

The Robson (1987) four-layer Denver Basin model served as the 

basis for all subsequent groundwater resource evaluations conducted by the USGS, CDWR, and various municipal and county 

governments until 1996. The four-layer Denver Basin model was 

superseded by a model constructed by CDWR between 1996 

and 1998 in response to Colorado Senate Bill 96–74 (SB96–74; 

Colorado Division of Water Resources, 1998). The CDWR 

SB96–74 model incorporated additional hydrogeologic data 

(Robson and Banta, 1993; Robson and others, 1998; VanSlyke 

and others, 1988a, 1988b, 1988c, 1988d) in a six-layer quasithree-dimensional groundwater flow model (Colorado Division 

of Water Resources, 1998) and used MODFLOW (McDonald 

and Harbaugh, 1988). This report compares modeling results 

from the current study to results of previous models (Robson, 

1987; Colorado Division of Water Resources, 1998) in the section “Comparisons between Current and Previous Models” in 

Chapter C. The CDWR SB96–74 model was further updated in 

2002 using MODFLOW-2000 (Harbaugh and others, 2000; Hill 

and others, 2000) as part of the South Metro Water Supply Study 

(SMWSS) funded by a consortium of water suppliers in the south 

Denver metropolitan area. The SMWSS model used the CDWR 

SB96–74 model and updated Douglas County water-use estimates to the year 2000 (South Metro Water Supply Study Board, 

written commun., 2003).

Two previous USGS investigations evaluated the potential 

effects of increased groundwater pumping from the bedrock aquifers in El Paso County. Banta (1989) simulated increased pumping in the southern Denver Basin, on the basis of projected water 

needs in the Colorado Springs vicinity, using the Robson (1987) 

four-layer quasi-three-dimensional groundwater flow model with 

a refined finite-difference grid. The Banta (1989) modeling study 

indicated increased pumping would result in drawdowns of as 

much as 1,300 ft and large decreases in groundwater storage after 

100 years of pumping. Model simulations also indicated a decline 

in groundwater discharge to streams through time and, eventually, pumping-induced stream recharge to aquifers (Banta, 1989). 

Buckles and Watts (1988) simulated the effects of continued 

pumping on the Black Squirrel Creek alluvial aquifer in El Paso 

County and emphasized the importance of evapotranspiration 

as a hydrologic process in the Denver Basin. Watts (1995) later 

developed a three-dimensional model of the Black Squirrel 

Creek Basin and simulated the effects of increased Denver Basin 

pumping on water levels in the alluvial aquifer. Model results 

indicated continued pumping of the Denver Basin bedrock aquifers would lead to water-level declines and groundwater storage 

decreases in the Black Squirrel Creek alluvial aquifer and could 

reverse the flow direction between alluvial and bedrock aquifers 

(Watts, 1995).
6 Groundwater Availability of the Denver Basin Aquifer System, Colorado

The most recent Denver Basin legislation, Senate 

Bill 96–74 (SB96–74) was implemented in 1996 and provided 

funds for a study of the Denver Basin and South Platte aquifer 

systems in anticipation of developing a SPDSS (Graham and 

Van Slyke, 2004). The SPDSS development currently (2010) 

is in progress under the direction of the Colorado Water 

Conservation Board (CWCB) and the CDWR and addresses 

many areas of water management in the South Platte River 

Basin, including surface-water diversions, consumptive water 

use, irrigated lands, land use, and groundwater (Colorado 

Water Conservation Board, 2004a). The SPDSS groundwater studies focus primarily on the alluvial aquifer of the 

South Platte River and its tributaries, and this study used data 

compilations from the SPDSS as the basis for aquifer characterization (Colorado Water Conservation Board, 2004b; 2006a, 

2006b, 2006c). This groundwater availability study in the 

Denver Basin began in 2004 as part of the USGS Groundwater 

Resources Program and focused on the bedrock aquifers. The 

present study and the SPDSS support each other in furthering 

the simulation of interaction between the bedrock and alluvial 

aquifers in the South Platte River Basin.

Study Methods

Study methods included compiling existing hydrologic 

data and developing a numerical groundwater flow model of the 

Denver Basin aquifer system using the U.S. Geological Survey 

(USGS) modular finite-difference groundwater flow computer 

program MODFLOW-2000 (Harbaugh and others, 2000; Hill 

and others, 2000). Model-input and model-calibration data sets 

were derived from geologic and hydrologic data collected since 

previous modeling efforts. The calibrated model was evaluated 

for model fit, and the simulation results were used to assess 

changes in groundwater availability through time. The model 

considers future groundwater conditions by simulating two 

hypothetical predictive scenarios and evaluating the sensitivity 

of the predictions to assess data needs and hydrologic monitoring. This section presents an overview of the methods used 

by the Denver Basin groundwater availability study and refers 

the reader to the appropriate Professional Paper chapter for 

detailed explanations.

Data Compilation

The hydrogeologic framework for the bedrock and alluvial aquifers, transient model boundary conditions (recharge, 

evapotranspiration, and well pumping), and calibration 

datasets of hydraulic head and stream base flow were developed from the data compilation. Additional hydrogeologic 

data collected since the SB96–74 model publication (Colorado 

Division of Water Resources, 1998) include updated geologic 

mapping of the extent of Denver Basin bedrock and alluvial 

aquifers (Robson, 1996; Robson and others, 1998); additional 

geophysical logs in the basin with which to further define 

aquifer layering; additional streamflow, precipitation, and 

land-use data; and an updated State well-permit database, 

which was used to revise pumping estimates for the bedrock 

and alluvial aquifers.

The hydrogeologic framework for the Denver Basin 

groundwater model consists of GIS data sets, which define the 

lateral extent, base altitude, and thickness of each aquifer and 

confining unit represented in the model. This study modified maps showing the extent of aquifers and confining units 

published by the CDWR (VanSlyke and others, 1988a, 1988b, 

1988c, 1988d) along the western basin margin to include geologic contacts mapped by Robson and others (1998), and the 

results defined the extent of active cells for each model layer. 

Topographic altitude data for land surface and streamflow 

routing were derived from the 30-meter-resolution National 

Elevation Dataset (NED) (U.S. Geological Survey, 1999).

Maps of the altitude of the base of the bedrock aquifers and confining units, silt-plus-sand thickness, and percent silt-plus-sand for bedrock aquifers were generated in 

ArcGIS 9.2 (Environmental Systems Research Institute, 

Inc., 1999–2004) from the CDWR Denver Basin geophysical log database (Brian Ahrens, Colorado Division of Water 

Resources, written commun., 2003) using ordinary kriging 

and inverse-distance weighed (IDW) methods. The CDWR 

geophysical-log database contains top and bottom elevations 

for each of the six bedrock aquifers (geophysical log “picks”) 

for about 4,400 geophysical logs. The geophysical log picks 

were processed into ArcGIS point-data shapefiles representing 

the base altitude for each of the six bedrock aquifers and five 

confining units, and the altitude data points were contoured 

using an ordinary kriging method available in ArcGIS. For 

many logs in the CDWR geophysical-log database, a silt-plussand thickness value was recorded for each bedrock aquifer. 

These silt-plus-sand-thickness values also were processed 

into ArcGIS point-data shapefiles, and the data points were 

contoured using either an ordinary kriging method or an IDW 

method available in ArcGIS, depending on which method 

resulted in the least error. Percent silt-plus-sand-thickness 

values were calculated from the silt-plus-sand-thickness point 

data for each of the six bedrock aquifers as the silt-plus-sand 

thickness divided by the total layer thickness, and the percent 

silt-plus-sand-thickness values were contoured using an IDW 

method available in ArcGIS. Averaged percent silt-plus-sand 

values for each model grid cell were used as multiplier arrays 

for hydraulic-conductivity and specific-yield values in the 

model as described in the “Hydrogeologic-Unit Properties” 

section of Chapter C. Contouring methods and model-fitting 

parameters are listed in table A1, and the results are presented 

in the “Hydrogeologic Framework” section of Chapter A.

The contours shown on the maps in this report and the 

data points on which they were based are available electronically as spatial data at http://water.usgs.gov/lookup/getgislist/. 

Table A2 lists file names and an abbreviated description of the 
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 7

Table A1. Geostatistical models, model-fitting parameters, and cross-validation statistics for Denver Basin bedrock units.[na, not applicable]

Variable Number of data points Contour method Model type

Model-fitting parameters Cross-validation statistics

Lag

(feet)

Range

(feet)

Sill

(square

feet)

Nugget

(square

feet)

Mean

error

(feet)

Root-meansquared error

(feet)

Average

standardized

error

(feet)

Mean

standardized

error

(feet)

Root-mean-squared

standardized

error

(feet)

Upper Dawson aquifer, 

base altitude

275 Ordinary 

kriging

Spherical 16,534 195,982 86,597 0.0 –2.10 69.4 54.4 –0.013 1.9

Upper Dawson aquifer, 

silt-plus-sand thickness

123 Inversedistance

weighted

na na na na na –0.62 62.7 na na na

Dawson confining unit, 

base altitude

331 Ordinary 

kriging

Spherical 15,484 18,3536 88,584 0.0 –0.75 64.9 54.1 0.0017 1.7

Lower Dawson aquifer, 

base altitude

631 Ordinary 

kriging

Spherical 23,012 272,767 259,340 0.0 –0.60 57.4 71.6 –0.0047 1.1

Lower Dawson aquifer, 

silt-plus-sand thickness

402 Ordinary 

kriging

Spherical 20,721 245,612 11,852 1,008 0.05 54.2 39.1 –0.0012 1.3

Denver upper confining unit, 

base altitude 

613 Ordinary 

kriging

Spherical 23,832 282,487 266,480 0.0 –0.24 59.9 72.2 –0.006 1.29

Denver aquifer, 

base altitude

1,180 Ordinary 

kriging

Spherical 36,370 431,103 297,480 0.0 –0.82 52.3 68.1 –0.009 1.07

Denver aquifer, 

silt-plus-sand thickness

763 Ordinary

kriging

Spherical 35,107 91,565 13,993 0.0 0.12 75.0 34 –0.015 4.08

Denver lower confining unit, 

base altitude

1,589 Ordinary 

kriging

Spherical 40,153 475,944 1,321,700 0.0 –0.38 50.4 57.6 –0.007 1.17

Upper Arapahoe aquifer, 

base altitude

637 Ordinary 

kriging

Spherical 20,954 248,373 68,871 0.0 –0.12 46.9 41.4 –0.002 1.38

Upper Arapahoe aquifer , 

silt-plus-sand thickness

520 Inversedistance 

weighted

na na na na na –1.39 29.9 na na na

Arapahoe confining unit, 

base altitude

617 Ordinary 

kriging

Spherical 21,893 236,396 78,553 0.0 0.16 49.6 46.6 0.0028 1.28

Lower Arapahoe aquifer, 

base altitude

1,770 Ordinary 

kriging

Spherical 42,846 507,865 309,540 0.0 0.05 54.2 65.1 0.00083 1.05

Lower Arapahoe aquifer, 

silt-plus-sand thickness

1,488 Inversedistance 

weighted

na na na na na 0.66 46.41 na na na

Laramie confining unit, 

base altitude

2,286 Ordinary 

kriging

Spherical 50,177 594,762 403,283 0.0 0.08 39.2 67.4 0.00086 2.57

Laramie–Fox Hills aquifer, 

base altitude

2,567 Ordinary 

kriging

Spherical 53,378 632,704 41,942 0.0 0.28 44.4 67.4 0.0023 2.17

Laramie–Fox Hills aquifer, 

silt-plus-sand thickness

1,674 Ordinary 

kriging

Spherical 46,032 362,451 1,187 560 0.07 24.2 25.2 0.0029 0.95
8 Groundwater Availability of the Denver Basin Aquifer System, Colorado

spatial data prepared for this study. The file names are a combination of the Professional Paper report number (PP1770) and 

a data descriptor. For example, the name of the spatial data 

set of contours of the generalized altitude of the base of the 

Laramie–Fox Hills aquifer is PP1770_alt_base_KLF_con.

The altitude of the base of the alluvial aquifer system 

was derived from a compilation of alluvial aquifer mapping. Substantial alluvial aquifers considered by this study 

include (1) present-day and prehistoric stream channels of the 

South Platte River and its tributaries, (2) the Black Squirrel 

Creek drainage, and (3) the Big Sandy Creek drainage and 

the Nussbaum Alluvium south of Big Sandy Creek. The 

base altitude of alluvial aquifers in the South Platte River 

basin was mapped for individual basins by various previous 

efforts (Bjorklund and Brown, 1957; Smith and others, 1964; 

Duke and Longenbaugh, 1966; Nelson and others, 1967; 

Hurr, Schneider, and others, 1972a, 1972b, 1972c; Robson, 

1996; Robson, Arnold, and Heiny, 2000a, 2000b; Robson, 

Heiny, and Arnold, 2000a, 2000b), which were compiled 

as part of the SPDSS (Colorado Water Conservation Board, 

2006b). Base-altitude contours for Black Squirrel Creek 

(Erker and Romero, 1967; Buckles and Watts, 1988), Big 

Sandy (Willard Owens Associates, 1971), and the Nussbaum 

Alluvium (Paschke, 1992) were compiled for this study and 

combined with the alluvium base-altitude contours from 

the Colorado Water Conservation Board (2006b) to depict a 

Table A2. Index of Denver Basin hydrogeologic framework spatial data.

Spatial data file name Data type Data description

PP1770_seo_geologs_point Points Location and selected attributes of geophysical logs (Colorado Division 

of Water Resources, 2004)

PP1770_NED30M_raster Raster Altitude of land surface

PP1770_alt_base_UTDW_point Points Altitude of the base of the upper Dawson aquifer 

PP1770_alt_base_TDWC_point Points Altitude of the base of the Dawson confining unit 

PP1770_alt_base_LTDW_point Points Altitude of the base of the lower Dawson aquifer 

PP1770_alt_base_TKDUC_point Points Altitude of the base of the Denver upper confining unit 

PP1770_alt_base_TKD_point Points Altitude of the base of the Denver aquifer 

PP1770_alt_base_TKDLC_point Points Altitude of the base of the Denver lower confining unit 

PP1770_alt_base_UKA_point Points Altitude of the base of the upper Arapahoe aquifer 

PP1770_alt_base_KAC_point Points Altitude of the base of the Arapahoe confining unit 

PP1770_alt_base_LKA_point Points Altitude of the base of the lower Arapahoe aquifer 

PP1770_alt_base_KLC_point Points Altitude of the base of the Laramie confining unit

PP1770_alt_base_KLF_point Points Altitude of the base of the Laramie–Fox Hills aquifer

PP1770_alt_base_UTDW_con Contours Generalized altitude of the base of the upper Dawson aquifer 

PP1770_alt_base_TDWC_con Contours Generalized altitude of the base of the Dawson confining unit 

PP1770_alt_base_LTDW_con Contours Generalized altitude of the base of the lower Dawson aquifer 

PP1770_alt_base_TKDUC_con Contours Generalized altitude of the base of the Denver upper confining unit 

PP1770_alt_base_TKD_con Contours Generalized altitude of the base of the Denver aquifer 

PP1770_alt_base_TKDLC_con Contours Generalized altitude of the base of the Denver lower confining unit 

PP1770_alt_base_UKA_con Contours Generalized altitude of the base of the upper Arapahoe aquifer 

PP1770_alt_base_KAC_con Contours Generalized altitude of the base of the Arapahoe confining unit 

PP1770_alt_base_LKA_con Contours Generalized altitude of the base of the lower Arapahoe aquifer 

PP1770_alt_base_KLC_con Contours Generalized altitude of the base of the Laramie confining unit

PP1770_alt_base_KLF_con Contours Generalized altitude of the base of the Laramie–Fox Hills aquifer

PP1770_thickness_sand_UTDW_point Points Silt-plus-sand thickness of the Dawson aquifer

PP1770_thickness_sand_LTDW_point Points Silt-plus-sand thickness of the lower Dawson aquifer 

PP1770_thickness_sand_TKD_point Points Silt-plus-sand thickness of the Denver aquifer 

PP1770_thickness_sand_UKA_point Points Silt-plus-sand thickness of the upper Arapahoe aquifer 

PP1770_thickness_sand_LKA_point Points Silt-plus-sand thickness of the lower Arapahoe aquifer 

PP1770_thickness_sand_KLF_point Points Silt-plus-sand thickness of the Laramie–Fox Hills aquifer

PP1770_thickness_sand_UTDW_con Contours Generalized silt-plus-sand thickness of the Dawson aquifer

PP1770_thickness_sand_LTDW_con Contours Generalized silt-plus-sand thickness of the lower Dawson aquifer 

PP1770_thickness_sand_TKD_con Contours Generalized silt-plus-sand thickness of the Denver aquifer 

PP1770_thickness_sand_UKA_con Contours Generalized silt-plus-sand thickness of the upper Arapahoe aquifer 

PP1770_thickness_sand_LKA_con Contours Generalized silt-plus-sand thickness of the lower Arapahoe aquifer 

PP1770_thickness_sand_KLF_con Contours Generalized silt-plus-sand thickness of the Laramie–Fox Hills aquifer
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 9

base-altitude surface for the major alluvial aquifers overlying 

the Denver Basin bedrock aquifers. The base-altitude contours 

were converted to a GIS raster, and alluvium thickness was 

computed as the difference between land surface (30-meter 

digital elevation model [DEM], U.S. Geological Survey, 1999) 

and the base altitude on 1,000-ft raster grid spacing.

Estimates of hydraulic conductivity, transmissisivity, 

storage coefficient, and specific yield for the Denver Basin 

bedrock and alluvial aquifers were recently compiled from 

previous studies by the SPDSS (Colorado Water Conservation 

Board, 2006c), and the results are discussed in the “Aquifer 

Hydraulic Properties” section of Chapter A.

Recharge and discharge estimates were derived by various methods, the details of which are described in Chapter C. 

Precipitation records from 1930 to 2003 were obtained for 

National Weather Service sites in and adjacent to the Denver 

Basin, and the data were contoured into long-term mean 

precipitation maps for the period of record and for stress 

periods simulated in the groundwater flow model. The resulting precipitation-distribution maps were used as input to the 

groundwater flow model recharge estimates as described in the 

“Recharge” subsection of “Hydrologic Stresses” in Chapter C. 

Areas of urban/suburban and agricultural irrigation were 

mapped for the periods 1880–1919, 1920–1939, 1940–1953, 

1954–1986, and 1987–2003 from ditch-diversion records, historical maps, and census records. Streamflow records for eight 

sites in the Denver Basin were used to estimate base flow from 

groundwater. Base flow for the period of record at each site 

was estimated using the Base Flow Index (BFI) method (Wahl 

and Wahl, 1988).

Actual pumping rates and volumes from wells in the 

Denver Basin currently (2010) are not recorded or compiled 

into a central database in the State of Colorado. Previous 

modeling efforts estimated transient groundwater pumping 

based on the CDWR well-permit database (Colorado Division 

of Water Resources, 2005), which records the owner, location, 

depth, completion interval, permitted pumping rate, use, and 

dates of first use and abandonment for all permitted wells in 

the State of Colorado. The CDWR well-permit database is the 

only basinwide well database and was used by this study to 

estimate pumping locations, depths, and rates through time. 

Detailed methods and results of the pumping analysis are 

described in the section “Denver Basin Pumping History” 

of Chapter B.

Calibration data for the Denver Basin groundwater flow 

model included hydraulic-head and streamflow measurements through time available from the USGS NWIS database 

(http://waterdata.usgs.gov/co/nwis) and the CDWR State 

database, Hydrobase (Colorado Division of Water Resources, 

2005). Hydraulic-head estimates and estimates of discharge 

from flowing wells in the 1890s were interpreted from data 

tables presented by Emmons and others (1896). Calibration 

datasets are documented in Chapter C.

Model Construction and Calibration

A transient (time-varying), fully three-dimensional 

groundwater flow model of the Denver Basin bedrock and alluvial aquifers, representing the time period from predevelopment 

(pre-1880) through 2003, was developed using the groundwater 

flow simulation code MODFLOW-2000 (Harbaugh and others, 

2000; Hill and others, 2000). Groundwater flow modeling capabilities and computer technology have advanced considerably 

since previous models of the Denver Basin were developed, 

and the Denver Basin model reported by this study is considered 

an enhancement over previous efforts because the following 

capabilities were implemented:

• A fully three-dimensional representation of the aquifer 

system that includes explicit definition of all Denver 

Basin aquifers and confining layers. Previous models 

used a quasi-three-dimensional approach, which 

represented confining layers by using a verticalconductance term.

• Explicit representation of the alluvial aquifer system 

as the uppermost layer in the model. Previous models 

simulated the alluvial aquifer as constant-head cells 

(Robson, 1987) or as head-dependent-flow boundaries using the MODFLOW River Package (Colorado 

Division of Water Resources, 1998).

• Updated streamflow routing simulation capabilities 

(SFR2, Niswonger and Prudic, 2005), which allow 

a more-detailed representation of streams than in 

previous models. Previous models did not explicitly 

represent streams.

• Inclusion of evapotranspiration as a simulated hydrologic process (ETS, Banta, 2000). Previous models did 

not explicitly simulate evapotranspiration as a hydrologic process.

• Availability of new parameter-estimation and sensitivity 

software and transient-calibration techniques. Parameter estimation was used to estimate model hydraulic 

parameters and guide transient model calibration (PEST, 

Doherty, 2006). Model sensitivity to parameters was calculated with UCODE_2005 (Poeter and others, 2005).

• Availability of new MODFLOW numerical solvers 

and parallel-computing capabilities. New numericalsolution software (GMG Package, Wilson and Naff, 

2004; Banta, 2006) available in MODFLOW-2000 

provided improved model convergence for this highly 

nonlinear model. The software Parallel PEST (Doherty, 

2006) was used with multiple networked computers 

to simultaneously simulate various combinations of 

model parameters during model calibration.

Chapter C presents the groundwater flow model conceptualization, construction, calibration, and sensitivity analysis. 

An overview of the groundwater flow model construction and 

calibration is presented in the following paragraphs.
10 Groundwater Availability of the Denver Basin Aquifer System, Colorado

A three-dimensional grid was defined to represent the 

Denver Basin aquifer system in MODFLOW-2000. Vertically, 

the model is discretized into 12 layers explicitly representing 

the six bedrock aquifers, five aquifer confining units, and the 

alluvial aquifer. The explicit representation of all hydrogeologic units is a fully three-dimensional approach, which allows 

simulation of lateral and vertical flow and calculation of storage 

changes in all model layers. The altitude of land surface and 

the base of each model layer were assigned from the surfaces 

developed during data compilation and described in the section “Hydrogeologic Framework” of Chapter A. In map view, 

the model area is divided into one-square-mile grid cells, and 

the model grid is composed of 124 rows by 84 columns with 

a north-south/east-west orientation. Aquifer specific-storage 

and specific-yield values were assigned on the basis of available data, and aquifer hydraulic conductivity was a modelcalibration parameter.

The Denver Basin model simulates steady-state groundwater conditions prior to 1880 and transient groundwater conditions from 1880 through 2003. Predevelopment conditions 

are represented by a steady-state stress periods, and the period 

1880–2003 is discretized into 15 stress periods with lengths 

varying from 2 to 27 years, depending on climatic and pumping conditions. Sources of recharge simulated by the model 

include time-varying precipitation recharge, agricultural irrigation return flow, and urban/suburban recharge in developed 

areas. Streams are simulated using the Streamflow-Routing 

package (SFR2, Niswonger and Prudic, 2005), springs are 

simulated using the Drain package (DRN), and evapotranspiration is simulated using the Evapotranspiration Segments 

package (ETS, Banta, 2000). Pumping is simulated using the 

Well (WEL) package for wells completed in a single aquifer 

and using the Multi-Node Well package (MNW, Halford and 

Hanson, 2002) for wells completed in more than one aquifer. 

Other simulated flows in the model are drain cells to represent 

pre-1900 flowing wells; General-Head Boundary (GHB) cells 

to represent groundwater flow out of the basin in the alluvial 

aquifers of Black Squirrel Creek and the Nussbaum Alluvium; 

and Flow and Head Boundary (FHB, Leake and Lilly, 1997) 

cells to represent groundwater underflow out of the study area 

in the South Platte River alluvial aquifer.

Model calibration was achieved using nonlinearregression methods implemented in PEST (Doherty, 2006) 

in combination with manual adjustment of parameters that 

proved resistant to calibration by automated methods. Model 

fit was evaluated by comparing model-simulated values for 

hydraulic head and streamflow to values observed in the 

field or derived by analysis of field data. For each pair of 

observed values and corresponding model-simulated values, 

a residual was defined by subtracting the simulated value 

from the observed value. Each observation was assigned 

a weight based on an estimate of the standard deviation 

or coefficient of variation associated with the observation. Hydraulic-conductivity and recharge parameters were 

adjusted within a reasonable range of values during model 

calibration until a best fit was obtained between modelsimulated and observed values. The resulting hydraulicconductivity and recharge distributions provide reasonable 

representations of aquifer conditions as compared to pumping-test results for hydraulic conductivity, the conceptual 

understanding of recharge in the basin, and previous groundwater modeling studies.

Evaluation of Modeling Results

Groundwater flow models can contribute substantially 

to understanding of groundwater availability in complex aquifer systems such as the Denver Basin. Models can represent 

multiple hydrologic processes that change through time, and 

they facilitate detailed calculations of groundwater budget and 

water levels that are too voluminous to calculate manually. 

Time-varying water budgets and hydraulic-head distributions calculated by the calibrated Denver Basin model were 

evaluated in detail as measures of groundwater availability 

and are described in the section “Effects of Development on 

Groundwater Availability in the Denver Basin” of Chapter B. 

A sensitivity analysis indicated which model-calibration 

parameters were most important to achieving a successful model calibration and indicates the model is suitable for 

reasonable basin-scale predictions. Model sensitivity calculations using Observation-Prediction (OPR) and ParameterPrediction (PPR) statistics (Hill and Tiedeman, 2007) were 

used to evaluate benefits of additional data collection, and the 

calibrated model was used in a predictive mode to estimate 

the hydrologic system response to two pumping scenarios for 

the period 2004 through 2053. Chapter C describes model 

sensitivity and prediction simulations along with a comparison 

of model results to previous models. The model developed by 

this study is not an end unto itself but rather a necessary tool 

for continued evaluations of basinwide groundwater resources. 

The results provide quantitative estimates of system changes 

through time; however, continued monitoring and updates to 

the model are considered necessary for future assessments of 

groundwater availability.

Conceptual Model of the 

Denver Basin Aquifer System

Following an overview of the Denver Basin hydrogeologic conceptual model, this report section presents the 

physiographic and climatic setting, hydrogeologic framework, 

and predevelopment groundwater budget and flow conditions 

for the aquifer system. The discussion synthesizes compiled 

hydrologic and geologic data with groundwater flow model 

input parameters and results. Chapter B of this Professional 

Paper describes temporal changes to groundwater budgets 

and flow conditions.
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 11

Overview

The Denver Basin aquifer system is composed of 

Tertiary- to Cretaceous-age bedrock sandstones that overlie 

the Cretaceous Pierre Shale (fig. A2) such that the basin 

extent is equivalent to the extent of the Upper Cretaceous 

Fox Hills Sandstone (Robson, 1987). From oldest to youngest, 

the four primary units are the Laramie–Fox Hills aquifer, 

the Arapahoe aquifer, the Denver aquifer, and the Dawson 

aquifer (Romero, 1976; Robson, 1983; Robson, 1987). The 

Dawson and Arapahoe aquifers are further divided into upper 

and lower units in parts of the basin (VanSlyke and others, 

1988a, 1988c) so that six bedrock aquifers are recognized 

for administrative purposes (Colorado Division of Water 

Resources, 1998). The Denver Basin aquifers comprise 

the uppermost layers of the larger structural Denver Basin 

with high-angle dips along the western basin margin and 

low-angle dips in the central and eastern parts of the basin 

(Anderman and Ackman, 1963; Nwangwu, 1977; Robson, 

1987; Robson and others, 1998). About 5,200 ft of lowpermeability Cretaceous Pierre Shale underlie the Laramie–Fox Hills aquifer, forming the base of the Denver Basin 

aquifer system (Nwangwu, 1977; Robson, 1983). The aquifer 

materials are synorogenic sediment deposited east of the 

Rocky Mountain Front Range during the Laramide Orogeny 

and exhibit substantial horizontal and vertical heterogeneity 

in the stratigraphic section (Scott, 1963b; Weimer, 1973; 

Epis and others, 1976; Weimer and LeRoy, 1987; Robson, 

1987; Raynolds, 2002). A detailed discussion of Denver 

Basin stratigraphy is provided in the section “Hydrogeologic 

Framework” of Chapter A.

Groundwater exists primarily under confined conditions 

in the bedrock sandstones, although unconfined conditions 

exist where bedrock aquifers lie near the land surface (fig. A2). 

Quaternary alluvial, colluvial, and eolian deposits overlie the 

Denver Basin bedrock aquifers primarily along present-day 

stream channels (Hunt, 1954; Scott, 1960), and these surficial 

materials form an unconfined alluvial aquifer where saturated 

(Robson, 1989; Robson, 1996). The alluvial aquifer is in 

hydraulic connection with streams draining the area as well as 

underlying bedrock aquifers where present (Robson, 1996). In 

general, regional groundwater flow in all aquifers is away from 

the topographic high and recharge area of the Palmer Divide, 

downgradient to the north toward the discharge area of the 

South Platte River and downgradient to the south toward the 

discharge areas of Monument, Black Squirrel, and Big Sandy 

Creeks (Robson, 1987). Recharge to the bedrock and alluvial 

aquifers is from infiltration of precipitation, lawn-irrigation 

return flow in developed areas, agricultural irrigation return 

flow in agricultural areas, streams, and interaquifer flow. On the 

basis of modeling results, recharge has increased through time 

as a result of irrigation return flow in developed and agricultural 

areas. Natural discharge from bedrock aquifers is to springs, 

streams, alluvial aquifers, and evapotranspiration. Groundwater 

development of the Denver Basin aquifer system has steadily 

increased since the 1950s, resulting in water-level declines 

and storage depletion in the bedrock aquifers as described in 

Chapter B of this Professional Paper.

Physiography

The Denver Basin is located in the Colorado Piedmont of 

the Great Plains physiographic province where rolling hills and 

dissected plains characterize the topography (Trimble, 1980a). 

The topographic high, approximately 8,000 ft, is located in the 

southern one-third of the Denver Basin along the Palmer Divide 

(fig. A3; Trimble, 1980a), which is the drainage divide between 

Figure A2. Conceptual block diagram of the Denver Basin aquifer system for predevelopment conditions.

Water

level

Denver aquifer

Dawson aquifer

Laramie-Fox Hills aquifer

Arapahoe aquifer

Pierre Shale

South Platte River
12 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A3. Land-surface alititude of the Denver Basin aquifer system, Colorado.

Box Elder Creek

Adobe Creek

BadgerCreek

MiddleBijouCreek

East Bi oj uC reek Middle Rush Creek

Mustang Creek

Horse Creek

Steels Fork

Brackett Cr

Black Squirrel Creek

Fountain Creek

Big Sandy Creek

Running

Cree k

Kiowa Creek

Wolf Creek

Comanche Creek

Dr y Creek

Antelope Cr

Bijou Creek

Lost Creek

Sou th Platte R iver

Big Thompson R

Little Thompson R

Cache la PoudreR

Saint Vrain Cr

Bould er Cr

Coal Creek

Big Dr y Cr

Clear Creek

West Sand Creek

LittleDryCr

Dry Cr

First Creek

Sand Creek

Cook Cr

West Plum Cr

Bear Cr

Kettle Cr

Cherry Cr West Kiowa Cr

West Bijou Cr

P ul m Cr

MuddyCreek

Rock Creek

PALMER

DIVIDE

PALMER

DIVIDE

!

!

!

!

! ! !

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

! !

!

!

! !

Boulder

Loveland

Brighton

Fort

Lupton

Lafayette

Golden

Lakewood Denver

Highlands 

Ranch

Aurora

Littleton

Parker

Longmont

Broomfield

Englewood

Kiowa

Morrison

Franktown

Elizabeth

Castle

Rock

Calhan

Monument

Colorado

Springs

Limon

Simla

Deer

Trail

Fort

Morgan

Wiggins

Greeley

Bennett

Boulder

Loveland

Brighton

Fort

Lupton

Lafayette

Golden

Lakewood Denver

Highlands 

Ranch

Aurora

Littleton

Parker

Longmont

Broomfield

Englewood

Kiowa

Morrison

Franktown

Elizabeth

Castle

Rock

Calhan

Monument

Colorado

Springs

Limon

Simla

Deer

Trail

Fort

Morgan

Wiggins

Greeley

Bennett

Barr 

Lake

Milton

Reservoir

Aurora

Reservoir

Cherry Creek

Reservoir

Barr 

Lake

Milton

Reservoir

Chatfield

Reservoir

Aurora

Reservoir

Cherry Creek

Reservoir

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

MORGAN

ARAPAHOE

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

ADAMS

BOULDER

TELLER

DENVER

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

MORGAN

ARAPAHOE

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

ADAMS

BOULDER

TELLER

DENVER

§ ̈¦70

§ ̈¦25

§ ̈¦76

104°

40°

39°

105°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Land-surface altitude, Denver Basin aquifer system extent

 in feet

High: 7,992

Low: 4,318

Stream

Perennial

Ephemeral
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 13

the tributaries of the South Platte River to the north and those 

of the Arkansas River to the south. Surface drainage generally 

is north toward the South Platte River in the northern two-thirds 

of the Denver Basin. The South Platte River flows east out of 

the mountains through Waterton Canyon and into Chatfield 

Reservoir, crosses the western part of the Denver Basin flowing 

north through Denver, and then flows northeast toward Greeley. 

The topographic low of approximately 4,300 ft lies along the 

South Platte River on the northeast edge of the basin (fig. A3). 

Approximately the southern one-third of the basin drains south 

toward the Arkansas River.

Native vegetation over much of the Denver Basin 

consists of grasslands with mixed-grass prairie and low 

shrubs (Keith and Maberry, 1973). Open stands of mountain 

mahogany, Gambel shrub oak, and other rangeland shrubs 

cover areas along the foothills (Keith and Maberry, 1973). At 

the higher elevations near the Palmer Divide, where precipitation is greatest and coarse sandy soils are present, native 

vegetation consists of open stands of Ponderosa pine and 

shrubs mixed with grass meadows (Keith and Maberry, 1973; 

Driscoll, 1975). Phreatophytes such as cottonwood trees and 

willows grow in valley bottom lands and flood plains (Keith 

and Maberry, 1973; Driscoll, 1974, 1975).

Climate

Topography of the Rocky Mountain Front Range 

affects the semiarid climate and weather over the plains in 

the Denver Basin east of the Rocky Mountain front (Robson, 

1989). In general, prevailing westerly winds carry moisture 

from the Pacific Coast toward Colorado. As air rises over 

the mountains of Colorado, it cools and deposits precipitation as rain and snow on the mountains along the Continental 

Divide. This orographic effect on weather also causes a rain 

shadow on the eastern slope of the mountains and the Denver 

Basin, which has a semiarid climate (Scott, 1963a; Robson, 

1989). Warm, moist air masses from the Gulf of Mexico also 

move into Colorado from the south, causing upslope storms 

that may deposit many feet of snow on the plains of eastern 

Colorado (recent blizzards of 2003 and 2006 are examples; 

http://severewx.atmos.uiuc.edu/index.15.html, accessed 

February 17, 2009). Wide variations in weather also are 

characteristic of the Denver Basin (Scott, 1963a; Robson, 

1989). Record high summer temperatures range from 100° to 

107°F, and record low winter temperatures range from –30° to 

–45°F (National Oceanic Atmospheric Administration, 2004). 

Downslope (chinook) winds frequently exceed 50 miles per 

hour (mi/hr) during the winter and have exceeded 135 mi/hr 

along the foothills of the Front Range (Robson, 1989). During 

the summer, orographic thunderstorms develop over the higher 

elevations of the Denver Basin and the Front Range, which 

can produce, on rare occasions, as much as 24 inches of rain in 

local areas in a few hours (Robson, 1989).

Analysis of long-term mean annual precipitation data from 

129 individual weather stations in the vicinity of the Denver 

Basin indicates mean annual precipitation over the basin ranges 

from approximately 11.5 to 20 inches for the period 1931–2003 

(National Oceanic Atmospheric Administration, 2004) (fig. A4). 

About 70 percent of the precipitation falls during the 6-month 

period from April through September, and the remaining 30 percent of precipitation is winter snowfall (Hansen and others, 

1978). The Palmer Divide creates a local orographic effect on 

storms in the Denver Basin, such that precipitation rates are 

greater at higher elevations near the divide (Hansen and others, 

1978; fig. A4). The long-term mean annual precipitation at the 

highest altitudes along the western basin margin and the Palmer 

Divide is about 20 inches per year, whereas the long-term average annual precipitation at lower elevations along the South 

Platte River and in the southernmost part of the basin is about 

11 to 12 inches per year.

Basinwide mean annual precipitation (mean of annual 

precipitation for all active weather stations within a given 

year) ranges from 8.7 to 21.5 inches per year for the period 

1930–2003 with a long-term annual mean of about 15.2 inches 

(fig. A5). The 1930–2003 period of record exhibits several 

periods defined as exhibiting variable to dry conditions (shown 

in brown in fig. A5). Basinwide average precipitation was 

less than the long-term annual mean from 1931 to about 1940 

(during the 1930s drought) and from 1948 to 1956 (during the 

1950s drought; fig. A5). Variable to dry conditions also are 

noted for 1962 through 1964, 1966, 1968, 1970 through 1971, 

and 1974 through 1978 (fig. A5). The period 1979–1999 had 

variable to wet precipitation conditions, which resulted in an 

overall upward trend for basinwide mean annual precipitation 

over the period of record (fig. A5). Variable to dry conditions 

began in 2000 when effects of a recent drought (McCabe and 

others, 2004) become apparent (fig. A5).

Surface-Water Hydrology

The Denver Basin contains perennial and ephemeral 

surface drainages. Streams originating in the Front Range 

enter the west side of the Denver Basin and are generally 

perennial because they receive water primarily from snowmelt 

runoff in the mountains. Streams that originate on the semiarid plains within the Denver Basin are generally ephemeral 

and intermittent because they receive water primarily from 

local precipitation runoff and groundwater discharge from 

the Denver Basin aquifers. Drainage within the Denver Basin 

east of the mountains is radial to the north, east, and south 

away from the drainage divide between the South Platte and 

Arkansas Rivers, known as the Palmer Divide. North of the 

Palmer Divide, streams drain toward the South Platte River, 

and south of the Palmer Divide, streams drain toward the 

Arkansas River (fig. A6). Perennial streams entering the western side of the basin include the main stem of the South Platte 

River and its major tributaries of Plum, Bear, Clear, Coal, and 
14 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A4. Spatial distribution of mean annual precipitation in the Denver Basin aquifer system, 

1931–2003, in inches.

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryCr

Plum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Creek

Steels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

PALMERPALMER DIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

MORGAN

ARAPAHOE

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

ADAMS

BOULDER

TELLER

DENVER

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

MORGAN

ARAPAHOE

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

ADAMS

BOULDER

TELLER

DENVER

Boulder

Loveland

Brighton

Fort

Lupton

Lafayette

Golden

Lakewood Denver

Highlands

Ranch

Aurora

Littleton

Parker

Longmont

Broomfield

Englewood

Kiowa

Morrison

Franktown

Elizabeth Castle

Rock

Calhan

Monument

Colorado

Springs

Limon

Simla

Deer

Trail

Fort

Morgan

Wiggins

Greeley

Bennett

Boulder

Loveland

Brighton

Fort

Lupton

Lafayette

Golden

Lakewood Denver

Highlands

Ranch

Aurora

Littleton

Parker

Longmont

Broomfield

Englewood

Kiowa

Morrison

Franktown

Elizabeth Castle

Rock

Calhan

Monument

Colorado

Springs

Limon

Simla

Deer

Trail

Fort

Morgan

Wiggins

Greeley

Bennett

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

( (

(

(

(

(

(

(

(

(!

!

!

!

!

!

!

!

!

!

! ! !

!

!

! !

!

!

! !

!

!

!

!

!

!

!

!

!

12.4

12.6

15.9

16.1

12.2

14.1

11.4

12.9

17.3

12.2

14.4

16

13.1

14.9

16.7

14.9 12.8

14.5 13

14

16

15.1

17.1 18.6

18.8

19.6

14.7 17.1

17.4

16.1

18.1

17.5

18.8

17

16

10.5

12.1

21.4

15.1

17.1

20.7

17.6

22.9

11.5

18.4

15.9

17.3

17.9

19

24.2 13.7

12.3

18.8

17.1

14.5

15.2

13.9

14.8

13.3 18.6

13.1

16.1

16.7

14.6

15.7

15.9

16.1

13.9

14.8

12.8

11.9

14.8

24

16.1

21.4

18.8

19.3

20.2

18.6

17

17

18.4

14.3

14.7

16.4

16.3

15.4

14.7

15.4

18.3

20.8

24.2

12.4

12.6

15.9

16.1

12.2

14.1

11.4

12.9

17.3

12.2

14.4

16

13.1

14.9

16.7

14.9 12.8

14.5 13

14

16

15.1

17.1 18.6

18.8

19.6

14.7 17.1

17.4

16.1

18.1

17.5

18.8

17

16

10.5

12.1

21.4

15.1

17.1

20.7

17.6

22.9

11.5

18.4

15.9

17.3

17.9

19

24.2 13.7

12.3

18.8

17.1

14.5

15.2

13.9

14.8

13.3 18.6

13.1

16.1

16.7

14.6

15.7

15.9

16.1

13.9

14.8

12.8

11.9

14.8

24

16.1

21.4

18.8

19.3

20.2

18.6

17

17

18.4

14.3

14.7

16.4

16.3

15.4

14.7

15.4

18.3

20.8

24.2

12

12

12 12

12

12

15

14

18

18

19

17

15

14

13

12

16 16

15

13

15

15

15

16

12

12

12 12

12

12

15

14

18

18

19

17

15

14

13

12

16 16

15

13

15

15

15

16

§ ̈¦70

§ ̈¦25

§ ̈¦76

104°

40°

39°

105°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W 

Precipitation data from National Oceanic and Atmopheric Administration, 2004.

EXPLANATION

Mean annual precipitation cokriged Denver Basin aquifer system extent

 with altitude, 1931–2003, in inches

11.5 to 12

12 to 13

13 to 14

14 to 15

15 to 16

16 to 17

17 to 18

18 to 19

19 to 20

Line of equal mean annual precipitation, 

 1931–2003, in inches. Contour interval 1 inch

Stream

Perennial

Ephemeral

Precipitation station and mean annual 

 precipitation, 1931–2003, in inches

16.1

0 10 20 MILES

0 10 20 KILOMETERS
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 15

Boulder Creeks (fig. A6). Ephemeral and intermittent streams 

that originate within the Denver Basin include the South Platte 

River tributaries of Cherry, Running/Box Elder, Lost, Kiowa, 

Comanche, and Bijou Creeks (fig. A6). Ephemeral and intermittent tributaries of the Arkansas River that originate in the 

Denver Basin include Monument, Black Squirrel, Adobe, and 

Big Sandy Creeks (fig. A6).

Surface water interacts with groundwater where streams 

flow across the Denver Basin. Surface-water diversions from 

perennial streams decrease streamflow as streams enter the 

basin, but some perennial flow is generally maintained year 

round in mountain-front streams, and they interact with 

groundwater in the alluvial and bedrock aquifers as they 

cross the Denver Basin. In general, groundwater discharge 

from bedrock and alluvial aquifers maintains perennial flow 

in upstream reaches of streams originating within the Denver 

Basin (fig. A7). Topographic gradients of these streams flatten 

in a downstream direction away from the Palmer Divide, and 

the streams lose water to the extensive and permeable alluvial 

aquifers along their channels (fig. A8); little to no perennial 

surface water flows in downstream reaches of Running/Box 

Elder, Kiowa, Bijou, and Black Squirrel Creeks. In developed 

drainages such as the lower South Platte River and Plum and 

Cherry Creeks, groundwater discharge, reservoir releases, 

wastewater-treatment plant discharge, agricultural irrigationreturn flow, and lawn-irrigation return flow contribute to 

perennial flow.

Streamflow records for eight sites in the Denver Basin 

(fig. A6) were used to estimate base flow from groundwater 

for comparison to model simulations (table C11 discussed in 

the “Calibration Targets” section of Chapter C). The Cherry 

Creek site near Franktown (06712000) is located upstream 

from urban/suburban development, represents natural baseflow conditions, and has the longest period of record of any 

of the Denver Basin surface-water gaging stations. Streamflow 

data and estimated base flow from this site indicate a range 

in streamflow from 0.2 to 1,400 ft3/s and a range in estimated base flow from 0.2 to 62 ft3

/s for 1939–2004 (fig. A9). 

Streamflow and estimated base flow at this site mimic the 

mean annual precipitation values and the wet and dry periods, 

indicating that streamflow at the site responds to variable 

climatic conditions. Decreases in streamflow and base flow 

are observed during the variable to dry periods of 1948–1957, 

1962–1965, 1970–1973, 1974–1979, and 2002–2004 (fig. A9). 

Similarly, there are increases in streamflow and base flow 

during the variable to wet periods of 1958–1961, 1973, and 

1980–2001 (fig. A9).

Figure A5. Temporal distribution of mean annual precipitation in the Denver Basin aquifer system, 1931–2003.

Basinwide annual mean

Basinwide long-term mean

Five-year moving average

Linear trend of annual mean

Variable to dry conditions

Variable to wet conditions

EXPLANATION

1931193319351937193919411943194519471949195119531955195719591961196319651967

19691971197319751977197919811983198519871989199119931995199719992001

2003

Basinwide mean annual precipitation, in inches

Year

5

10

15

20

25
16 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A6. Locations of streams, stream-gaging stations, and extent of alluvial aquifer for the 

Denver Basin aquifer system.

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Creek

Steels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

PALMER DIVIDE

!(

!(

!(

!(

!(

!(

!(

!( !( !(

!(

!(

!(

!(

!( !(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

#

#

#

#

#

#

#

#

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

07103990

06759000

06758200

06720460

06712000

07103990

06759000

06758200

06720460

06712000

Barr 

Lake

Milton 

Reservoir

Chatfield

Reservoir

Aurora

Reservoir

Cherry Creek

Reservoir

Barr 

Lake

Milton 

Reservoir

Chatfield

Reservoir

Aurora

Reservoir

Cherry Creek

Reservoir

104°

40°

39°

105°

Black Squirrel Creek

alluvial aquifer

Nussbaum

alluvial aquifer

Big Sandy Creek

alluvial aquifer

South Platte River

and tributaries

alluvial aquifer

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000 

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W

EXPLANATION

Denver Basin aquifer system extent

Alluvial aquifer extent

Stream

Perennial

Ephemeral

# Stream-gaging station used for model 

 calibration and site identification number
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 17

Figure A7. Photograph of Cherry Creek in Castlewood Canyon 

State Park near Franktown.

Figure A8. Photograph looking north along Cherry Creek 

downstream from Castlewood Canyon State Park where stream 

and topographic gradients flatten.

Hydrogeologic Framework

The hydrogeologic framework of an aquifer system 

describes the extent, thickness, depth, and layering of the 

encompassed aquifers. Numerous workers have mapped and 

conceptualized the hydrogeologic framework of the Denver 

Basin aquifer system beginning with Emmons and others 

(1896) as discussed in the “Previous Work” section of this 

chapter. This “Hydrogeologic Framework” section provides 

an overview of the structural geology and geologic history 

compiled from previous studies followed by detailed descriptions of the geometry and characteristics of the Denver Basin 

aquifers as developed by this study and used as input to the 

groundwater flow model.

The structural Denver Basin is a bowl-shaped, doubleplunging syncline located at the base of the Rocky Mountain 

Front Range in eastern Colorado (Anderman and Ackman, 

1963). The basin axis is oriented approximately north-south 

and parallel to the mountain front of the Front Range (Soister, 

1978a). The basin axis is located near the western margin of 

the asymmetric basin, so that sedimentary rocks on the western basin margin dip steeply (as much as 70°) off the Front 

Range toward the basin axis (Anderman and Anderson, 1963; 

Nwangwu, 1977; Robson, 1987), and sedimentary rocks on 

the eastern margin dip gently toward the axis (Tweto, 1980). 

Away from the mountain front, the dip of sedimentary rocks 

is generally less than 5° toward the basin axis (Nwangwu, 

1977; Tweto, 1980). The eastern flank of the Front Range is an 

east-dipping monocline formed during the Laramide Orogeny 

by crustal shortening beneath the range (Nesse, 2006). South 

of Boulder and along the western edge of the Denver Basin, 

east-directed (west-dipping) thrust faults form the structural 

boundary between the mountains and the Denver Basin 

(Robson and others, 1998; Nesse, 2006). In the northwest 

part of the Denver Basin, near Boulder County, numerous 

northeast-trending faults have offset the Fox Hills Sandstone 

and Laramie Formation (Weimer, 1973; Colton and Lowrie, 

1973; Davis and Weimer, 1976; Tweto, 1979; Schneider, 

1980; Kittleson, 2004; Sterne, 2006). The faulting affects only 

a small area of the Denver Basin; this and previous studies assume that the faulting has no substantial effect on the 

Upper Cretaceous to Tertiary aquifer system at a regional scale 

(Robson, 1987; Robson and others, 1998).

Sedimentary rocks forming the structural Denver Basin 

range from Pennsylvanian to Tertiary in age (Anderman 

and Ackman, 1963; Scott, 1963b). Sandstone of the 

Pennsylvanian- to Permian-age Fountain Formation crops 

out along the western basin margin (DeVoto, 1980), forming prominent hogbacks (Scott, 1963b; Weimer and LeRoy, 

1987) at well-known locations such as the Flatirons near 

Boulder, Red Rocks Amphitheatre near Morrison (fig. A10), 

and Garden of the Gods Park near Colorado Springs. The 

Early Cretaceous Dakota Group, which represents transgression of the Cretaceous sea in eastern Colorado (Berman and 

others, 1980), contains sandstones that also form a prominent 

hogback parallel to the mountain front (the Dakota Hogback) 

along the western basin margin (Scott, 1963b, fig. A10). Overlying the Dakota Group are Late Cretaceous marine shales and 

limestones with minor amounts of sandstone, deposited by the 

Cretaceous sea (Scott, 1963b; Berman and others, 1980). The 

Late Cretaceous Pierre Shale, a marine shale about 5,200 ft 

thick, is the uppermost Cretaceous shale in the Denver Basin 

(Scott, 1963b; Weimer, 1973) and is considered the base of 

the Denver Basin aquifer system because of its low permeability and great thickness (Romero, 1976). Late Cretaceousto Tertiary-age sandstone aquifers and intervening claystone 

confining units overlying the Pierre Shale comprise the Denver 

Basin aquifer system (Scott, 1963b, Weimer, 1973; Romero, 

1976). Alluvial deposits along tributaries to the South Platte 

River north of the Palmer Divide and along tributaries to the 

Arkansas River south of the Palmer Divide form unconfined 

alluvial aquifers where saturated (Robson, 1989, 1996).
18 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Bedrock Aquifers and Confining Units

Colorado State legislation administrating the Denver 

Basin recognizes and defines four primary bedrock aquifers 

(Romero, 1976; Robson, 1987; Graham and VanSlyke, 2004). 

From oldest to youngest (bottom to top), the four primary units 

are the Laramie–Fox Hills aquifer, the Arapahoe aquifer, the 

Denver aquifer, and the Dawson aquifer. In parts of the basin, 

confining units further divide the Arapahoe and Dawson aquifers into upper and lower units (table A3). Thus, the six Denver 

Basin bedrock aquifers and five intervening confining units used 

to develop geologic layers for the groundwater flow model, 

from oldest to youngest, are the Laramie–Fox Hills aquifer, 

Laramie confining unit, lower Arapahoe aquifer, Arapahoe 

confining unit, upper Arapahoe aquifer, lower Denver confining unit, Denver aquifer, upper Denver confining unit, lower 

Dawson aquifer, Dawson confining unit, and upper Dawson 

aquifer (table A3).

Structural geology, layered stratigraphy, and topography control the areas of outcrop, and thus, areas of potential 

recharge and discharge for the Denver Basin aquifers and 

confining units. The bedrock aquifers and confining units crop 

out in a ring pattern because of the synclinal structure of the 

Denver Basin, with the oldest rocks of the Laramie–Fox Hills 

aquifer cropping out around the outer margins of the basin and 

the youngest rocks of the Dawson aquifer cropping out in the 

center of the basin (fig. A11). The alluvial aquifer is the uppermost stratigraphic layer in the basin and overlies parts of all 

bedrock units. All (100 percent) of the simulated alluvial aquifer 

is exposed at the land surface, and the alluvial aquifer comprises 

about 30 percent of the total basin area (table A4). The upper 

Dawson aquifer is the uppermost bedrock unit of the basin, and 

only a small part of the simulated unit (2 percent) is overlain by 

the alluvial aquifer, such that 98 percent of the simulated upper 

Figure A9. Streamflow, estimated base flow, and 180-day moving average of estimated base flow for site Cherry Creek at Franktown.

Figure A10. Photograph of hogbacks along the western margin 

of the Denver Basin. View looking south.

100

1,000

10,000

0

1

10

Date

Flow, in cubic feet per second 1/1/1939

1/1/19441/1/19491/1/19541/1/19591/1/19641/1/19691/1/19741/1/19791/1/19841/1/19891/1/1994

1/1/19991/1/2004

Total streamflow

Estimated base flow

180-day moving average base flow

Variable to dry conditions

Variable to wet conditions

EXPLANATION
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 19

Table A3. Geology, hydrogeology, and water-use characteristics of the Denver Basin aquifers, Colorado (adapted from Kirkham and Ladwig [1979]; Robson [1987]; Nichols and Fleming [2002]; Raynolds [2002]; Lapey-Woodard and others [2002]; Thorson, [2003]).

Age Stratigraphic unit Hydrogeologic unit Model layer

Stratigraphic

nomenclature

of Raynolds

(2002)

Stratigraphic

nomenclature

in Black Forest 

quadrangle

(Thorson, 2003)

Lithologic

description

Range of

thickness for

entire section

(feet)

Hydrogeologic

description

Quaternary Alluvial, flood plain, terrace, 

colluvial, and eolian sand, gravel, and clay deposits

Alluvial aquifer 

where saturated

1 Unconsolidated sand and 

gravel with clay lenses

0 to 175 Productive unconfined alluvial 

aquifer where saturated.

Eocene Dawson Arkose upper Dawson 

aquifer

2 D2 sequence Dawson Formation

facies unit 5

Arkosic fluvial sandstone 

and conglomerate with interbedded claystone

200 to 840 Productive unconfined to 

confined aquifer.

Paleocene Dawson Arkose Dawson

confining unit

3 Paleosol Paleosol Claystone 1 to 350 Claystone confining unit in northern 

part of Dawson extent.

Paleocene Dawson Arkose lower Dawson 

aquifer

4 D1 sequence Dawson Formation

facies unit 4

Mixed arkosic and andesitic 

fluvial sandstone with interbedded claystone, lignite, and volcanics

100 to 570

where

undifferentiated

Productive confined aquifer.

Cretaceous/Paleocene

(K/T) boundary

Denver Formation upper Denver 

confining unit

5 Dawson Formation

facies unit 3/2

Predominantly claystone with 

interbedded sandstone, lignite, and volcanics

1 to 150 Fine-grained confining unit in 

upper Denver Formation.

Late Cretaceous Denver Formation Denver aquifer 6 Dawson Formation

facies unit 1

Mixed arkosic and andesitic 

fluvial sandstone 

280 to 1,100 Confined to unconfined aquifer. 

Lower hydraulic conductivity and less production than Arapahoe aquifers.

Denver Formation lower Denver 

confining unit

7 Predominantly claystone with 

andesitic fluvial sandstone 

5 to 150 Fine-grained confining unit at base 

of Denver Formation. 

Arapahoe Formation upper

Arapahoe

aquifer

8 Lower part of 

Dawson Formation

Arkosic fluvial sandstone 

with interbedded claystone

50 to 300 Productive confined aquifer above 

Arapahoe confining unit in northern one-third of basin.

Arapahoe Formation Arapahoe

confining

unit

9 Predominantly claystone 50 to 180 Claystone confining unit in 

northern one-third of basin.

Arapahoe Formation Lower 

Arapahoe

aquifer

10 D1 sequence/ 

Wildcat

Mountain fan

Alluvial fan conglomerate and 

sandstone with interbedded claystone

200 to 600

where

undifferentiated

Productive confined aquifer. 

Greatest thickness and hydraulic conductivity in west-central part of basin.

Laramie Formation Laramie 

confining unit

11 Laramie

Formation

Laramie Formation Gray to black shale, coal, 

siltstone, and sandstone

100 to 500 Thick confining unit limits 

downward movement of water from Arapahoe to Laramie–Fox Hills aquifer.

Laramie Formation Laramie–Fox 

Hills aquifer

12 Laramie

Formation

Laramie Formation Clean, well-sorted, nearlywhite, poorly consolidated 

fluvial delta-front sandstone

100 to 500 Productive confined to unconfined 

aquifer composed of sandstones from lower Laramie Formation and Fox Hills Sandstone. Fox Hills Sandstone Fox Hills

Sandstone

Fox Hills Sandstone Yellow brown marine deltafront and beach sandstone

Pierre Shale Pierre Shale 

confining unit

Not

applicable

Pierre Shale Pierre Shale Dark to light gray marine 

shale

5,200 Low permeability base of Denver 

Basin aquifer system.
20 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A11. Location of springs and extent of Denver Basin aquifers and confining units.

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryCr

Plum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

PALMER

DIVIDEDIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Barr 

Lake

Milton 

Reservoir

Chatfield

Reservoir

Aurora

Reservoir

Cherry Creek

Reservoir

Barr 

Lake

Milton 

Reservoir

Chatfield

Reservoir

Aurora

Reservoir

Cherry Creek

Reservoir

§ ̈¦70

§ ̈¦25

§ ̈¦76

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Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

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Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

104°

40°

39°

105°

Base from U.S. Geological Survey digital data, 2009, 1:100,000 

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Alluvial aquifer Denver Basin aquifer system extent

Upper Dawson aquifer

Lower Dawson aquifer

Denver aquifer

Upper Arapahoe aquifer

Lower Arapahoe aquifer

Laramie–Fox Hills aquifer

Confining unit

Stream

Perennial

Ephemeral

Mapped spring
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 21

Dawson aquifer crops out at the land surface (table A4). The 

Denver aquifer underlies much of the topographically flatter 

parts of the basin and thus exhibits the greatest outcrop area of 

the bedrock aquifers (about 1,310 mi2; table A4). About 40 percent of the Denver aquifer crops out at the land surface, and 

its outcrop area represents the greatest proportion of bedrock 

outcrop in the basin (about 20 percent of the total basin area, 

table A4).

The synorogenic sedimentary sequence above the Pierre 

Shale records regression of the Interior Cretaceous Seaway in 

the Late Cretaceous followed by episodic uplift of the Rocky 

Mountain Front Range during the Laramide Orogeny (Emmons 

and others, 1896; Scott, 1963b, Weimer, 1973; Tweto, 1980; 

Raynolds, 2002). As the Cretaceous Seaway gradually retreated 

from what is now central Colorado, the Fox Hills Sandstone 

marine beach sand was deposited as a series of shingles interbedded with the upper Pierre Shale (Weimer, 1973; Raynolds, 

2002). As the seaway regressed farther, the Cretaceous Laramie 

Formation was deposited in a backswamp and deltaic environment dominated by overbank claystones and coal with interbedded fluvial channel sandstones (Weimer, 1973; Raynolds, 2002). 

The Laramie–Fox Hills aquifer includes both the marine Fox 

Hills Sandstone and nonmarine sandstones of the lower Laramie 

Formation (Romero, 1976), and these sandstones are relatively 

continuous across the basin. The upper Laramie Formation forms 

an overlying confining unit (Romero, 1976).

Above the Laramie Formation confining unit are the 

Arapahoe, Denver, and Dawson aquifers. Because of depositional 

facies changes across the basin and the resulting heterogeneity, 

previous workers have variously defined stratigraphic nomenclature for these sediments (see for example, Raynolds (2002) 

or Thorson (2003) for a synopsis of stratigraphic nomenclature). 

Raynolds (2002) describes a nomenclature reflective of depositional history for the Arapahoe, Denver, and Dawson aquifers 

where two pulses of synorogenic sedimentation are recognized. 

The lower (D1) sediment sequence, composed of the Arapahoe, 

Denver, and lower Dawson aquifers, represents the first pulse of 

sedimentation (Raynolds and Johnson, 2003). The D1 sequence 

includes basal conglomerates of the Arapahoe Formation near 

the mountain front, such as the Wildcat Mountain fan, which are 

alluvial fan deposits shed off the rising Front Range (Raynolds, 

2004). Farther east, away from the mountain front, the D1 

sequence contains claystone, coal, and volcanic ash deposited 

in terrestrial backswamp environments of the Denver Formation 

(Kirkham and Ladwig, 1979). The appearance of andesitic 

volcanic clasts and volcanic rocks within the sediments traditionally defined the mapped Denver Formation, although questions 

remain regarding the lateral continuity of the volcanic input to 

the D1 sequence (Thorson, 2003). The upper (D2) sedimentary 

sequence is equivalent to the upper Dawson aquifer and represents a separate pulse of sedimentation (Raynolds and Johnson, 

2003). The Pikes Peak area southwest of the Denver Basin is considered the source area for the arkosic sand in the D2 sequence 

(Raynolds, 2002), although Thorson (2003) notes that there may 

be multiple sources for the D2 sediments. The Dawson paleosol 

(Soister and Tschudy, 1978) represents an erosional unconformity 

between the D1 and D2 sequences (Raynolds, 2002) and generally correlates to the Dawson confining unit. The most recent 

geologic mapping by the CGS (for example, Thorson, 2003) uses 

an older nomenclature (Richardson, 1915; Scott, 1963b), which 

uses the term Dawson Formation to describe the entire D1 and 

D2 interval (Arapahoe, Denver, and Dawson aquifers). Depositional facies within the upper Dawson sediments are used as the 

basis for ongoing mapping rather than differentiating between 

Arapahoe, Denver, and Dawson Formations (Thorson, 2003; 

Thorson, 2007; Peter Barkmann, Colorado Geological Survey, 

oral commun., 2009).

Laramie–Fox Hills Aquifer

The Laramie–Fox Hills aquifer is the deepest and most 

extensive of the bedrock aquifers. It underlies an area of 

approximately 6,900 mi2 and the geographic outline of its base 

defines the lateral extent of the Denver Basin aquifer system 

(fig. A12). The Laramie–Fox Hills aquifer is composed of 

poorly to moderately consolidated sandstone units of the Upper 

Cretaceous Fox Hills Sandstone and the lower Laramie Formation (Romero, 1976; Schneider, 1980; Robson, Wacinski, and 

Table A4. Summary of simulated aquifer areas.

Aquifer or confining

unit name

Aquifer or confining 

unit abbreviation

Model

layer

Simulated

outcrop area

(square miles)

Simulated

layer area

(square miles)

Percentage

of layer that

crops out

Percentage

of simulated outcrop 

area within

basin modeled area

Alluvium QAL 1 2,212 2,212 100.0 31.3

Upper Dawson aquifer UTDW 2 696 710 98.0 9.9

Dawson confining unit TDWC 3 38 749 5.1 0.5

Lower Dawson aquifer LTDW 4 558 1,350 41.3 7.9

Denver upper confining unit TKDUC 5 92 1,447 6.4 1.3

Denver aquifer TKD 6 1,310 3,246 40.4 18.6

Denver lower confining unit TKDLC 7 158 3,490 4.5 2.2

Upper Arapahoe aquifer UKA 8 86 1,136 7.6 1.2

Arapahoe confining unit KAC 9 45 1,198 3.8 0.6

Lower Arapahoe confining unit LKA 10 538 4,668 11.5 7.6

Laramie confining unit KLC 11 733 5,841 12.5 10.4

Laramie–Fox Hills aquifer KLF 12 592 6,966 8.5 8.4

Basin modeled area 7,060
22 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A12. Altitude of the base and total thickness of the Laramie–Fox Hills aquifer.

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

4,500

4,700

4,900

5,500

5,100

4,100

4,300

5,300

3,900

5,700

3,500

3,700

5,900

4,500

4,700

4,900

5,500

5,100

4,100

4,300

5,300

3,900

5,700

3,500

3,700

5,900

PALMER

DIVIDEDIVIDE

104°

40°

39°

105°

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000 

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W

EXPLANATION

Total thickness of Laramie–Fox Hills 

 aquifer, in feet

100 to 200

200 to 300

300 to 400

400 to 500

Laramie–Fox Hills aquifer extent

Line of equal base altitude, in feet.

 Contour interval 100 feet

Base-altitude data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 23

others, 1981; Robson, 1987). The Fox Hills Sandstone is a 

marine beach and delta-front sand deposited during regression 

of the Cretaceous sea through Colorado (Scott, 1963b; Weimer, 

1973). The Fox Hills Sandstone generally is 150–200 ft thick, 

a dusky yellow-brown in color, and contains marine fossils (Scott, 1963b; Robson, 1987). Sandstones of the lower 

Laramie Formation were deposited in freshwater delta-front 

fluvial channels subsequent to the Cretaceous sea regression 

(Weimer, 1973). The Laramie Formation sandstones generally 

are composed of clean, well-sorted, very fine- to mediumgrained sandstones with minor amounts of silt and clay and are 

nearly white (Scott, 1963b). A 10- to 20-ft-thick shale bed may 

separate the Laramie sandstones into two units, and a shale bed 

5 to 20 ft thick generally separates the Laramie and Fox Hills 

sandstones (Robson, 1987). Local sandstones also are present 

in the upper 100 to 200 ft of the Pierre Shale and represent a 

transitional depositional environment between the Cretaceous 

marine shale and Fox Hills sandstones (Scott, 1963b; Kiteley, 

1977; Nwangwu, 1977). Where present, sandstones in the upper 

Pierre Shale are considered part of the Laramie–Fox Hills aquifer (Robson, 1987).

The base-altitude configuration of the Laramie–Fox 

Hills aquifer shows the bowl shape and dipping margins of the 

Denver Basin geologic structure (fig. A12). The lowest part 

of the Laramie–Fox Hills aquifer base, and therefore also the 

lowest part of the Denver Basin bedrock aquifer system, is in 

Douglas County just north of Parker, Colorado, at an altitude of 

approximately 3,410 ft. Depth to the base of the Laramie–Fox 

Hills aquifer near the structural center of the basin is approximately 2,200 to 2,300 ft below land surface, and the total thickness of the aquifer system reaches a maximum of approximately 

3,200 ft beneath the topographic high of the Palmer Divide. 

Total thickness of the Laramie–Fox Hills aquifer ranges from 

0 to about 500 ft, and two distinct northwest- to southeasttrending areas of greatest aquifer thickness (300 to 400 ft) are 

noted (fig. A12). One area having aquifer thickness greater than 

300 ft trends northwest to southeast near the center of the basin, 

and a second area having aquifer thickness greater than 400 ft 

is present in the northeast part of the basin. The silt-plus-sand 

thickness of the Laramie–Fox Hills aquifer ranges from about 

100 to 250 ft (fig. A13). Areas having the greatest silt-plus-sand 

thickness (fig. A13) and the areas simulated with the greatest 

percent silt-plus-sand (fig. A14) coincide with the northwest- to 

southeast-trending area near the center of the basin having the 

greatest total aquifer thickness (fig. A12).

Laramie Confining Unit

The Laramie confining unit overlies the Laramie–Fox 

Hills aquifer and consists of the Cretaceous-age upper Laramie 

Formation, which is composed of gray to black shale, coal 

seams, and minor amounts of siltstone and sandstone (Robson, 

1987). The lowermost coal seams of the Laramie Formation 

occur just above the Laramie sandstones of the Laramie–Fox 

Hills aquifer and form the upper boundary of the aquifer unit 

(Murray, 1980; Robson, 1987). The subbituminous to lignitic 

coal seams are as much as 10 ft in thickness and have been 

extensively mined along the northwest and southwest margins of the basin (Colton and Lowrie, 1973; Kirkham, 1978; 

Kirkham and Ladwig, 1979).

The base-altitude configuration of the Laramie confining 

unit (fig. A15) is similar to that of the underlying Laramie–Fox 

Hills aquifer (fig. A12) and reflects the bowl shape and dipping 

margins of the Denver Basin. The lowest part of the Laramie 

confining unit is in Douglas County just north of Parker at an 

altitude of approximately 3,700 ft. The Laramie confining unit 

forms a wedge-shaped layer that is thickest (as much as 700 ft) 

along the mountain front and thins southeastward to less than 

100 ft (fig. A15).

Arapahoe Aquifers and Confining Unit

The Arapahoe aquifer consists of a 400- to 600-ft-thick 

sequence of interbedded conglomerate, sandstone, siltstone, 

and shale (Romero, 1976; Robson, Romero, and Zawistowski, 

1981; Robson, 1987). In the northern one-third of the basin, 

shale is more prevalent, and the Arapahoe is divided into the 

upper Arapahoe aquifer and the lower Arapahoe aquifer with 

an intervening shale that forms the Arapahoe confining unit 

(Robson, 1987). The upper and lower Arapahoe aquifers each 

generally consist of 150 to 200 ft of sandstone and siltstone with 

interbedded shale. Conglomerate and coarse-grained sandstone 

common in the lower Arapahoe aquifer are feldspar- and quartzrich, reflecting an erosional source from nearby Front Range 

Precambrian and Paleozoic rocks and deposition at the mountain 

front during the Laramide Orogeny (Scott, 1963b; Weimer, 1973; 

Trimble, 1980a; Tweto, 1980, Raynolds, 2002).

The base-altitude configuration of the lower Arapahoe 

aquifer (fig. A16) has a bowl-shaped form similar to the bases 

of the underlying Laramie confining unit and Laramie–Fox Hills 

aquifer. The altitude of the base of the lower Arapahoe aquifer 

ranges from 6,200 ft along the southwest basin margin to about 

4,100 ft at the basin center, just north of Parker, Colorado 

(fig. A16). Depth to the base of the lower Arapahoe aquifer near 

the structural center of the basin is approximately 1,700 ft below 

land surface, and the maximum depth to the base of the aquifer 

is approximately 2,600 ft below land surface near the Palmer 

Divide. Total thickness of the Arapahoe aquifer (fig. A16) represents the entire thickness of the Arapahoe aquifers and confining 

unit and ranges from less than 100 to as much as 745 ft with a 

mean of about 475 ft based on mapping by this study.

The base altitude of the Arapahoe confining unit (fig. A17) 

ranges from 5,400 ft along the northwest edge to about 4,400 ft 

on the southern edge of the confining unit, and the contours 

indicate a structural low toward the south and the center of the 

Denver Basin. Thickness of the Arapahoe confining unit ranges 

from 0 up to about 250 ft with a mean of about 90 ft. The greatest 

thickness of the Arapahoe confining unit is near the center of the 

confining-unit extent in southwest Adams County. The base of the 

upper Arapahoe aquifer (fig. A18) is similar in shape and altitude 

to that of the Arapahoe confining unit and ranges in altitude from 

more than 5,400 ft along the northwest edge to about 4,400 ft on 

the southern edge of the aquifer. Thickness of the upper Arapahoe 

aquifer ranges from 0 to about 350 ft with a mean of about 170 ft.
24 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A13. Silt-plus-sand thickness of the Laramie–Fox Hills aquifer.

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

150

200

100

PALMER

DIVIDEDIVIDE

105° 104°

40°

39°

0 6 12 MILES

0 6 12 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000 

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W

EXPLANATION

Silt-plus-sand thickness, in feet Laramie–Fox Hills aquifer extent

Line of equal silt-plus-sand thickness, 

 in feet. Contour interval 50 feet

Silt-plus-sand thickness data point

93 to 100

100 to 125 200 to 225

225 to 250

175 to 200

125 to 150

150 to 175

!
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 25

Figure A14. Simulated silt-plus-sand fraction of the Laramie–Fox Hills aquifer.

DIVIDE

PALMER

DIVIDE

PALMER

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

§ ̈¦70

§ ̈¦25

§ ̈¦76

Limon

Simla

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Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

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Morrison

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Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

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105° 104°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000 

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W

EXPLANATION

Simulated silt-plus-sand fraction of 

 total thickness

0.15 to 0.2

0.2 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.6

0.6 to 0.7

0.7 to 0.8

0.8 to 0.94

Laramie–Fox Hills aquifer 

 simulated extent (model 

 layer 12)
26 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A15. Altitude of the base and total thickness of the Laramie confining unit.

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PALMER

DIVIDE

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

4,900

4,700

4,800

4,600

5,000

5,100

5,300

5,200

5,400

5,500

4,500

5,600

4,400

4,300

4,200

5,800

4,100

5,700

3,900

4,000

3,800

5,900

4,900

4,700

4,800

4,600

5,000

5,100

5,300

5,200

5,400

5,500

4,500

5,600

4,400

4,300

4,200

5,800

4,100

5,700

3,900

4,000

3,800

5,900

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000 

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27' N and 37°45' N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Total thickness of Laramie confining 

 unit beneath the lower Arapahoe 

 aquifer, in feet

0 to 100

100 to 200

200 to 300

300 to 400

400 to 500

500 to 600

600 to 700

Lower Arapahoe aquifer extent

Laramie–Fox Hills aquifer extent

Laramie confining unit extent

Line of equal base altitude, in feet.

 Contour interval 100 feet ! Base-altitude data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 27

Figure A16. Altitude of the base of the lower Arapahoe aquifer and total thickness of the 

combined Arapahoe aquifers and confining unit.

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

PALMER

DIVIDEDIVIDE

§ ̈¦70

§ ̈¦25

§ ̈¦76

4,800

5,200

5,100

5,300

5,500

5,400

5,600

5,000

5,700

4,900

5,800

4,700

5,900

4,600

4,500

4,300

4,400

4,200

6,000

6,100

4,100

4,800

5,200

5,100

5,300

5,500

5,400

5,600

5,000

5,700

4,900

5,800

4,700

5,900

4,600

4,500

4,300

4,400

4,200

6,000

6,100

4,100

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Denver lower confining unit extent

Line of equal base altitude, in feet.

 Contour interval 100 feet ! Base-altitude data point

Total thickness of the Arapahoe aquifers

 and confining unit beneath the Denver 

 lower confining unit, in feet

0 to 100

100 to 200

200 to 300

300 to 400

400 to 500

500 to 600

600 to 700

700 to 745
28 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A17. Altitude of the base and thickness of the Arapahoe confining unit.

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Aurora

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4,900

4,700

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4,600

4,500

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4,600

4,500

5,100

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105° 104°30'

40°

39°30'

0 5 10 MILES

0 5 10 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Total thickness of Arapahoe confining 

 unit, in feet

0 to 50

50 to 100

100 to 150

150 to 200

200 to 250

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Arapahoe confining unit extent

Line of equal base altitude, in feet.

 Contour interval 100 feet

Base-altitude data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 29

Figure A18. Altitude of the base and thickness of the upper Arapahoe aquifer.

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4,800

5,000

4,700

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4,500

4,400

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4,700

5,100

4,600

4,500

4,400

105° 104°30'

40°

39°30'

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Total thickness of upper 

 Arapahoe aquifer, in feet

0 to 100

100 to 200

200 to 300

Line of equal base altitude, in feet.

 Contour interval 100 feet

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Upper Arapahoe aquifer extent

Base-altitude data point
30 Groundwater Availability of the Denver Basin Aquifer System, Colorado

The silt-plus-sand thickness of the lower Arapahoe 

aquifer ranges from about 8 to as much as 490 ft with a mean 

of about 175 ft (fig. A19). The greatest silt-plus-sand thickness for the lower Arapahoe aquifer is along the western 

basin margin in Douglas and El Paso Counties and coincides 

with simulated areas of greatest percent silt-plus-sand (up to 

about 85 percent silt-plus-sand; fig. A20). The area of greatest silt-plus-sand thickness and greatest percent silt-plussand corresponds with the location of the Wildcat Mountain 

alluvial fan reported by Raynolds (2004). The lower Arapahoe 

sandstones of the Wildcat Mountain fan were deposited at the 

mountain front as distributary alluvial fans during the Late 

Cretaceous (Raynolds, 2004). The coarse-grained sandstones 

of the lower Arapahoe aquifer form a transmissive bedrock 

aquifer in Douglas and El Paso Counties that is heavily used 

for municipal and domestic water supply. The silt-plus-sand 

thickness for the upper Arapahoe aquifer ranges from about 

5 to about 250 ft with a mean of about 90 ft (fig. A21), and 

the percent silt-plus-sand ranges from about 16 to 80 percent 

(fig. A22). There is no discernible alluvial-fan pattern in the 

upper Arapahoe aquifer.

Denver Aquifer and Confining Units

The Denver aquifer and confining units consist of 

a 600- to 1,200-ft-thick sequence of Late Cretaceous- to 

Tertiary-age interbedded shale, claystone, siltstone, sandstone, 

coal, and volcanic rocks (Romero, 1976; Robson, Wacinski, 

and others 1981; Robson, 1987). The depositional environment during Denver Formation sedimentation consisted of 

continued alluvial fan deposition near the mountain front, 

where sandstone beds are more prevalent (Crifasi, 1992; 

Kirkham and Ladwig, 1979), with overbank and swamp 

deposition of finer grained sediments and coal farther from 

the mountain front (Kirkham and Ladwig, 1979). The prominent Wildcat Mountain alluvial fan mapped in the lower 

Arapahoe aquifer (lower D1 sequence; Raynolds, 2004) also 

is present in the Denver and lower Dawson portions of the 

sequence as mapped by this study. Because volcanic activity 

increased in the Rocky Mountain Front Range during deposition of the Denver Formation, the sequence contains andesitic 

sandstones and volcanic ash layers (Epis and others, 1976; 

Kirkham and Ladwig, 1979; Tweto, 1980; Robson, Wacinski, 

and others; 1981; Thorson, 2007). The Denver Formation 

commonly is distinguished from the underlying Arapahoe 

Formation and overlying Dawson Arkose by its olive, greengray, brown, and tan sediment colors, which are caused by 

the presence of iron-rich sediment derived from erosion of 

basalts and andesitic lava (Weimer, 1973; Robson, Wacinski, 

and others, 1981; Robson, 1987). Some researchers define 

the Denver Formation by the appearance of volcanic debris 

in the stratigraphic section (Weimer, 1973). The Cretaceous/

Tertiary boundary is in the upper part of the Denver aquifer 

sequence and is closely established by vertebrate fossils 

(Tweto, 1980) and pollen (Barclay and others, 2003; Nichols 

and Fleming, 2002).

The Denver aquifer is, in general, finer grained than the 

underlying Arapahoe aquifer sequence and is composed of isolated channel sandstones contained in extensive fine-grained 

deposits (Robson, Wacinski, and others, 1981; Robson, 1987; 

Raynolds, 2004). Because of the difficulty correlating individual sandstones between wells even on a local scale, it 

also is difficult to map the Denver aquifer on a regional scale 

(Raynolds, 2002, 2004). For administrative purposes, the 

Denver stratigraphic interval is divided into a lower confining unit (Denver lower confining unit), an aquifer (Denver 

aquifer), and an upper confining unit (Denver upper confining 

unit), where the confining units contain a larger proportion of 

fine-grained sediments than the aquifer. Despite the limitations 

of sandstone correlations within the Denver interval, the basealtitude maps of the Denver aquifer and confining units depict 

the regional structure of the Denver Basin, similar to other 

aquifer base-altitude maps, and provide reasonable representations of layering within the Denver aquifer and confining units 

(figs. A23–A25). The base altitudes of the Denver aquifer and 

confining units range from about 4,600 to 6,300 ft, and the 

maximum depth of the base is about 2,100 ft below land surface near the Palmer Divide (figs. A23–A25). Total thickness 

of the Denver lower confining unit ranges from 0 to as much 

as 300 ft with a mean of about 50 ft (fig. A23), and total thickness of the Denver aquifer ranges from about 20 to 1,055 ft 

with a mean of about 550 ft (fig. A24). The Denver upper confining unit has a smaller extent than that of the Denver aquifer 

and occurs at higher altitudes. The Denver upper confining 

unit base altitude ranges from about 5,400 ft near the center of 

the basin to 6,800 ft on the southern edge of the confining unit 

(fig. A25). Total thickness of the Denver upper confining unit 

ranges from 0 to as much as 200 ft, with a mean of about 50 ft.

Silt-plus-sand thickness for the Denver aquifer ranges 

from about 5 to about 665 ft with a mean of about 190 ft 

(fig. A26). The area with the greatest silt-plus-sand thickness 

(400 to 600 ft) is along the western basin margin in southern 

Douglas County (fig. A26) and corresponds with an area where 

40 to 72 percent of the total thickness is silt-plus-sand (fig. A27) 

as well as the location of the Wildcat Mountain fan in the D1 

synorogenic deposition sequence (Raynolds, 2004). These siltplus-sand thickness results indicate that the lower Arapahoe and 

Denver aquifers, as administratively defined by the CDWR and 

mapped herein, contain the lower and upper parts, respectively, 

of the Wildcat Mountain Fan. The Denver lower confining unit 

is thin (generally less than 50 ft thick) where Denver aquifer 

silt-plus-sand thickness is greatest, indicating a potential for 

hydraulic connection between the Denver aquifer and the lower 

Arapahoe aquifer especially on the west-central margin of the 

basin. The Denver aquifer contains less than 40 percent sand 

over much of its extent (Robson, 1987; fig. A27).
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 31

Figure A19. Silt-plus-sand thickness of the lower Arapahoe aquifer.

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EL PASO

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LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦25

§ ̈¦76

§ ̈¦70

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105° 104°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Silt-plus-sand thickness of lower 

 Arapahoe aquifer, in feet

8 to 100

100 to 150

150 to 200

200 to 250

250 to 300

300 to 400

400 to 500

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Arapahoe confining unit extent

Line of equal silt-plus-sand thickness, 

 in feet. Variable contour interval

Silt-plus-sand data point !
32 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A20. Simulated silt-plus-sand fraction of the lower Arapahoe aquifer.

PALMER

DIVIDEDIVIDE

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EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

§ ̈¦70

§ ̈¦25

§ ̈¦76

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

105° 104°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

Hydraulic conductivity data from Colorado Water Conservation Board, 2006

EXPLANATION

Simulated silt-plus-sand fraction 

 of total thickness

0.08 to 0.2

0.2 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.6

0.6 to 0.7

0.7 to 0.85

Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 33

Figure A21. Silt-plus-sand thickness of the upper Arapahoe aquifer.

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ARAPAHOE

JEFFERSON

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Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Fort

Lupton

Castle

Rock

Highlands 

Ranch

Aurora

Littleton

Lafayette

Englewood

Kiowa

Parker

Golden

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Fort

Lupton

Castle

Rock

Highlands 

Ranch

Aurora

Littleton

Lafayette

Englewood

§ ̈¦25

§ ̈¦76

105° 104°30'

40°

39°30'

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES 

0 5 10 KILOMETERS

EXPLANATION

Silt-plus-sand thickness of upper 

 Arapahoe aquifer, in feet

6 to 50

50 to 100

100 to 150

150 to 200

200 to 256

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Upper Arapahoe aquifer extent

Line of equal silt-plus-sand thickness, 

 in feet. Contour interval 50 feet

Silt-plus-sand data point
34 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A22. Simulated silt-plus-sand fraction of the upper Arapahoe aquifer.

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ELBERT

LARIMER

ADAMS

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ARAPAHOE

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Fort

Lupton

Castle

Rock

Highlands

Ranch

Aurora

Littleton

Lafayette

Englewood

Kiowa

Parker

Golden

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Fort

Lupton

Castle

Rock

Highlands

Ranch

Aurora

Littleton

Lafayette

Englewood

§ ̈¦25

§ ̈¦76

105° 104°30'

40°

39°30'

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Simulated silt-plus-sand fraction 

 of total thickness

0.16 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.6

0.6 to 0.7

0.7 to 0.8 Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Upper Arapahoe aquifer simulated extent 

 (model layer 8)
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 35

Figure A23. Altitude of the base and total thickness of the Denver lower confining unit.

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WELD

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

PALMER

DIVIDEDIVIDE

Limon

Simla

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Golden

Calhan

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Greeley

Boulder

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Loveland

Morrison

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Longmont

Brighton

Franktown

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Deer

Trail

Fort

Lupton

Fort

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Rock

Highlands

Ranch

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Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

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§ ̈¦70

§ ̈¦25

§ ̈¦76

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5,700

5,300

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4,700

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5,700

5,300

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5,900

5,500

5,400

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5,200

5,800

5,000

4,900

4,800

4,700

6,100

6,200

4,600

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Total thickness of Denver lower 

 confining unit, in feet

0 to 50

50 to 100

100 to 200

200 to 300

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Denver lower confining unit extent

Line of equal base altitude, in feet.

 Contour interval 100 feet ! Base-altitude data point
36 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A24. Altitude of the base and total thickness of the Denver aquifer.

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Calhan

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Morrison

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Franktown

Elizabeth

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Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

5,600

5,100

5,800

5,700

5,500

5,400

5,300

6,000

5,900

5,200

5,000

4,900

4,800

6,100

6,200

4,700

6,300

5,600

5,100

5,800

5,700

5,500

5,400

5,300

6,000

5,900

5,200

5,000

4,900

4,800

6,100

6,200

4,700

6,300

105° 104°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Total thickness of Denver aquifer, in feet

17 to 200

200 to 400

400 to 600

600 to 800

800 to 1,055 Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Denver aquifer extent

Line of equal base altitude, in feet.

 Contour interval 100 feet ! Base-altitude data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 37

Figure A25. Altitude of the base and total thickness of the Denver upper confining unit.

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WASHINGTON

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LINCOLN

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ADAMS

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ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

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Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

6,000

6,100

6,500

6,200 6,300

5,900

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5,900

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105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Total thickness of Denver upper 

 confining unit, in feet

0 to 50

50 to 100

100 to 150

150 to 200

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Denver aquifer extent

Denver upper confining unit extent

Line of equal base altitude, in feet.

 Contour interval 100 feet

! Base-altitude data point
38 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A26. Silt-plus-sand thickness of the Denver aquifer.

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

100

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PALMER

DIVIDEDIVIDE

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105° 104°

40°

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0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Silt-plus-sand thickness of the Denver 

 aquifer, in feet

5 to 50

50 to 100

100 to 150

150 to 200

200 to 250

250 to 300

300 to 350

350 to 400

400 to 665

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Denver aquifer extent

Line of equal silt-plus-sand thickness, 

 in feet. Variable contour interval ! Silt-plus-sand data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 39

Figure A27. Simulated silt-plus-sand fraction of the Denver aquifer.

PALMER

DIVIDEDIVIDE

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WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Simulated silt-plus-sand fraction of 

 total thickness

0.06 to 0.1

0.1 to 0.2

0.2 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.6

0.6 to 0.72 Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Denver aquifer simulated extent (model 

 layer 6)
40 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Dawson Aquifers and Confining Unit

The Dawson aquifer sequence consists of a 100- to 

1,100-ft-thick sequence of interbedded fluvial conglomerate, sandstone, siltstone, and shale (Romero, 1976; Robson, 

Romero, and Zawistowski, 1981; Robson, 1987). In general, the conglomerates and sandstones are coarse-grained, 

feldspathic, and poorly to moderately consolidated (Robson, 

1987). In the northern two-thirds of its extent, a clay and 

shale confining layer is present in the Dawson, and the aquifer 

is separated into an upper and lower aquifer. The confining 

layer is difficult to identify in the southern part of the Dawson extent, and the entire 600-ft-thick sequence of sediment 

in this area is assigned to the lower Dawson aquifer.

The lower Dawson aquifer forms the upper part of the 

D1 sequence described by Raynolds (2002) and represents 

continued Laramide synorogenic fluvial deposition in an 

environment similar to that which existed during deposition of 

the Denver Formation (Soister, 1978b; Raynolds, 2004). The 

base altitude of the lower Dawson aquifer ranges from about 

5,400 ft at the northern edge of its extent to about 6,800 ft on 

the southern edge of its extent, and the shape of the aquifer 

base again reflects the synclinal shape of the Denver Basin 

(fig. A28). The base of the lower Dawson aquifer is 1,100 ft 

below land surface near the Palmer Divide. Total thickness of 

the lower Dawson aquifer ranges from about 75 to about 790 ft 

with a mean of about 310 ft. Silt-plus-sand thickness ranges 

from 0 to about 375 ft with a mean of 85 ft, and the area with 

the greatest silt-plus-sand thickness (200 to 375 ft) is in the 

southwestern part of the basin along the Douglas–El Paso 

County line (fig. A29). The simulated percent silt-plus-sand is 

greatest along the northern and western aquifer margins and in 

the southern part of Douglas County (fig. A30).

The Dawson confining unit consists of a shale-rich 

layer at the top of the D1 sequence (Raynolds, 2002) and the 

Dawson paleosol series that caps the shale-rich layer (Soister, 

1978b; Soister and Tschudy, 1978; Trimble, 1980b; Bryant 

and others, 1981; Raynolds, 2002). The Dawson confining unit 

base altitude ranges from about 5,600 ft at the northern edge 

of its extent to about 6,650 ft on the southern edge of its extent 

(fig. A31). Total thickness of the Dawson confining unit ranges 

from 0 to about 300 ft with a mean of about 80 ft. The Dawson 

confining unit is thinnest in the southern part of its extent and 

thickens to the north (fig. A31).

The upper Dawson aquifer is equivalent to the D2 

sequence of Raynolds (2002). The upper Dawson aquifer 

consists of arkosic, coarse-grained, poorly consolidated sandstones with some interbedded overbank mudstone deposits in 

a distribution pattern quite different from that of the underlying D1 sequence (Raynolds, 2002). Sandstones of the upper 

Dawson aquifer are more uniform and less variable in composition than those from the underlying layers and likely were 

derived from the Pikes Peak area of the Front Range (Morse, 

1979; Crifasi, 1992; Raynolds, 2002; Thorson, 2003). The 

base altitude of the upper Dawson aquifer ranges from about 

5,650 ft at the northern edge of its extent to about 6,800 ft on 

the southern edge of its extent. Paleochannels at the base of 

the lower Dawson aquifer are evident from the base-altitude 

map, and one such paleochannel aligns with the presentday course of Cherry Creek (fig. A32), indicating that some 

present-day drainage patterns began to develop in Eocene 

time (Bader and others, 2007). The upper Dawson aquifer 

is the uppermost bedrock unit of the Denver Basin, so its 

thickness is calculated as the land-surface altitude minus the 

aquifer-base altitude where alluvium is not present (fig. A32). 

Thickness is greatest at the southern edge of the aquifer extent 

(about 840 ft) and thinnest along drainages of Plum and 

Cherry Creeks where deposits are incised by present-day erosion. Mean thickness is about 350 ft. Silt-plus-sand thickness 

for the upper Dawson aquifer ranges from 3 to about 275 ft 

with a mean of about 107 ft (fig. A33). The greatest silt-plussand thicknesses in the upper Dawson are located in an area 

that trends northeast from Douglas County (fig. A33), which 

the model represents with a northeast-trending area where 

simulated percent silt-plus-sand ranges from 30 to 60 percent 

(fig. A34).

Alluvial Aquifer

The alluvial aquifer overlies parts of the Denver Basin 

bedrock aquifers and consists of Quaternary alluvial, colluvial, and eolian deposits. These deposits form an extensive, 

transmissive, unconfined alluvial aquifer where interconnected 

and saturated, primarily along present-day stream channels 

(Robson, 1989). Substantial alluvial aquifers considered by 

this study include (1) present-day and prehistoric stream channels of the South Platte River and its tributaries (Bjorklund 

and Brown, 1957; Smith and others, 1964); (2) the Black 

Squirrel Creek drainage (Erker and Romero, 1967; Bingham 

and Klein, 1973; Buckles and Watts, 1988; Watts, 1995); 

and (3) the Big Sandy Creek drainage (Coffin, 1962; Willard 

Owens Associates, 1971) and the Nussbaum Alluvium south 

of Big Sandy Creek (Scott, 1963c; Soister, 1967; Paschke, 

1992) (fig. A35). Eolian and colluvial deposits outside of 

stream channels were not included in this study because 

they generally are insufficiently saturated to form a productive aquifer. Unconsolidated, coarse-grained sand and gravel 

deposits composed of granitic and gneissic rock fragments, 

sedimentary rock fragments, quartz, and feldspar characterize 

alluvial deposits in the Denver Basin (Scott, 1963a). Interbedded clays are present in some areas (Scott, 1963a). Alluvial 

deposits along the main channel of the South Platte River 

and tributaries draining directly from the Front Range tend to 

be coarser grained than alluvial deposits in tributaries draining from the Palmer Divide because streams draining from 

the mountains receive coarser-grained source materials from 

steeper terrain than streams draining the sedimentary rocks of 

the Denver Basin. The Nussbaum Alluvium is a terrace gravel 

deposit of coarse-grained sand and gravel derived from erosion of the upper Dawson sequence to the north (Scott, 1963c; 

Soister, 1967; Paschke, 1992).
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 41

Figure A28. Altitude of the base and total thickness of the lower Dawson aquifer.

PALMER DIVIDE

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ADAMS

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TELLER

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DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

§ ̈¦25

6,100

6,200

6,300

6,000

6,400

6,600

5,900

6,500

5,800

5,700

6,700

5,600

6,800

5,500

6,100

6,200

6,300

6,000

6,400

6,600

5,900

6,500

5,800

5,700

6,700

5,600

6,800

5,500

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

!

Total thickness of the lower Dawson 

 aquifer, in feet

75 to 100

100 to 200

200 to 300

300 to 400

400 to 500

500 to 600

600 to 700

700 to 786

Lower Dawson aquifer extent

Denver aquifer extent

Lower Arapahoe aquifer extent

Laramie-Fox Hills aquifer extent

Line of equal base altitude, in feet. 

 Contour interval 100 feet

Base-altitude data point
42 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A29. Silt-plus-sand thickness of the lower Dawson aquifer.

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ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

PALMER DIVIDEPALMER DIVIDE

§ ̈¦25

50

100

150

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200

250

300

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105° 104°30'

39°30'

39°

0 5 10 MILES

0 5 10 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Silt-plus-sand thickness of the lower

 Dawson aquifer, in feet

0 to 50

50 to 100

100 to 150

150 to 200

200 to 250

250 to 300

300 to 374

Lower Dawson aquifer extent

Denver aquifer extent

Lower Arapahoe aquifer extent

Laramie–Fox Hills aquifer extent

Line of equal silt-plus-sand thickness, 

 in feet. Contour interval 50 feet ! Silt-plus-sand data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 43

Figure A30. Simulated silt-plus-sand fraction of the lower Dawson aquifer.

PALMER

DIVIDEDIVIDE

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

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Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

§ ̈¦25

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Simulated silt-plus-sand fraction 

 of total thickness

0.2 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.6

0.6 to 0.7

0.7 to 0.77

Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Denver aquifer simulated extent (model 

 layer 6)

Lower Dawson aquifer simulated extent 

 (model layer 4)
44 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A31. Altitude of the base and total thickness of the Dawson confining unit.

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TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

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JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

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PALMER

DIVIDEDIVIDE

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

§ ̈¦25

6,200 6,100

6,300

6,000

6,400

6,500

5,900

5,800

6,600

5,700

5,600

6,200 6,100

6,300

6,000

6,400

6,500

5,900

5,800

6,600

5,700

5,600

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Total thickness of Dawson confining 

 unit, in feet

1 to 25

25 to 50

50 to 100

100 to 150

150 to 200

200 to 300

Dawson confining unit extent

Lower Dawson aquifer extent

Denver aquifer extent

Lower Arapahoe aquifer extent

Laramie-Fox Hills aquifer extent

Line of equal base altitude, in feet.

 Contour interval 100 feet ! Base-altitude data point
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 45

Figure A32. Altitude of the base and total thickness of the upper Dawson aquifer.

PALMER

DIVIDEDIVIDE

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DENVER

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DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

§ ̈¦25

6,250

6,050

6,400

6,200

6,350

6,100

6,300

6,450

6,500

6,550

6,000

5,950

5,900

6,650

5,800

6,700

5,700

6,250

6,050

6,400

6,200

6,350

6,100

6,300

6,450

6,500

6,550

6,000

5,950

5,900

6,650

5,800

6,700

5,700

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Total thickness of upper 

 Dawson aquifer, in feet

3 to 200

200 to 400

400 to 600

600 to 840

Laramie–Fox Hills aquifer extent

Lower Arapahoe aquifer extent

Denver aquifer extent

Lower Dawson aquifer extent

Upper Dawson aquifer extent

Line of equal base altitude, in feet.

 Contour interval 100 feet

Base-altitude data point
46 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A33. Silt-plus-sand thickness of the upper Dawson aquifer.

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!

!!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

! !!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

! !

!

!

!

!

! !

!

!

!

!

!

!

!

! !

!

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands 

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

PALMER

DIVIDE

PALMER

DIVIDE

§ ̈¦25

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Silt-plus-sand thickness of the upper 

 Dawson aquifer, in feet

3 to 25

25 to 50

50 to 75

75 to 100

100 to 125

125 to 150

150 to 175

175 to 200

200 to 225

225 to 250

250 to 275

Upper Dawson aquifer extent

Lower Dawson aquifer extent

Denver aquifer extent

Lower Arapahoe aquifer extent

Laramie-Fox Hills aquifer extent

Line of equal silt-plus-sand thickness, 

 in feet. Variable contour interval ! Silt-plus-sand data point

0 5 10 MILES

0 5 10 KILOMETERS
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 47

Figure A34. Simulated silt-plus-sand fraction of the upper Dawson aquifer.

PALMER

DIVIDEDIVIDE

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

!

! ! !

!

!

!

!

!

!

!

! !

!

!

!

!

!

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

§ ̈¦25

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Simulated silt-plus-sand fraction 

 of total thickness

0.12 to 0.2

0.2 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.6

Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Denver aquifer simulated extent (model 

 layer 6)

Lower Dawson aquifer simulated extent 

 (model layer 4)

Upper Dawson aquifer simulated extent 

 (model layer 2)
48 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A35. Altitude of the base and total thickness of alluvial sand and gravel deposits in the 

Denver Basin.

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryCr

Plum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Creek

Steels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

PALMER

DIVIDEDIVIDE

!

!

!

!

!

!

!

! ! !

!

!

!(

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

6,000

4,400

4,500

6,100

5,900

6,200

5,800

6,300

6,400

4,600

4,300

4,900

5,100

4,700

6,500

4,800

5,000

5,700

6,600

5,200

5,300

5,400

5,600

6,700

5,500

7,000

6,900

4,800

5,000

5,800

5,700

5,700

5,700

6,000

4,500

5,600

5,900

5,000

4,700

4,700

5,800

5,800

4,600

5,200

5,300

4,600

5,200

4,900

5,800

6,100

5,900

5,100

4,700

5200

4,800

5,000

4,800

4,800

5,800

4,600

4,900

4,600

5,300

4,500

5,000

5,500

5,800

5,100

4,700

4,900

6,000

4,400

4,500

6,100

5,900

6,200

5,800

6,300

6,400

4,600

4,300

4,900

5,100

4,700

6,500

4,800

5,000

5,700

6,600

5,200

5,300

5,400

5,600

6,700

5,500

7,000

6,900

4,800

5,000

5,800

5,700

5,700

5,700

6,000

4,500

5,600

5,900

5,000

4,700

4,700

5,800

5,800

4,600

5,200

5,300

4,600

5,200

4,900

5,800

6,100

5,900

5,100

4,700

5200

4,800

5,000

4,800

4,800

5,800

4,600

4,900

4,600

5,300

4,500

5,000

5,500

5,800

5,100

4,700

4,900

105° 104°

40°

39°

Black Squirrel Creek 

alluvial aquifer

Nussbaum

alluvial aquifer

Big Sandy Creek

alluvial aquifer

South Platte River

and tributaries

alluvial aquifer

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Thickness of alluvial aquifer, in feet Alluvial aquifer extent

3 to 50

50 to 100

100 to 150

150 to 200

200 to 250

250 to 376 Stream

Perennial

Ephemeral

Laramie–Fox Hills aquifer extent

Equal base altitude of alluvial aquifer, 

 in feet. Contour interval 100 feet
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 49

In general, the base of the alluvial aquifer slopes 

outward in a radial fashion away from the Palmer Divide, 

reflecting paleo- and present-day topography (fig. A35). The 

base of alluvium in the South Platte River and its tributaries 

slopes northward, and the base of alluvium in Black Squirrel 

Creek slopes southward. The base of the Nussbaum Alluvium 

and Big Sandy Creek alluvial aquifers slopes southeast away 

from the Palmer Divide. Paleochannels are evident in the 

alluvial base configuration, and some present-day stream 

channels align with paleochannels, indicating renewed 

headward erosion along prehistoric drainages (Paschke, 

1992). Alluvial deposit thickness ranges from 3 to about 

376 ft (fig. A35). The thickest part of the South Platte alluvial aquifer (more than 250 ft) is located in the downstream 

part of the aquifer near Wiggins, Colorado (Hurr, Schneider, 

and others, 1972b); unconsolidated alluvial deposits over 

300 ft in thickness also are found in the Nussbaum Alluvium 

(Paschke, 1992).

Aquifer Hydraulic Properties

The ability of an aquifer to transmit and store water 

is a function of its thickness, hydraulic conductivity, and 

storage properties. Hydraulic conductivity (K) describes the 

capacity of a unit volume of permeable material to transmit 

water (Lohman, 1979). Transmissivity (T) is a measure of the 

capacity of a unit width of the aquifer to transmit water and 

is equal to the hydraulic conductivity multiplied by aquifer 

saturated thickness (b) (T = Kb) (Lohman, 1979). A greater 

thickness of water-yielding material of a given hydraulic 

conductivity allows larger volumes of water to move through 

the aquifer.

Storage properties of an aquifer, described in terms of 

storage coefficient or specific storage for confined aquifers and 

specific yield for unconfined aquifers, represent the volume of 

water released from or taken into aquifer storage as hydraulic 

head changes. These properties can vary substantially depending on whether the aquifer is under confined or unconfined 

conditions. The storage coefficient is the volume of water 

released from or taken into storage by an aquifer because of 

expansion of water and compression of the aquifer per unit 

surface area of the aquifer in response to a unit change in 

hydraulic head (Lohman, 1979). The capacity of a confined 

aquifer to release water from storage depends on the thickness 

of the aquifer, the porosity of the water-bearing material, and 

to a lesser extent, the compressibility of the water and rock 

(Lohman, 1979). If the potentiometric surface remains above 

the top of the water-bearing zone as for a confined aquifer, 

there is no physical draining of the aquifer when water is 

released from storage. Confined storage release is a result of 

the elastic properties of the water and the rock and is measured 

by the aquifer property of confined storage coefficient or specific storage (storage coefficient divided by aquifer thickness) 

(Lohman, 1979; Fetter, 1994). The specific yield is the ratio of 

the volume of water that will drain by gravity from a unit volume of the aquifer (Lohman, 1979). When the potentiometric 

surface of an aquifer drops below the top of the water-bearing 

zone, pore spaces in the rock are physically drained, and the 

aquifer at that location is considered unconfined. Specific yield 

is the aquifer parameter used to describe the capacity of an 

unconfined aquifer to release water as the water table is lowered and is defined as the ratio of the volume of water drained 

by gravity to the volume of drained rock (Lohman, 1979). The 

ratio of the volume of water not drained by gravity to the volume of drained rock is the specific retention. Porosity, which is 

the ratio of the volume of void spaces to the volume of rock, is 

the sum of specific yield and specific retention.

Hydraulic Conductivity and Transmissivity

Hydraulic-conductivity (K) values were derived from 

aquifer pumping tests, specific-capacity tests, and laboratory analyses. Hydraulic-conductivity values on which the 

discussion in this section is based are available in Hydrobase 

(Colorado Division of Water Resources, 2005; Colorado 

Water Conservation Board, 2006c), and the spatial distributions of hydraulic conductivity values are compared to modelcalibrated spatial distributions in the section “Calibrated 

Hydraulic-Conductivity Distribution” of Chapter C. 

Hydraulic-conductivity values from pumping tests for the 

upper Dawson aquifer range from 0.1 to about 6.2 ft/d, and 

hydraulic-conductivity values for the lower Dawson aquifer 

range from 0.01 to about 6.1 ft/d (fig. A36; table A5; Robson, 

1983). For the Denver aquifer, hydraulic-conductivity values 

from pumping tests range from a minimum of 0.03 ft/d to 

a maximum of 24 ft/d (fig. A36), and the maximum values 

are in the south Denver metropolitan area (fig. C6), similar 

to the hydraulic-conductivity distributions in the Arapahoe 

and Laramie–Fox Hills aquifers. For the Arapahoe aquifer, 

hydraulic-conductivity values from pumping tests range 

from a maximum of 9.9 ft/d south of Denver to a minimum 

of 0.06 ft/d in other areas (fig. A36). Hydraulic-conductivity 

values for the lower Arapahoe aquifer are greatest in the 

south Denver metropolitan area (fig. C7), but no pattern to 

hydraulic-conductivity values was discernible in the upper 

Arapahoe aquifer. For the Laramie–Fox Hills aquifer, Robson 

(1983) reported hydraulic-conductivity values ranging from 

about 0.05 ft/d along the northwest margin of the basin 

to about 6 ft/d south of Denver. The greatest hydraulicconductivity values from pumping tests (maximum of 7.2 ft/d) 

in the Laramie–Fox Hills aquifer (table A5; fig. A36) are 

observed in the south Denver metropolitan area, as noted by 

Robson (1983), and in the eastern part of the aquifer (fig. C8). 

The lowest hydraulic-conductivity values from pumping tests 

(minimum of 0.003 ft/d) in the Laramie–Fox Hills aquifer 

occur in the northwestern part of the aquifer, again consistent 

with results of Robson (1983). Few hydraulic-conductivity 

values are available for the Denver, Arapahoe, and Laramie–

Fox Hills aquifer in the areas overlain by the Dawson aquifer.
50 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Pumping

Specific capacity

Specific capacity

Specific capacity

Specific capacity

Laboratory

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

1,000

10,000

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

1,000

10,000

Hydraulic conductivity, in feet per day

Aquifer-test category

Pumping

Laboratory

Pumping

Laboratory

Pumping

Laboratory

90th percentile

EXPLANATION

25th percentile

50th percentile/mean

75th percentile

10th percentile

median

minimum

maximum

outliers

99th percentile

n=53 n=729 n=16 n=79 n=233 n=29 n=248 n=205 n=23 n=147 n=171 n=39

Dawson Denver Arapahoe Laramie–Fox Hills

Figure A36. Distribution of hydraulic-conductivity values from pumping tests for the Denver Basin bedrock aquifers.

A summary of hydraulic-conductivity values from 

pumping tests for the bedrock aquifers indicates the Arapahoe 

aquifer is the most permeable of the bedrock units, with a 

median hydraulic conductivity of 1.8 ft/d and geometric mean 

of 1.6 ft/d. The Dawson aquifer is the secondmost permeable 

bedrock unit, with a median hydraulic conductivity of 0.80 ft/d 

and a geometric mean of 0.71 ft/d (table A5; fig. A36). The 

Denver aquifer, with a median hydraulic conductivity of 

0.45 ft/d and a geometric mean of 0.42 ft/d, and the Laramie–

Fox Hills aquifer, with a median hydraulic conductivity of 

0.40 ft/d and a geometric mean of 0.40 ft/d, are nearly equal in 

hydraulic conductivity and are typically less permeable than 

either the Arapahoe or Dawson aquifers (fig. A36). Hydraulicconductivity values for the alluvial aquifer are two to more 

than three orders of magnitude greater than those of the bedrock aquifers and range from about 18 ft/d to about 2,560 ft/d 

with a median value of about 480 ft/d on the basis of pumpingtest results (table A5).

The median hydraulic-conductivity values derived from 

specific-capacity tests are generally similar to the medians 

of values from pumping tests for each aquifer (table A5). 

Hydraulic-conductivity values calculated from laboratory tests 

typically exhibit a larger range and have smaller geometricmean values than values from either pumping or specific 

capacity tests for all aquifers (table A5).

The transmissivity of the bedrock aquifers, as described 

by Robson (1983, 1987), ranged from zero at the limit of each 

of the aquifers to more than 1,000 ft2/d in the Laramie–Fox 

Hills aquifer, more than 2,100 ft2/d in the Arapahoe aquifer, 

more than 400 ft2/d in the Denver aquifer, and more than 

1,200 ft2/d in the Dawson aquifer. The Arapahoe aquifer was 

the most transmissive of the bedrock aquifers, followed by the 

Dawson and Laramie–Fox Hills aquifers (Robson, 1987). The 

Denver aquifer was considered the least transmissive of the 

bedrock aquifers (Robson, 1987). The alluvial aquifer is much 

more transmissive than the bedrock aquifers, as evidenced by 

an average transmissivity of about 13,000 ft2/d along the South 

Platte River downstream from Brighton (Hurr, Schneider, and 

others, 1972a). The difference in transmissivity between the 

alluvial and bedrock aquifers accounts for the larger yields of 

wells completed in the alluvial aquifer relative to the yields 

typical of wells completed in the bedrock aquifers (Robson, 

1989). The difference in hydraulic properties of the alluvial 

and bedrock aquifers is an important part of the Denver Basin 

conceptual model because it affects water movement and the 

water budget in the basin.

Storage Parameters

Storage-coefficient estimates for the Denver Basin 

bedrock aquifers were reported by Robson (1983), and 

additional storage-coefficient values based on subsequent 

aquifer tests were compiled as part of the SPDSS (Colorado 

Water Conservation Board, 2006c) with results reported 

in the CDWR Hydrobase database (Colorado Division of 
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 51

Water Resources, 2005). Reported storage-coefficient values 

ranged from 0.00002 to 0.004 for the Laramie–Fox Hills 

aquifer, from 0.0001 to 0.008 for the Arapahoe aquifer, and 

from 0.0002 to 0.0006 for the Denver and Dawson aquifers. 

In general, estimated storage-coefficient values are largest 

in the central and western parts of the basin (Robson, 1987; 

Colorado Water Conservation Board, 2006c). For this study, 

specific-storage values for the Denver Basin groundwater 

flow model were estimated separately for aquifers and 

confining units during model calibration. A specific-storage 

value of 5.2×10–7/ft was estimated for aquifers, and a value 

of 1.7×10–6 /ft was estimated for confining units as described 

in the “Specific Storage and Specific Yield” section of 

Chapter C.

Specific-yield estimates for the Denver Basin bedrock 

aquifers for this study tabulated from laboratory tests of hand 

and core samples (Robson, 1983; McConaghy and others, 

1964; Lapey, 2001; Robson and Banta, 1993; Raynolds and 

others, 2001) provided the basis for specific-yield values 

assigned in the groundwater flow model. Specific-yield values 

reported by Robson (1983) are from laboratory tests of hand 

and core samples, and specific-yield values reported by 

McConaghy and others (1964) are for hand samples collected 

from outcrops. Specific-yield values from Robson and Banta 

(1993) are for tests on core samples from the Castle Pines 

core hole located on the western margin of the basin near 

Castle Rock, and specific-yield values from Lapey (2001) are 

for tests on core samples from the Kiowa core hole located 

near the center of the Denver Basin near Kiowa. All four 

data sources classified specific-yield values as being from the 

Dawson, Denver, Arapahoe, or Laramie–Fox Hills aquifers; 

specific-yield values were not differentiated for the upper and 

lower Dawson or Arapahoe aquifer units.

Table A6 presents a summary of mean specific-yield 

values for bedrock aquifers tabulated from each of the 

original data sources and facilitates geographic and sampletype comparisons. Mean specific-yield values for the Dawson, 

Denver, and Arapahoe aquifers provided by McConaghy 

and others (1964) typically are greater than values provided 

by subsequent workers, probably owing to testing of hand 

samples from outcrops, which tend to be more weathered and 

porous than core samples. Mean specific-yield values for the 

Arapahoe and Laramie–Fox Hills aquifers from the Castle 

Pines core (Robson and Banta, 1993) are greater than mean 

specific-yield values for the Arapahoe and Laramie–Fox Hill 

aquifers from the Kiowa core hole (Lapey, 2001; Raynolds 

and others, 2001), likely owing to the eastward fining of sediment shed off the Front Range during the Late Cretaceous 

(Lapey-Woodard and others, 2002). Mean specific-yield 

values for the Dawson and Denver aquifers from the Castle 

Pines core (Robson and Banta, 1993) are less than mean 

specific-yield values for the Dawson and Denver aquifers 

from the Kiowa core hole (Lapey, 2001; Raynolds and others, 

2001), suggesting varying source areas and that the Dawson 

and Denver sandstones do not necessarily fine eastward 

between these two core holes. The Denver aquifer exhibits the 

smallest mean specific yield of the bedrock aquifers, which is 

consistent with the conceptualized depositional environment 

and lower hydraulic conductivity of the Denver aquifer.

Table A5. Summary of hydraulic-conductivity values calculated from pumping tests (adapted from Colorado Division of Water 

Resources [2005]).

[ft/d, feet per day]

Alluvial

aquifer

Upper

Dawson

aquifer

Lower

Dawson

aquifer

All

Dawson

aquifers

Denver

aquifer

Upper

Arapahoe

aquifer

Lower

Arapahoe

aquifer

All

Arapahoe

aquifers

Laramie–Fox

Hills aquifer

Hydraulic-conductivity values from pumping tests

Number of tests 132 22 52 53 79 45 244 248 147

Minimum, ft/d 18 0.1 0.01 0.01 0.03 0.06 0.06 0.06 0.003

Maximum, ft/d 2,558 6.2 6.2 6.2 24.0 4.7 9.9 9.9 7.2

Median, ft/d 479 1.2 0.78 0.80 0.45 1.2 1.8 1.8 0.40

Geometric mean, ft/d 1.1 0.69 0.71 0.42 0.9 1.6 1.6 0.40

Hydraulic-conductivity values for specific-capacity tests

Number of tests 490 265 464 729 233 46 159 205 171

Minimum, ft/d 0.67 0.06 0.04 0 0.01 0.07 0.02 0.02 0.03

Maximum, ft/d 7,194 32 191 191 430 6.7 42 42 262

Median, ft/d 290 1.40 0.88 0.98 0.71 0.69 0.79 0.75 0.54

Geometric mean, ft/d 1.75 1.08 1.3 0.71 0.78 0.68 0.70 0.58

Hydraulic conductivity values for laboratory tests

Number of tests 73 10 14 16 29 6 23 23 39

Minimum, ft/d 0.008 0.02 0.01 0.01 1.0E–04 0.01 3.0E–04 3.0E–04 4.0E–04

Maximum, ft/d 3,877 11 11 11 989 6.0 9.0 8.96 70

Median, ft/d 92.3 1.15 1.0 1 0.10 4.7 1.6 1.6 0.50

Geometric mean, ft/d 64.7 0.83 0.37 0.54 0.30 2.9 0.47 0.47 0.23
52 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Overall arithmetic mean values for specific yield calculated by this study from the original data sources are compared 

to specific-yield values assigned by the SB85–5 model of 

the Denver Basin aquifers in table A6. The SB85–5 model 

uses a constant specific-yield value for each aquifer as shown 

in table A6. For the Dawson and Denver aquifers, SB85–5 

specific-yield values are greater than arithmetic mean values 

calculated by this study. The Arapahoe SB85–5 specific-yield 

value of 17 percent is approximately equal to the overall 

arithmetic mean of 17.8 percent, and the Laramie–Fox Hills 

SB85–5 value of 15 percent is less than the overall arithmetic mean of 18.6 percent. Lapey-Woodard and others (2002) 

assumed a log-normal distribution to the specific-yield data, 

determined that geometric mean values from the Castle Pines 

and Kiowa core holes were less than SB85–5 specific-yield 

values, and concluded that previous estimates of specific yield 

may lead to overestimating the volume of recoverable water 

in the Denver Basin aquifers. However, the Shapiro-Wilks test 

for normality (Shapiro and Wilk, 1965) indicates the specificyield values for each aquifer are not log-normally distributed 

at a 95-percent confidence level.

This study assigned arithmetic mean specific-yield values 

(table A6) to model layers in the groundwater flow model as 

described in the “Specific Storage and Specific Yield” section 

of Chapter C. For bedrock aquifers, the mean specific-yield 

values from table A6 were multiplied by the fraction of siltplus-sand for the given aquifer (figs. A14, A20, A22, A27, 

A30, A34) to obtain an effective specific yield for the overall aquifer thickness for each model grid cell. The resulting 

spatial distributions of specific yield assigned to each bedrock 

aquifer (figs. A37–A42) thus reflect the spatial distributions 

of the silt-plus-sand fraction and assume that materials finer 

than silt sized provide a minimal contribution to specific yield. 

Because estimated specific-yield values were multiplied by 

the silt-plus-sand fraction, simulated specific-yield values 

for all bedrock aquifers are less than mean values reported in 

table 6. Specific yield for the alluvial aquifer ranges from less 

than 1 percent to about 37 percent based on pumping tests and 

laboratory analysis of samples from previous work (Bjorklund 

and Brown, 1957; McConaghy and others, 1964; Smith and 

others, 1964; and Wilson, 1965; Colorado Water Conservation 

Board, 2006c), and a specific yield of 0.165 was assigned to 

the simulated alluvial aquifer (model layer 1; table C13).

Predevelopment Groundwater Availability

Interpretations and results from the initial, steady-state 

stress period of the calibrated groundwater flow model 

(Chapter C) as well as previous studies form the basis for this 

description of the predevelopment (pre-1880) Denver Basin 

groundwater availability. Predevelopment conditions represent 

aquifer conditions at the end of 1879, prior to any groundwater 

withdrawals and are discussed in terms of the water budget, 

hydraulic heads, and groundwater flow directions.

Predevelopment Groundwater Budget

A groundwater budget accounts for all of the inflows and 

outflows for a hydrologic system. For predevelopment conditions in the Denver Basin, inflow components of the groundwater budget are recharge from precipitation plus irrigation 

return flow, recharge from streams, and interaquifer flow 

between hydrogeologic units. Predevelopment outflow components of the groundwater budget are discharge to evapotranspiration, streams, springs, and interaquifer flow. Discharge 

to pumping wells and associated changes in aquifer storage 

were assumed negligible before 1880 and are not included in 

the predevelopment groundwater budget. Both inflows and 

outflows exist for some budget components, and the difference between the inflow and the outflow is herein referred 

to as the “net” inflow or outflow. For example, net recharge 

equals recharge minus evapotranspiration, and the net flow 

to streams equals the inflow from streams minus the outflow 

to streams. The groundwater budgets presented herein uses 

the MODFLOW convention that positive values are inflows 

to, and negative values are outflows from, the groundwater 

system. The discussion is based on the simulated, cell-by-cell 

water budget for the initial (steady-state) model stress period, 

which represents hydrologic conditions at the end of 1879. 

Transient groundwater budget results from predevelopment to 

2003 are presented in section “Changes to the Groundwater 

Budget” in Chapter B. Groundwater budgets are presented for 

the entire model (sum of all model layers), the alluvial aquifer 

(model layer 1), and the bedrock units (layers 2 through 

12) and are discussed in terms of individual groundwater 

budget components.

Table A6. Summary of specific-yield values.

Aquifer

Specific yield

(percent)

Mean value

from McConaghy 

and others

(1964)

Mean value

from Robson 

(1983)

Mean value 

Castle Pines core 

(Robson and 

Banta, 1993)

Geometric mean

Kiowa core

(Raynolds and others, 

2001; Lapey, 2001)

Mean value

of all

available

data1

Senate

Bill 85–5

assigned

value

Percent difference

of mean values

from Senate

Bill 85–5

assigned values

Dawson 19.9 16.8 12.6 18.7 15.2 20 –27

Denver 33.7 13.8 11.9 14.4 13.3 17 –24

Arapahoe 20.8 17.9 18.3 10.8 17.8 17 5

Laramie–Fox Hills 17.3 21.2 22.0 12.9 18.6 15 21 1

Values used in U.S. Geological Survey Denver Basin groundwater flow model.
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 53

Figure A37. Simulated specific-yield distribution for the Laramie–Fox Hills aquifer.

PALMER

DIVIDEDIVIDE

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

§ ̈¦70

§ ̈¦25

§ ̈¦76

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Specific-yield value of 0.186 multiplied by 

 fraction silt-plus-sand, dimensionless

0.03 to 0.05

0.05 to 0.07

0.07 to 0.09

0.09 to 0.11

0.11 to 0.13

0.13 to 0.15

0.15 to 0.17

Laramie–Fox Hills aquifer simulated 

 extent (model layer 12)

Extent of active model cells
54 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A38. Simulated specific-yield distribution for the lower Arapahoe aquifer.

PALMER

DIVIDEDIVIDE

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

§ ̈¦70

§ ̈¦25

§ ̈¦76

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

105° 104°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

EXPLANATION

Specific-yield value of 0.178 multiplied 

 fraction silt-plus-sand, dimensionless

0.014 to 0.02

0.02 to 0.04

0.04 to 0.05

0.05 to 0.06

0.06 to 0.07

0.07 to 0.08

0.08 to 0.09

0.09 to 0.11

0.11 to 0.13

0.13 to 0.15

Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 55

Figure A39. Simulated specific-yield distribution for the upper Arapahoe aquifer.

!

!

!

! !

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

! !

WELD

ELBERT

LARIMER

ADAMS

BOULDER

ARAPAHOE

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

WELD

ELBERT

LARIMER

ADAMS

BOULDER

ARAPAHOE

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Fort

Lupton

Castle

Rock

Highlands

Ranch

Aurora

Littleton

Lafayette

Englewood

Kiowa

Parker

Golden

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Fort

Lupton

Castle

Rock

Highlands

Ranch

Aurora

Littleton

Lafayette

Englewood

§ ̈¦25

§ ̈¦76

105° 104°30'

40°

39°30'

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Specific-yield value of 0.178 multiplied by 

 fraction-silt-plus sand, dimensionless

0.028 to 0.04

0.04 to 0.05

0.05 to 0.06

0.06 to 0.07

0.07 to 0.08

0.08 to 0.09

0.09 to 0.1

0.1 to 0.11

0.11 to 0.12

0.12 to 0.137

Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Upper Arapahoe aquifer simulated extent 

 (model layer 8)
56 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A40. Simulated specific-yield distribution for the Denver aquifer.

PALMER

DIVIDEDIVIDE

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Specific-yield value of 0.133 multiplied by 

 fraction silt-plus-sand, dimensionless

0.008 to 0.02

0.02 to 0.03

0.03 to 0.04

0.04 to 0.05

0.05 to 0.06

0.06 to 0.07

0.07 to 0.08

0.08 to 0.1

Laramie–Fox Hills aquifer simulated 

 extent (model layer 12)

Lower Arapahoe aquifer simulated 

 extent (model layer 10)

Denver aquifer simulated extent 

 (model layer 6)
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 57

Figure A41. Simulated specific-yield distribution for the lower Dawson aquifer.

PALMER

DIVIDEDIVIDE

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

!

! ! !

!

!

!

!

!

!

!

! !

!

!

!

!

!

§ ̈¦25

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Specific-yield value of 0.152 multiplied by 

 fraction silt-plus-sand, dimensionless

0.03 to 0.04

0.04 to 0.05

0.05 to 0.06

0.06 to 0.07

0.07 to 0.08

0.08 to 0.09

0.09 to 0.1

0.1 to 0.12 Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Denver aquifer simulated extent (model 

 layer 6)

Lower Dawson aquifer simulated extent 

 (model layer 4)
58 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A42. Simulated specific-yield distribution for the upper Dawson aquifer.

PALMER

DIVIDEDIVIDE

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

!

! ! !

!

!

!

!

!

!

!

! !

!

!

!

!

!

§ ̈¦25

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Specific yield value of 0.152 multiplied by 

 fraction silt-plus-sand, dimensionless

0.019 to 0.03

0.03 to 0.04

0.04 to 0.05

0.05 to 0.06

0.06 to 0.07

0.07 to 0.08

0.08 to 0.1

Laramie–Fox Hills aquifer simulated extent 

 (model layer 12)

Lower Arapahoe aquifer simulated extent 

 (model layer 10)

Denver aquifer simulated extent (model 

 layer 6)

Lower Dawson aquifer simulated extent 

 (model layer 4)

Upper Dawson aquifer simulated extent 

 (model layer 2)
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 59

Groundwater Budget for the Entire Modeled Area

Precipitation plus irrigation recharge was the largest inflow 

component (630 ft3/s or 98 percent of inflow) for the predevelopment groundwater budget of the entire Denver Basin aquifer 

system (alluvial and bedrock units), and the primary outflow 

components were to evapotranspiration (442 ft3/s or 68 percent 

of outflow) and streams (182 ft3/s or 28 percent of outflow; 

table A7, figs. A43, A44). Stream inflow (16 ft3/s), spring 

outflow (4 ft3/s), and groundwater flow out of the basin through 

alluvial aquifers (19 ft3/s) were minor components of the predevelopment groundwater budget (table A7, figs. A43, A44). 

Groundwater flow out of the basin through the alluvial aquifer 

includes underflow to the alluvial aquifer and specified flux to 

the South Platte alluvial aquifer (table A7). Subtracting evapotranspiration from recharge results in a net recharge rate of 

189 ft3/s; net discharge to streams was about 166 ft3/s (table A7, 

figs. A43, A44). These results indicate that before development, 

precipitation plus irrigation recharge was the primary source 

of water to the entire Denver Basin aquifer system, and evapotranspiration and discharge to streams were the primary outflows from the system. There was no outflow to wells or inflow 

from aquifer storage for the predevelopment period. The total 

predevelopment water budget, defined as the total inflow or 

outflow, was about 650 ft3/s (table A7).

Groundwater Budget for the Alluvial Aquifer 

and Bedrock Units

Separating the water budget of the alluvial aquifer 

from that of the bedrock aquifers and confining units (herein 

referred to collectively as bedrock units) indicates that for 

predevelopment conditions, about three-fourths of the water 

moved in and out of the system through the alluvial aquifer 

(about 500 ft3/s), and about one-fourth of the water moved 

in and out of the system through the bedrock units (about 

187 ft3/s; table A7, fig. A45). This difference in water budgets between the alluvial and bedrock units is caused by the 

large difference in hydraulic-conductivity values between the 

unconsolidated and permeable alluvial materials as compared to the consolidated, less-permeable bedrock sandstones 

(Robson, 1989), which allows water to move more easily in 

and out of the alluvial aquifer. For the predevelopment period, 

the majority of the simulated recharge (72 percent), evapotranspiration (70 percent), and net recharge (78 percent) were 

directly to and from the alluvial aquifer, while 28 percent of 

recharge, 30 percent of evapotranspiration, and 22 percent of 

net recharge were directly to and from the bedrock aquifers 

(table A7). The majority of net outflow to streams also was 

from the alluvial aquifer (91 percent), and 9 percent of net 

outflow to streams was directly from bedrock units (table A7).

Precipitation plus irrigation recharge was the primary 

inflow groundwater budget component for the alluvial aquifer 

(454 ft3/s or 91 percent of alluvial aquifer inflow) and evapotranspiration was the primary outflow groundwater budget 

component for the alluvial aquifer (307 ft3/s or 61 percent 

of outflow) for the predevelopment period (fig. A45). The 

predevelopment net water budget for the alluvial aquifer 

indicates that 84 percent (147 ft3/s) of the net inflow was from 

precipitation plus irrigation recharge minus evapotranspiration and that the remaining 16 percent (26 ft3

/s) of inflow was 

from the bedrock aquifers (table A7; fig. A45). Net outflow 

from the alluvial aquifer was primarily to streams (151 ft3/s 

or 88 percent of net outflow). Groundwater flow out of the 

basin (19 ft3/s or 10 percent of net outflow) and flow to springs 

(2 percent of net outflow) were minor components of outflow 

from the alluvial aquifer (table A7; fig. A45).

Precipitation plus irrigation recharge was the primary 

inflow groundwater budget component for bedrock units 

(176 ft3/s or 94 percent of bedrock units inflow) (fig. A45), and 

evapotranspiration was the primary outflow groundwater budget component for the bedrock units (135 ft3

/s or 72 percent of 

outflow) for the predevelopment period (fig. A45). The predevelopment net water budget for the bedrock units (table A7; 

fig. A45) indicates that all of the net inflow was from precipitation plus irrigation recharge minus evapotranspiration 

(41 ft3/s) and that net outflow from the bedrock units was to 

the alluvial aquifer (26 ft3/s or 64 percent of net outflow) and 

streams (14 ft3/s or 34 percent of net outflow). There was no 

groundwater outflow out of the basin from bedrock units, and 

outflow to springs was a minor outflow component (2 percent 

of net outflow) for the bedrock units (figs. A1, A45).

Groundwater Budget Components

This discussion presents details of each groundwater budget component for the predevelopment Denver Basin groundwater system. Simulated inflow was from precipitation plus 

irrigation recharge, and simulated outflow was to evapotranspiration, springs, and model boundaries simulating groundwater 

flow out of the basin. Simulated streams were both sources and 

sinks of groundwater.

Recharge and Evapotranspiration

Sources of recharge to the predevelopment Denver 

Basin aquifer system included infiltration of incident precipitation and agricultural irrigation in excess of consumptive use 

(irrigation return flows). Precipitation infiltration and runoff 

is an important source of water in this semiarid region for 

streams and groundwater recharge. Mean annual precipitation 

in the basin supplies an estimated 6,900 ft3/s of water to the 

surface (Robson, 1987). However, most of this water runs off 

or is evapotranspired, and only a small fraction of precipitation infiltrates to the water table to provide recharge to the 

bedrock aquifers (Robson, 1987). In areas where bedrock 

crops out with little or no overlying surficial deposits, deep 

infiltration of precipitation provides direct recharge to the 

bedrock aquifers. Previous studies indicated that precipitation recharge is, in general, greatest over the upper Dawson 

aquifer because (1) precipitation is greatest at the higher 

elevations where the upper Dawson aquifer is located, 

(2) the Dawson aquifers have a larger outcrop area than the 

other bedrock aquifers, and (3) the upper Dawson aquifer 
60 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Table A7. Simulated predevelopment (pre-1880) groundwater budget for entire Denver Basin aquifer system, the alluvial aquifer, and 

the bedrock units.

[In some cases values do not sum to total because of rounding; ft3/s, cubic feet per second; --, not calculated]

Groundwater budget component

Entire model

flow rate

(ft3/s)

Alluvial aquifer Bedrock units

Flow rate

(ft3/s)

Percentage

of component

Flow rate

(ft3/s)

Percentage

of component

Inflow

Recharge (precipitation + irrigation) 630 454 72 176 28

Streams 16.2 12.7 78 3.5 22

Interaquifer flow -- 34.1 -- 7.8 --

Storage 0.0 0.0 -- 0.0 --

Total inflow

646 501 187

Outflow

Evapotranspiration –442 –307.1 70 –135 30

Streams –182 –164.2 90 –17.6 10

Springs –4.2 –3.2 76 –1.0 24

Underflow to alluvial aquifers –4.8 –4.8 100 0.0 0

Specified flux to South Platte alluvial aquifer –14.1 –14.1 100 0 0

Interaquifer flow -- –7.8 -- –34.1 --

Wells 0.0 0.0 -- 0.0 --

Storage 0.0 0.0 -- 0.0 --

Total outflow

–646 –501 –187

Net flows

Net recharge (recharge + evapotranspiration) 189 147 78 41.5 22

Net stream discharge (stream inflow + stream outflow) –166 –152 91 –14.2 9

Net interaquifer flow 0 26.3 -- –26.3 --

has a high hydraulic conductivity relative to the surrounding 

Denver aquifer (Robson, 1987). In areas overlain by alluvial 

aquifers, precipitation recharge is directly to permeable sand 

and gravel deposits.

Because agricultural irrigation, primarily by surfacewater diversions, began in the South Platte River valley prior 

to the 1880s (Nadler and Schumm, 1981), irrigation recharge 

is included in the predevelopment recharge estimates. The 

pre-1880 extent of irrigated agriculture was determined on the 

basis of irrigation ditch service areas, which were delimited as 

part of the SPDSS (Riverside Technology, Inc., 2005). Recharge 

to the pre-1880 irrigation ditch service areas downstream from 

Denver (fig. A46) were assigned a constant irrigation recharge 

rate of 10 in/yr on the basis of preliminary results from irrigation return-flow studies (L.R. Arnold, U.S. Geological Survey, 

oral commun., 2005). Total recharge in irrigated areas is equal 

to precipitation recharge plus the assigned irrigation recharge.

The simulated recharge distribution for predevelopment 

conditions (fig. A47) depicts the salient recharge features in 

the basin. The variation in permeability of geologic materials 

at the land surface, greater precipitation at higher altitudes, 

and the application of irrigation recharge are all factors that 

conceptually are expected to affect the spatial distribution 

of recharge in the Denver Basin and are represented in the 

groundwater flow model. Just as in the hydrologic system, 

simulated recharge is applied to the model layer that occurs 

at land surface, so that the variation in aquifer hydraulic conductivity at the land surface affects recharge rates. To represent this conceptual spatial variation of infiltration potential, 

the percentage of precipitation reaching the water table as 

recharge was an estimated model parameter for each bedrock 

aquifer during model calibration. The results indicate that of 

the bedrock aquifers, the Dawson aquifers and confining units 

receive the greatest percentage of precipitation (2 percent, 0.3 

to 0.5 in/yr), followed by the Arapahoe and Laramie–Fox Hills 

aquifers and confining units (1.4 percent, 0.2 to 0.4 in/yr), and 

the Denver aquifer and confining units (1.1 percent, 0.1 to 

0.3 in/yr). The model-estimated percentages and the resulting simulated distribution of recharge are consistent with the 

conceptual understanding of recharge in the basin. Recharge 

rates to the Dawson units are greater than for the other bedrock units because the Dawson units are composed of coarsegrained materials and crop out in the upland parts of the basin 

where precipitation is greatest and the overlying alluvial aquifer is sparse (allowing direct recharge to the bedrock units).

In the simulated system, the alluvial aquifer was assigned 

a recharge rate equal to 7 percent of precipitation, assuming alluvial aquifers are more permeable at the land surface 

than bedrock aquifers, which enables a larger percentage of 

precipitation to infiltrate the alluvial aquifer as compared to 

the bedrock aquifers. The resulting simulated recharge rates 

for the alluvial aquifer are thus greater than for the bedrock 

aquifers and range from 0.75 to 1.3 in/yr for predevelopment 

conditions outside the areas of irrigated agriculture (fig. A47). 

Irrigated agriculture areas in the South Platte River valley 

and the alluvial aquifer are located in proximity to streams 

such that recharge to the water table from agricultural return 

flows is primarily to the alluvial aquifer. Recharge to aquifers 
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 61

Figure A43. Predevelopment (pre-1880) groundwater budget for the Denver Basin aquifer system.

underlying irrigated agriculture is equal to the assigned 

recharge from irrigated recharge (10 in/yr) plus the percentage 

of precipitation recharge for the underlying units and ranges 

from 10 to 11.1 in/yr.

Evapotranspiration from groundwater is the primary 

process for natural discharge in the Denver Basin. Potential 

pan evaporation values near the Denver Basin range from 45 

to 48 in/yr (National Oceanic Atmospheric Administration 

[NOAA], 2004), greatly exceeding precipitation and estimated recharge rates. Measured pan evaporation data for three 

NOAA weather stations in eastern Colorado (National Oceanic 

Atmospheric Administration, 2004) indicate that evapotranspiration begins in March of each year, peaks in July at an average maximum value of approximately 10 inches per month, 

declines through the late summer and falls to zero, and remains 

at zero during the winter, when vegetation is dormant (fig. A48). 

Total average evaporation calculated on the basis of the three 

weather stations near the Denver Basin is 55.6 in/yr. These 

results indicate a potential for evapotranspiration from the 

Denver Basin aquifers in areas where the water table is shallow and phreatophytes grow. Because the climate is semiarid, 

potential also exists for evaporation to occur in nonriparian 

and nonirrigated areas, if the water table is sufficiently near 

land surface. Few evapotranspiration studies and data are 

available for native vegetation in the Denver Basin; however, 

studies from a similar climatic setting in the San Luis Valley 

of Colorado indicate evapotranspiration is a function of depth 

to water and suggest mathematical functions to describe the 

relation (David Groeneveld and Michael Prescott, HydroBio, 

Inc., written commun., 2006). Results from the San Luis Valley 

were used as a basis for the relation of groundwater evapotranspiration as a function of depth to water for the Denver Basin 

model (section “Hydrologic Stresses”, “Evapotranspiration” in 

Chapter C).

The simulated spatial distribution of groundwater 

evapotranspiration for predevelopment conditions (fig. A49) 

represents groundwater discharge from the water table to 

evapotranspiration in areas where the water table is less than 

30 ft below land surface. The map of evapotranspiration rates 

indicates small predevelopment evapotranspiration rates (less 

than 0.5 in/yr) over most of the basin (fig. A49). Evapotranspiration rates in the range of 25 to 45 in/yr were simulated 

for areas along stream valleys and in the areas of irrigated 

agriculture along the South Platte River (fig. A49). Although 

evapotranspiration rates are small over most of the basin, large 

volumes of water are discharged from the aquifers by evapotranspiration because of the large area over which evapotranspiration occurs, making evapotranspiration a primary outflow 

component of the groundwater budget.

The difference between recharge and evapotranspiration (net recharge) for predevelopment conditions is shown in 

figure A50. The predevelopment net recharge rate over much 

of the area where Denver Basin bedrock aquifers are exposed 

at land surface is less than 0.5 in/yr, while net recharge in 

most alluvial-aquifer areas is in the range of 0.5 to 2 in/yr 

(fig. A50). In areas of irrigated agriculture along the South 

Platte River, net recharge ranges from 10 to 11.1 in/yr. In 

Precipitation 

plus irrigation

recharge

(176 cubic feet

per second)

Evapotranspiration

(135 cubic feet

per second)

Stream 

and spring 

outflow

(19 cubic feet

per second)

Stream

inflow

(13 cubic feet

per second)

Flow from 

bedrock units

(34 cubic feet

per second)

Flow from

alluvial aquifer

(8 cubic feet

per second)

Bedrock

units

Precipitation 

plus irrigation

recharge

(454 cubic feet

per second)

Evapotranspiration

(307 cubic feet

per second)

Stream

inflow

(3 cubic feet 

per second)

Stream 

and spring

outflow

(167 cubic feet

per second) Groundwater flow 

out of basin

(19 cubic feet

per second)

Groundwater flow 

out of basin

(19 cubic feet

per second)

Precipitation 

plus irrigation

recharge

(630 cubic feet 

per second)

Evapotranspiration

(442 cubic feet

per second)

DENVER BASIN AQUIFER SYSTEM

Alluvial

aquifer

Stream

inflow

(16 cubic feet

per second)

Stream 

and spring

outflow

(186 cubic feet

per second)
62 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A44. Predevelopment (pre-1880) groundwater budget 

inflow and outflow components for the entire Denver Basin 

aquifer system.

some areas of the basin, primarily in agricultural recharge 

areas overlying the alluvial aquifer in the South Platte River 

valley, evapotranspiration exceeds recharge from precipitation 

plus irrigation such that net discharge results (represented as a 

negative net recharge value in figure A50).

Predevelopment recharge, evapotranspiration, and 

net recharge rates for individual aquifers (tables A7, A8; 

figs. A51A, A51B) indicate 78 percent of the net recharge was 

to the alluvial aquifer (147 ft3/s), while 22 percent of the net 

recharge was directly to bedrock units (41 ft3/s). Net recharge 

to individual bedrock aquifers and confining units was as follows: 9 percent (16 ft3

/s) to the Dawson aquifer and confining 

units, 7 percent (14 ft3/s) to the Denver units, 5 percent (9 ft3/s) 

to the Laramie–Fox Hills units, and 1 percent (2 ft3/s) to the 

Arapahoe units (fig. A51B). These values are generally consistent with previous studies (for example, Robson, 1987), which 

indicate that the Dawson units receive the greatest recharge 

because precipitation is greatest over their outcrop areas. The 

Arapahoe units receive the least recharge because of their limited outcrop area. However, because the present study includes 

the effects of predevelopment irrigation recharge in the South 

Platte River basin, and some of the irrigated areas overlie the 

Denver aquifer and confining units, results from the present 

study indicate predevelopment recharge to the Denver units 

was nearly equal to that for the Dawson units.

Groundwater/Surface-Water Interaction

Groundwater from bedrock aquifers and confining units 

interacts with streams and the alluvial aquifer. There also is 

hydraulic connection between streams and the alluvial aquifer 

where streams overlie alluvium. Surface water in the Denver 

Basin flows in perennial streams that originate as snowmelt 

runoff from the Front Range and in ephemeral and intermittent streams that originate on the plains east of the mountains. 

Conceptually, ephemeral streams that originate in the basin 

gain flow in the upland areas near the Palmer Divide where 

alluvium is thin or absent and bedrock groundwater discharges 

directly to streams. In downstream parts of the basin where 

the alluvial aquifer is present, ephemeral streams can lose 

water to the alluvial aquifer as topographic gradients flatten, 

and streambeds can be dry. Bedrock groundwater also can 

discharge to the overlying alluvial aquifer. Simulation results 

for predevelopment groundwater flow to and from streams 

(fig. A52) are consistent with this conceptual understanding. 

Negative flow values displayed in figure A52 (pale yellow and 

blue colors) represent areas where groundwater discharges to 

streams, and positive flow values (red colors) represent areas 

where groundwater receives streamflow (streams lose water 

to the groundwater system). Simulated rates of groundwater 

flow to streams are less than 0.5 ft3/s in upland areas near the 

Palmer Divide where the alluvial aquifer is absent (fig. A52), 

and groundwater discharge to streams is directly from bedrock units. Streamflow loss (maximum rate of 0.8 ft3

/s) to the 

groundwater system is evident in downstream areas underlain 

by the alluvial aquifer (fig. A52). There is little to no observed 

or simulated perennial streamflow in downstream reaches of 

Running/Box Elder, Kiowa, and Bijou Creeks (fig. A52). Predevelopment streamflow gains from groundwater flow also are 

simulated for the main stem of the South Platte River, again 

consistent with the conceptual understanding of historical and 

regional conditions (fig. A52).

The predevelopment net flow rate to streams from the 

bedrock aquifers and confining units (about 14 ft3/s or 9 percent 

of the net groundwater flow to streams) was small compared 

INFLOW

NETFLOW

OUTFLOW

Inflow

Outflow

Streams

16 cubic feet per second

(2 percent of inflow)

Recharge

630 cubic feet per second

(98 percent of inflow)

Springs 

4 cubic feet per second

(<1 percent of outflow)

Groundwater flow

out of basin

19 cubic feet per second

(3 percent of outflow)

Streams

182 cubic feet 

per second

(28 percent 

of outflow)

Evapotranspiration

442 cubic feet per second

(68 percent of outflow)

Net recharge

189 cubic feet per second

(100 percent of net inflow)

Groundwater flow

out of basin

19 cubic feet per second

(10 percent of net outflow)

Springs 

4 cubic feet per second

(2 percent of net outflow)

Net to streams

166 cubic feet per second

(88 percent of net outflow)

ALLUVIAL AQUIFER AND BEDROCK UNITS
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 63

Figure A45. Predevelopment (pre-1880) groundwater budget inflow and outflow components of the Denver 

Basin alluvial aquifer and bedrock units.

to discharge to streams from the alluvial aquifer (151 ft3/s or 

91 percent of net flow to streams; figs. A53A, A53B). About 

one-half of the net flow to streams directly from bedrock units 

was from the Dawson aquifer and confining units (7.6 ft3/s or 

53 percent of the bedrock net outflow to streams; fig. A53A) 

located where land-surface altitude and precipitation recharge 

are greatest. These results indicate that groundwater/surfacewater interaction is predominantly between the alluvial aquifer 

and streams, with lesser direct groundwater/surface-water interaction between the bedrock units and streams.

Discharge to Springs

Numerous springs and seeps occur in the Denver Basin 

where alluvial and bedrock groundwater discharges to surface 

water. Springs generally are contact springs located along topographic breaks in slope where coarse-grained materials overlie finer grained shales and(or) claystones (Richardson, 1915). 

Approximately 150 springs have been mapped in the Denver 

Basin, most of which are in the southern one-half of the basin 

and are associated with discharge from the bedrock aquifers 

Recharge

176 cubic feet per second

(94 percent of inflow)

Streams

13 cubic feet per second

(3 percent of inflow)

Streams

4 cubic feet per second

(2 percent of inflow)

Flow from 

bedrock units

34 cubic feet per second

(7 percent of inflow)

Flow from 

alluvial aquifer

8 cubic feet per second

(4 percent of inflow)

Streams

14 cubic feet per second

(34 percent of net outflow)

Streams

18 cubic feet per second

(10 percent of outflow)

Flow from 

bedrock units

26 cubic feet per second

(15 percent of net inflow)

Groundwater 

flow out of basin

19 cubic feet per second

(11 percent of net outflow)

Springs

3 cubic feet per second

(2 percent of net outflow)

Springs

3 cubic feet per second

(<1 percent of outflow)

Springs

1 cubic feet per second

(<1 percent of outflow)

Springs

1 cubic feet per second

(3 percent of net outflow)

Flow to

alluvial aquifer

26 cubic feet per second

(63 percent of net outflow)

Groundwater

flow out of basin

19 cubic feet per second

(4 percent of outflow)

Evapotranspiration

135 cubic feet per second

(72 percent of outflow)

Flow to

bedrock units

8 cubic feet per second

(2 percent of outflow)

Flow to

alluvial aquifer

34 cubic feet per second

(18 percent of outflow)

Net recharge

42 cubic feet per second

(100 percent of net inflow)

Recharge

454 cubic feet per second

(90 percent of inflow)

Streams

164 cubic feet per second

(33 percent of outflow)

Streams

151 cubic feet per second

(87 percent of net outflow)

Net recharge

147 cubic feet per second

(85 percent of net inflow)

Evapotranspiration

307 cubic feet per second

(61 percent of outflow)

INFLOW

NETFLOW

OUTFLOW

Inflow

Outflow

ALLUVIAL AQUIFER BEDROCK UNITS
64 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A46. Estimated areas of predevelopment (pre-1880) agricultural irrigation as simulated in 

the Denver Basin model.

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Creek

Steels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

PALMER

DIVIDEDIVIDE

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Area of simulated agricultural irrigation, pre-1880–1906

Agricultural ditch service areas, 1880

Alluvial aquifer extent

Extent of active model cells

Laramie–Fox Hills aquifer simulated extent (model layer 12)

Lower Arapahoe aquifer simulated extent (model layer 10)

Denver aquifer simulated extent (model layer 6)

Lower Dawson aquifer simulated extent (model layer 4)

Stream

Perennial

Ephemeral
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 65

Figure A47. Spatial distribution of simulated predevelopment (pre-1880) precipitation plus 

irrigation recharge for the Denver Basin modeled area.

PALMER

DIVIDEDIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryCr

Plum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

§ ̈¦70

§ ̈¦25

§ ̈¦76

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Pre-1880 simulated precipitation plus

 irrigation recharge, in inches per year

0.13 to 0.2

0.2 to 0.3

0.3 to 0.4

0.4 to 0.5

0.5 to 0.75

0.75 to 1

1.0 to 1.3

10 to 11.1

Laramie–Fox Hills aquifer simulated extent (model layer 12)

Lower Arapahoe aquifer simulated extent (model layer 10)

Denver aquifer simulated extent (model layer 6)

Lower Dawson aquifer simulated extent (model layer 4)

Alluvial aquifer simulated extent (model layer 1)

Stream

Perennial

Ephemeral
66 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A48. Monthly average measured evaporation for three 

NOAA weather stations near the Denver Basin.

(fig. A11). There are few historical flow measurements for 

springs, although Richardson (1915) reported flow of as much 

as 125 gallons per minute from a spring in the upper Cherry 

Creek basin. In general, simulated flow to springs was a small 

percentage (4 ft3/s or 2 percent of the net outflow; table A7) of 

the predevelopment outflow from the basin.

Interaquifer Flow

Interaquifer flow, the amount of flow between aquifers, 

is an important component of water exchange for confined 

portions of the Denver Basin aquifers because of their limited 

outcrop at the land surface and thus limited recharge (Robson, 

1987; this study). The lower potentiometric altitude in the 

underlying bedrock aquifers promotes downward water movement through bedrock aquifers and confining units. The rate of 

downward vertical movement is considered small compared to 

lateral movement in the bedrock aquifers because of horizontal 

bedding planes; however, the large area over which downward 

water movement occurs results in large volumes of water moving between aquifers (Robson, 1987; this study). An exception 

is the Laramie confining unit, which effectively separates the 

underlying Laramie–Fox Hills aquifer from the overlying layers because of its large thickness and small hydraulic conductivity (Robson, 1987; fig. A15). For predevelopment conditions, interaquifer flow between all bedrock units is downward 

with flow to underlying aquifers (table A8; fig. A54). The 

greatest downward flow is from the Denver aquifer (about 

14 ft3/s net flow) and Denver lower confining unit (about 

16 ft3/s net flow) into the Arapahoe aquifers (fig. A54) because 

the Denver lower confining unit is generally thin (less than 

50 ft thick) over much of its extent (fig. A25).

Groundwater Storage

The quantity of groundwater in storage can be calculated 

as the product of the specific yield of an aquifer and aquifer volume plus the confined storage available from aquifer 

and water compressibility. In the Denver Basin, the quantity 

of groundwater in unconfined bedrock-aquifer storage was 

previously estimated by multiplying the SB85–5 specific-yield 

values (table A9) times the volume of aquifer materials, and 

the result is an estimate of 295 million acre-ft of groundwater 

in storage (Colorado Division of Water Resources, 2004). An 

estimate of 259 million acre-ft of theoretically recoverable 

groundwater is cited by the South Metro Water Supply Study 

Board (written commun., 2004). However, many investigators, including Alley (2006, 2007), have pointed out the large 

uncertainty and possible misleading interpretations associated with estimating total recoverable groundwater in this 

manner because it is impossible to remove all groundwater 

from storage with pumping wells. While estimates of groundwater in storage are speculative, digital representation of the 

aquifer framework does facilitate storage calculations for the 

Denver Basin. This study estimated the volume of unconfined bedrock-aquifer storage as the product of aquifer area, 

simulated predevelopment (pre-1880) saturated thickness, 

and the spatial distribution of specific yield as presented in 

figures A37–A42. The calculation, which accounts for spatial 

heterogeneity of specific yield as a function of the silt-plussand aquifer fraction, results in about 247 million acre-ft of 

groundwater in unconfined storage, which is about 18 percent 

less than the SB85–5 estimate (table A9). About 45 million 

acre-ft of unconfined groundwater storage, or about 18 percent 

or the basin unconfined storage, underlies the south Denver 

metropolitan area (fig. B3). The section “Change in Aquifer 

Storage” in Chapter B discusses aquifer-storage depletion in 

terms of storage-loss rates and the cumulative amount of water 

removed from storage over the model-calibration period.

Predevelopment Hydraulic Heads and 

Groundwater Flow Directions

Potentiometric-surface maps illustrate the distribution of hydraulic heads in an aquifer using contour lines of 

equal hydraulic head. Groundwater flows from areas of high 

hydraulic head to areas of low hydraulic head, in a direction 

generally perpendicular to contours of equal hydraulic head 

on the potentiometric-surface maps. The groundwater flow 

model calculates heads at the center of every active model 

cell, which represent the average altitude of the potentiometric 

surface in the cell. Contour lines of equal hydraulic head 

prepared from simulated heads for each model cell compose 

a potentiometric-surface map for each aquifer layer in the 

Denver Basin model.

The extent, depth, thickness, and aquifer properties of the 

geologic units, the climate and resulting effects on the rate and 

location of recharge, and the location and altitude of surface 

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Average evapotranspiration, in inches

Month

Station AKRON 4 E

Station FORT COLLINS

Station WIGGINS 7 SW

Average of 3 stations

EXPLANATION
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 67

Figure A49. Spatial distribution of simulated predeveopment (pre-1880) evapotranspiration for the 

Denver Basin modeled area.

PALMER

DIVIDEDIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryCr

Plum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Creek

Steels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

§ ̈¦70

§ ̈¦25

§ ̈¦76

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Pre-1880 simulated evapotranspiration, 

 in inches per year

–45 to –25

–25 to –10

–10 to –5

–5 to –2

–2 to –1

–1 to –0.5

–0.5 to 0

0

Laramie–Fox Hills aquifer simulated extent (model layer 12)

Lower Arapahoe aquifer simulated extent (model layer 10)

Denver aquifer simulated extent (model layer 6)

Lower Dawson aquifer simulated extent (model layer 4)

Alluvial aquifer simulated extent (model layer 1)

Stream

Perennial

Ephemeral
68 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A50. Spatial distribution of simulated predevelopment (pre-1880) recharge minus 

evapotranspiration for the Denver Basin modeled area.

PALMER

DIVIDEDIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek

West P ul m Creek

Bear Creek

Running Creek

Dry Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

§ ̈¦70

§ ̈¦25

§ ̈¦76

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Pre-1880 simulated recharge minus

 evapotranspiration, in inches per year

–44 to –25

–25 to –10

–10 to –3

–3 to 0

0

0 to 0.5

0.5 to 1

1 to 2

2 to 10

10 to 11

Laramie–Fox Hills aquifer simulated extent (model layer 12)

Lower Arapahoe aquifer simulated extent (model layer 10)

Denver aquifer simulated extent (model layer 6)

Lower Dawson aquifer simulated extent (model layer 4)

Alluvial aquifer simulated extent (model layer 1)

Stream

Perennial

Ephemeral
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 69 Table A8. Simulated predevelopment (pre-1880) groundwater budget for individual bedrock aquifer and confining units in the Denver Basin. [In some cases values do not sum because of rounding; all flow rates are in cubic feet per second; positive values indicate inflow to groundwater; negative values indicate outflow from groundwater] Groundwater budget component Upper Dawson aquifer Dawson confining unit Lower Dawson aquifer Denver upper confining unit Denver aquifer Denver lower confining unit Upper Arapahoe aquifer Arapahoe confining unit Lower Arapahoe aquifer Laramie confining unit Laramie– Fox Hills aquifer Inflow Recharge (precipitation + irrigation) 18.3 1.3 14.9 2.1 18.3 8.7 9.6 2.4 10.6 37.4 52.6 Stream leakage 0.15 0.0 0.65 0.07 2.1 0.03 0.006 0.01 0.17 0.25 0.06 Inflow from alluvial aquifer 0.8 0.0 0.01 0.01 1.1 0.10 1.3 0.01 0.27 0.98 4.0 Inflow from overlying bedrock unit 0.0 3.4 3.7 5.8 6.4 14.2 16.1 1.9 2.2 2.5 7.2 Storage change 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Total inflow 19.3 6.4 20.3 8.5 31.1 29.8 35.9 14.3 18.6 42.4 64.8 Outflow Evapotranspiration –8.9 –0.5 –8.8 –1.1 –8.4 –5.9 –8.4 –2.0 –9.9 –33.1 –47.5 Stream leakage –4.3 –0.41 –3.7 –0.53 –3.0 –0.13 –2.1 –0.01 –0.32 –0.42 –2.6 Outflow to alluvial aquifer –1.6 –0.001 –0.6 0.0 –2.2 –0.80 –14.6 –0.02 –0.62 –0.36 –13.3 Outflow to underlying bedrock unit –3.4 –3.7 –5.8 –6.4 –14.2 –16.1 –1.9 –2.2 –2.5 –7.2 0.0 Springs –0.08 –0.08 –0.33 –0.02 –0.02 –0.05 –0.06 0.0 0.0 –0.003 –0.39 Wells 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Storage change 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Total outflow –19.3 –6.4 –20.3 –8.5 –31.1 –29.8 –35.9 –14.3 –18.6 –42.4 –64.8 Net flows Net recharge (recharge + evapotranspiration) 9.4 0.8 6.1 1.0 9.8 2.8 1.2 0.4 0.7 4.3 5.1 Net stream leakage (stream leakage inflow + stream leakage outflow) –4.2 –0.41 –3.0 –0.46 –0.9 –0.1 –2.2 0.0 –0.15 –0.17 –2.5 Net to alluvial aquifer –0.8 0.0 –0.6 0.0 –1.1 –0.7 –13.3 –0.01 –0.35 –0.11 –9.3
70 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A51. Simulated (A) predevelopment (pre-1880) recharge, evapotranspiration, and net recharge for the Denver Basin alluvial 

aquifer and individual bedrock units, and (B) net recharge for the Denver Basin alluvial aquifer and individual bedrock units.

drainages control the predevelopment distribution of hydraulic 

heads in the Denver Basin aquifers. Potentiometric-surface 

maps for predevelopment conditions for the seven aquifers 

show areas of high hydraulic head near the recharge area of the 

Palmer Divide and areas of low hydraulic head near discharge 

areas (figs. A55–A61). Groundwater flows downgradient from 

the Palmer Divide toward streams in all directions. Groundwater 

moves laterally through the permeable sandstones and alluvial 

454 176 34 29 23 90

–307 –134 –18 –15 –20 –81

147

42 16 14

3

9

–500

–400

–300

–200

–100

0

100

200

300

400

500

Alluvial

aquifer

Total for

bedrock units

Dawson

units

Denver

units

Arapahoe

units

Laramie−Fox Hills

units

–250

–200

–150

–100

–50

0

50

100

150

200

250

Flow rate, in cubic feet per second

Hydrogeologic unit

Net flow rate, in cubic feet per second

Recharge

Evapotranspiration

Net recharge

A

EXPLANATION

materials from areas of recharge toward areas of discharge on 

both a local and regional scale. On a local scale, water moves 

from upland recharge areas through the near-surface aquifer to 

discharge areas in nearby stream valleys. On a regional scale, 

bedrock groundwater moves laterally and downward from the 

topographic high of the Palmer Divide in the south-central part 

of the basin and ultimately discharges to alluvial aquifers or 

streams (Robson, 1987). 

Alluvial aquifer

147 cubic feet per second

(78 percent of net recharge)

Arapahoe units

3 cubic feet per second

(1 percent of net recharge)

Laramie–Fox Hills units

9 cubic feet per second

(5 percent of net recharge)

Denver units

14 cubic feet per second

(7 percent of net recharge) Dawson units

16 cubic feet per second

(9 percent of net recharge)

Total for bedrock units

42 cubic feet per second

(22 percent of net recharge)

B
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 71

Figure A52. Spatial distribution of simulated predevelopment (pre-1880) groundwater flow to and 

from streams for the Denver Basin modeled area.

PALMER

DIVIDEDIVIDE

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

§ ̈¦70

§ ̈¦25

§ ̈¦76

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Pre-1880 groundwater flow to and from 

 streams, in cubic feet per second

–3.4 to –2

–2 to –1

–1 to –0.5

–0.5 to 0

0

0 to 0.5

0.5 to 0.8 Laramie–Fox Hills aquifer simulated extent (model layer 12)

Lower Arapahoe aquifer simulated extent (model layer 10)

Denver aquifer simulated extent (model layer 6)

Lower Dawson aquifer simulated extent (model layer 4)

Alluvial aquifer simulated extent (model layer 1)

Stream

Perennial

Ephemeral
72 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A53. Simulated (A) predevelopment (pre-1880) groundwater flow to and from streams for the Denver Basin alluvial aquifer and 

individual bedrock units, and (B) net flow to streams from the Denver Basin alluvial aquifer and individual bedrock units.

Groundwater flow in the alluvial aquifer is downvalley 

to the north and east in alluvium of the South Platte River 

and tributaries and to the south and southeast in alluvium of 

Arkansas River Basin tributaries. The simulated predevelopment potentiometric surface for the alluvial aquifer (fig. A55) 

generally follows topography and locally is controlled by 

stream-channel elevations.

Groundwater in the bedrock aquifers and confining units 

flows laterally away from the Palmer Divide and downward into 

underlying bedrock units (Robson, 1987). Flow in the Dawson 

and Denver aquifers away from the Palmer Divide recharge 

area is toward discharge areas of Plum and Cherry Creeks to 

the northwest, Monument and Black Squirrel Creeks to the 

south, and Box Elder, Bijou, Kiowa, and Big Sandy Creeks on 

A

12.7 3.5 0.8 2.2 0.2 0.3

–164.2 –17.6 –8.4 –3.7 –2.5 –3.1

–14.2

–7.6

–1.5 –2.3 –2.8

–151.5

Alluvial

aquifer

Total for 

bedrock units

Dawson

units

Denver

units

Arapahoe

units

Laramie−Fox Hills

units

Hydrogeologic unit

–200

–150

–100

–50

0

50

Flow rate, in cubic feet per second

–200

–150

–100

–50

0

50

Net flow rate, in cubic feet per second

Streams inflow

Streams outflow

Net to streams

EXPLANATION

Alluvial aquifer

–152 cubic feet per second

 (91 percent of net to streams)

Dawson units

–8 cubic feet per second

(5 percent of net to streams)

Denver units

–2 cubic feet per second

(1 percent of net to streams)

Arapahoe units

–2 cubic feet per second

(1 percent of net to streams) Total for bedrock units

–14 cubic feet per second

(9 percent of net to streams)

Laramie–Fox Hills units

–3 cubic feet per second

(2 percent of net to streams)

B
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 73

Figure A54. Simulated predevelopment (pre-1880) interaquifer flow for individual Denver Basin bedrock units.

the north and east (figs. A56–A58). Groundwater also moves 

downward to underlying bedrock aquifers from the Dawson and 

Denver aquifers because of the downward hydraulic gradients 

between aquifers (Robson, 1987; section “Interaquifer Flow” 

in this chapter). Predevelopment potentiometric surfaces for the 

Denver, upper Arapahoe, lower Arapahoe, and Laramie–Fox 

Hills aquifers (figs. A59–A61) depict low hydraulic head (potentiometric-surface troughs) along the South Platte River, Bijou 

Creek, Big Sandy Creek, Black Squirrel Creek, and Monument 

Creek, which are areas of regional discharge to streams or alluvial aquifers where the sandstones lie near land surface.

Prior to about 1885, wells completed in the Denver, 

Arapahoe, and Laramie–Fox Hills aquifers in the Denver area 

typically flowed at land surface (Robson, 1987; Cross and 

others 1884; Emmons and others, 1896). However, hydraulic 

heads rapidly declined within a few years of groundwater 

development. Emmons and others (1896) reported water levels 

20 to 50 ft below land surface in the Denver area by 1894, and 

Darton (1905) reported that heads in the Denver and Arapahoe 

aquifers were 50 to 200 ft below land surface in the Denver 

area by 1905, requiring that water be pumped to the surface 

(Robson, 1987). Outside of the Denver area, two flowing wells 

were reported in the Plum Creek drainage in the early 1900s 

(Richardson, 1915).

Groundwater in Denver Basin bedrock aquifers is, in general, confined where overlain by less-permeable claystone and 

shale confining units and unconfined in the near-surface parts 

of the bedrock aquifers and in the alluvial aquifer (Robson, 

1987). Predevelopment confined and unconfined conditions 

for the Denver (fig. A58), lower Arapahoe (fig. A60), and 

Laramie–Fox Hills aquifers (fig. A61) were tabulated from 

model results and generally are consistent with the conceptual 

understanding of conditions described by Robson (1987). 

Areas of confined groundwater were interpreted where the 

calculated potentiometric head was higher than the base 

altitude of the overlying confining layer. Areas of unconfined 

groundwater were interpreted where the calculated potentiometric head was lower than the base altitude of the overlying 

confining layer or where the overlying confining unit is absent. 

Unconfined parts of all three aquifers are located around the 

margins of the aquifer extents. Water released from storage in 

confined parts of the aquifer system is from elastic compressibility of the rocks and water and is expressed as the aquifer 

storage coefficient. Water released from storage in unconfined 

parts of the aquifer system is from that portion of the rock 

porosity drained by gravity flow and is represented by the 

specific yield. In general, greater volumes of water can be 

extracted from unconfined storage than from confined storage.

–25

–20

–15

–10

–5

0

5

10

15

Flow rate, in cubic feet per second

Upper

Dawson

aquifer 

Dawson

confining

unit

Lower

Dawson

aquifer 

Denver upper

confining

unit

Denver 

aquifer

Upper

Arapahoe

aquifer

Arapahoe

confining

unit

Lower

Arapahoe

aquifer

Laramie

confining

unit

Denver lower 

confining

unit

Hydrogeologic unit

–25

–20

–15

–10

–5

0

5

10

15

Net flow rate, in cubic feet per second

0.8 0.9 0.2 0.3 2.9 3.9 5.0 5.0 0.3 1.0

–4.2 –4.5 –6.1 –6.7

–17.1 20.0

–6.9 –7.3

–2.7

–8.2 –3.4 –3.7

–5.8 –6.4

–14.2

–16.1

–1.9 –2.3 –2.5

–7.2

Outflow to underlying unit

Inflow from underlying unit

Net to underlying unit

EXPLANATION
74 Groundwater Availability of the Denver Basin Aquifer System, Colorado Table A9. Estimated unconfined groundwater storage for individual bedrock aquifers in the Denver Basin. Aquifer Model layer Senate Bill 85–5 specific-yield assigned value Denver Basin model specific-yield assigned value (this study) Percent difference of Senate Bill 85–5 specific-yield assigned value from this study Senate Bill 85–5 estimated storage volume (millions of acre-feet) Denver Basin model estimated storage volume using spatial distribution of specific-yield values (figs. A37–A42; millions of acre-feet) Percent difference of Senate Bill 85–5 estimated storage volume from this study Denver Basin model estimated storage volume in the south Denver metropolitan area (millions of acre-feet) Percentage of storage volume in the south Denver metropolitan area Upper Dawson 2 20 15.2 –27 22 5 –121 1.9 35 Lower Dawson 4 20 15.2 –27 10 16 46 4.1 26 Denver 6 17 13.3 –24 71 44 –46 12.9 29 Upper Arapahoe 8 17 17.8 5 12 11 –12 2.1 20 Lower Arapahoe 10 17 17.8 5 78 46 –51 9.3 20

Laramie–Fox Hills 12 15 18.6 21 102 125 20 14.3 11Total 295 247 –18 45 18
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 75

Figure A55. Simulated predevelopment (pre-1880) potentiometric surface and groundwater flow 

directions for the alluvial aquifer (model layer 1).

PALMER

DIVIDEDIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr

Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Creek

Steels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

Black Squirrel Cr

§ ̈¦70

§ ̈¦25

§ ̈¦76

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

6,100

5,900

6,200

6,000

6,300

6,500

6,400

4,500

4,700

5,800

4,800

4,400

5,000

5,700

4,900

4,600

5,200

5,100

4,900

6,200

5,000

4,900

5,100

4,600

4,800

4,600

5,800

6,000

5,900

4,800

6,400

6,000

4,600

6,300

4,700

5,000

4,700

4,900

6,100

5,900

6,200

6,000

6,300

6,500

6,400

4,500

4,700

5,800

4,800

4,400

5,000

5,700

4,900

4,600

5,200

5,100

4,900

6,200

5,000

4,900

5,100

4,600

4,800

4,600

5,800

6,000

5,900

4,800

6,400

6,000

4,600

6,300

4,700

5,000

4,700

4,900

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879. 

 Datum NAVD88. Contour interval 100 feet

Perennial

Ephemeral

Stream

Generalized groundwater flow direction

Alluvial aquifer simulated extent (model layer 1)

EXPLANATION

Extent of active model cells
76 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A56. Simulated predevelopment (pre-1880) potentiometric surface and groundwater flow 

directions for the upper Dawson aquifer (model layer 2).

PALMER

DIVIDEDIVIDE

Coal Creek

Clear Creek

West Sand Creek

First Cree

Sa

k

nd Creek

Little DryCr

Plum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Antelope Cr

Steels Fork

Horse Creek

West Bijou Cree k

Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr

Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

§ ̈¦25

6,700

6,600

6,800

6,900

6,400

6,300

7,000

6,100

6,000

6,200

7,100

5,900

6,800

6,700

6,600

6,800

6,900

6,400

6,300

7,000

6,100

6,000

6,200

7,100

5,900

6,800

105° 104°30'

39°30'

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 5 10 MILES

0 5 10 KILOMETERS

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879.

 Datum NAVD88. Contour interval 100 feet

Generalized groundwater flow direction

EXPLANATION

Stream

Perennial

Ephemeral

Extent of active model cells

Upper Dawson aquifer simulated extent (model layer 2)
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 77

Figure A57. Simulated predevelopment (pre-1880) potentiometric surface and groundwater flow 

directions for the lower Dawson aquifer (model layer 4).

PALMER

DIVIDE

Coal Creek

Clear Creek

West Sand Creek

First Cree

Sa

k

nd Creek

Little DryCr

Plum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Antelope Cr

Steels Fork

Horse Creek

West Bijou Cree k

Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr

Cherry Cr

Kettle Cr

Cook Creek Dry Cr

§ ̈¦25

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

EL PASO

ELBERT

ADAMS

ARAPAHOE

TELLER

JEFFERSON

DENVER

BROOMFIELD

DOUGLAS

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

Kiowa

Parker

Golden

Calhan

Denver

Bennett

Morrison

Lakewood

Monument

Franktown

Elizabeth

Broomfield

Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Englewood

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

! !

!

6,100

6,200

6,300

6,400

6,500

6,600

6,700

6,800

6,000

6,900

7,000

7,100

5,900

5,800

7,200

7,300

5,700

7,400

7,500

5,600

6,600

6,700

6,800

5,600

6,000

5,900

5,900

6,700

6,900

6,800

5,800

7,400

6,100

6,200

6,300

6,400

6,500

6,600

6,700

6,800

6,000

6,900

7,000

7,100

5,900

5,800

7,200

7,300

5,700

7,400

7,500

5,600

6,600

6,700

6,800

5,600

6,000

5,900

5,900

6,700

6,900

6,800

5,800

7,400

6,900

105° 104°30'

39°30'

39°

0 5 10 MILES

0 5 10 KILOMETERS

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879.

 Datum NAVD88. Contour interval 100 feet

Generalized groundwater flow direction

EXPLANATION

Stream

Perennial

Ephemeral

Extent of active model cells

Lower Dawson aquifer simulated extent (model layer 4)
78 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A58. Simulated predevelopment (pre-1880) potentiometric surface, groundwater flow directions, 

and calculated areas of predevelopment confined and unconfined conditions for the Denver aquifer 

(model layer 6).

PALMER

DIVIDEDIVIDE

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryCr

Plum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

§ ̈¦70

§ ̈¦25

§ ̈¦76

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

5,800

5,700

5,600

5,400

5,500

5,500

5,400

5,700

5,600

5,300

5,500

5,200

5,100

5,500

5,800 5,500

6,100

5,900 6,000

6,400

6,200

6,500

6,600

6,900

6,700

6,100

6,300

6,000

6,200

5,900 6,000

6,400

6,200

6,500

6,600

6,800

7,000

7,200

6,400

6,300

6,200

6,800

6,900

6,700

7,000

7,200

6,200

7,100

7,300

7,500

6,700

6,400 6,200

5,500

6,300

6,000

6,400

5,400

6,200

6,200

7,100

7,300

7,500

6,700

6,400

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879.

 Datum NAVD88. Contour interval 100 feet

Generalized groundwater flow direction

EXPLANATION

Stream

Perennial

Ephemeral Extent of active model cells

Denver aquifer simulated extent (model layer 6)

Denver upper confining unit simulated extent (model layer 5)

Simulated extent of confined conditions

Simulated extent of unconfined conditions
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 79

Figure A59. Simulated predevelopment (pre-1880) potentiometric surface and groundwater flow 

directions for the upper Arapahoe aquifer (model layer 8).

PALMER

DIVIDEDIVIDE

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

§ ̈¦70

§ ̈¦25

§ ̈¦76

5,300

5,200

5,100

5,400

5,000

5,5005,600

4,900

5,500

5,000

5,000

5,400

5,300

5,200

5,100

5,400

5,000

5,5005,600

4,900

5,500

5,000

5,000

5,400

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879.

 Datum NAVD88. Contour interval 100 feet

Generalized groundwater flow direction

EXPLANATION

Stream

Perennial

Ephemeral

Extent of active model cells

Upper Arapahoe Dawson aquifer simulated extent (model layer 8)
80 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Figure A60. Simulated predevelopment (pre-1880) potentiometric surface, groundwater flow directions, 

and calculated areas of predevelopment confined and unconfined conditions for the lower Arapahoe 

aquifer (model layer 10).

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek

West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

§ ̈¦70

§ ̈¦25

§ ̈¦76

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

PALMER

DIVIDEDIVIDE

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

! 5,400

6,000

5,300

5,900

5,600

5,800

6,100

5,200

5,500

5,700

6,200

5,100

6,300

6,400

6,500

6,600

5,000

6,700

6,800

6,900

7,000

4,900

7,100

7,200

7,300

4,800

7,400

6,000

6,000

5,000

6,200

5,000

5,700

5,600

5,600

5,900

5,500

5,700

5,900

6,100

6,300

5,000

5,900

4,900

5,400

6,000

5,300

5,900

5,600

5,800

6,100

5,200

5,500

5,700

6,200

5,100

6,300

6,400

6,500

6,600

5,000

6,700

6,800

6,900

7,000

4,900

7,100

7,200

7,300

4,800

7,400

6,000

6,000

5,000

6,200

5,000

5,700

5,600

5,600

5,900

5,500

5,700

5,900

6,100

6,300

5,000

5,900

4,900

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879.

 Datum NAVD88. Contour interval 100 feet

Generalized groundwater flow direction

EXPLANATION

Stream

Perennial

Ephemeral Extent of active model cells

Lower Arapahoe aquifer simulated extent (model layer 10)

Denver lower confining unit simulated extent (model layer 7)

Simulated extent of confined conditions

Simulated extent of unconfined conditions
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 81

Figure A61. Simulated predevelopment (pre-1880) potentiometric surface, groundwater flow directions, 

and calculated areas of predevelopment confined and unconfined conditions for the Laramie–Fox Hills 

aquifer (model layer 12).

South Platte

River

Cac he la Poudre R

Little ThompsonR

Boulder C r Coal Creek

Big D ry

Cr Clear Creek

West Sand Creek

First Cree k

Sand Creek

Little DryC

P

r

lum Creek West P ul m Creek

Bear Creek

Running

Creek

Dry

Creek

Kiow a Creek

Wolf Creek

Comanche Creek

Rock Creek

MuddyCreek

Badger C reek

Bijou C reek

Antelope Cr

Middle Bijou Creek

East BijouCreek

Middle Rush Creek

Mustang Creek

Adobe Cree

S

k

teels Fork

Horse Creek

West Bijou Cree k Big Sandy Cree k

Brac kett Cr

Black Squirrel Creek

West Kiowa Cr Cherry Cr

Kettle Cr

Cook Creek Dry Cr

Fountain Cree k

Saint Vrain Creek

Box Elder Creek

Lost Creek

Big Thompson R

PALMER

DIVIDEDIVIDE

§ ̈¦70

§ ̈¦25

§ ̈¦76

!

!

!

!

!

!

!

! ! !

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

5,400

5,300

5,200

5,100

5,600

5,700

5,800

5,900

5,000

4,800

5,500

4,900

4,700

6,000

4,600

6,100

6,200

6,400

6,300

6,500

6,600

6,700

6,800

6,900

7,000

7,100

4,500

7,200

7,300

7,400

4,400

5,600

6,300

5,400

4,400

6,000

4,600

5,700

5,800

4,500

5,900

4,900

5,700

5,000

5,500

5,800

5,600

5,800

5,900

5,500

5,000

4,600

5,400

5,300

5,200

5,100

5,600

5,700

5,800

5,900

5,000

4,800

5,500

4,900

4,700

6,000

4,600

6,100

6,200

6,400

6,300

6,500

6,600

6,700

6,800

6,900

7,000

7,100

4,500

7,200

7,300

7,400

4,400

5,600

6,300

5,400

4,400

6,000

4,600

5,700

5,800

4,500

5,900

4,900

5,700

5,000

5,500

5,800

5,600

5,800

5,900

5,500

5,000

4,600

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DENVER

BROOMFIELD

DOUGLAS

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

Limon

Simla

Kiowa

Parker

Golden

Calhan

Denver

Greeley

Boulder

Bennett

Loveland

Morrison

Lakewood

Monument

Longmont

Brighton

Franktown

Elizabeth

Broomfield

Deer

Trail

Fort

Lupton

Fort

Morgan

Castle

Rock

Highlands

Ranch

Colorado

Springs

Wiggins

Aurora

Littleton

Lafayette

Englewood

105° 104°

40°

39°

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27'N and 37°45'N, central meridian 105°00'W

0 10 20 MILES

0 10 20 KILOMETERS

Simulated potentiometric contour, shows altitude at which 

 water level would have stood in tightly cased wells, 1879.

 Datum NAVD88. Contour interval 100 feet

Generalized groundwater flow direction

EXPLANATION

Stream

Perennial

Extent of active model cells Ephemeral

Laramie–Fox Hills aquifer simulated extent (model layer 12)

Laramie confining unit simulated extent (model layer 11)

Simulated extent of confined conditions

Simulated extent of unconfined conditions
82 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Summary

The Denver Basin aquifer system, located in eastcentral Colorado, is an important source of water supply for 

municipalities along the semiarid Front Range urban corridor, 

including suburbs of Denver and Colorado Springs. It also is 

an important resource for rural residents on the eastern plains 

of Colorado. Denver Basin groundwater is administratively 

recognized as nonrenewable because the aquifers are primarily confined and receive little precipitation recharge. Permitted 

pumping rates are based on a 100-year aquifer life in areas 

of the basin where groundwater is considered not tributary 

to surface water, and there are ongoing needs in the Denver 

Basin to understand and monitor groundwater availability as 

pumping and storage depletion continue.

In 2004, the U.S. Geological Survey initiated large-scale 

regional studies to provide updated groundwater availability 

assessments of important principle aquifers across the United 

States, including the Denver Basin. This study of the Denver 

Basin aquifer system evaluates the hydrologic effects of continued pumping and documents an updated groundwater flow 

model useful for appraisal of hydrologic conditions. Study 

results are presented as three chapters in this Professional 

Paper. This chapter, Chapter A, presents an introduction, 

previous work, study methods, the hydrogeologic framework 

of the aquifer system, and a description of predevelopment 

(pre-1880) water budgets and groundwater flow conditions 

based on modeling results. Chapter B describes the effects of 

development on groundwater availability including changes 

in land and water use, changes to the recharge, discharge, and 

storage components of the water budget, and effects of these 

changes on groundwater levels and flow directions from 1880 

through 2003. Chapter B also includes results from predictive 

simulations and a discussion of monitoring needs. Chapter C 

documents the groundwater flow model.

The Denver Basin is located in the Colorado Piedmont 

of the Great Plains physiographic province where rolling 

hills and dissected plains characterize the topography. The 

topographic high of about 8,000 ft is located in the southern 

one-third of the Denver Basin along the Palmer Divide, which 

is the drainage divide between the tributaries of the South 

Platte River to the north and those of the Arkansas River to 

the south. The topographic low of approximately 4,300 ft 

lies along the South Platte River on the northeast edge of 

the basin. Topography of the Rocky Mountain Front Range 

affects the semiarid climate and weather over the plains in the 

Denver Basin east of the Rocky Mountain front. The Rocky 

Mountains create an orographic effect on weather moving 

eastward across Colorado, causing a rain shadow and semiarid 

climate on the eastern slope of the mountains. Analysis of 

long-term mean annual precipitation data from 129 individual 

weather stations in and around the Denver Basin indicates 

mean annual precipitation over the basin ranges from approximately 11.5 to 20 inches for the period 1931–2003. The 

Palmer Divide creates a local orographic effect on storms in 

the Denver Basin, such that precipitation rates are greater at 

higher elevations near the divide. The long-term mean annual 

precipitation at the highest altitudes along the western basin 

margin and the Palmer Divide is about 20 inches per year, 

whereas the long-term average annual precipitation at lower 

elevations along the South Platte River and in the southernmost part of the basin is about 11 to 12 inches per year. The 

1930–2003 period of record for precipitation records exhibits 

several periods defined as exhibiting variable to dry (drought) 

conditions. Basinwide average precipitation was less than 

the long-term annual mean from 1931 to about 1940 (during 

the 1930s drought) and from 1948 to 1956 (during the 1950s 

drought). The period 1979–2001 had variable to wet precipitation conditions, which resulted in an overall upward trend for 

basinwide mean annual precipitation over the period of record. 

Variable to dry conditions existed from 2001 to 2003 when 

effects of a recent drought become apparent.

The Denver Basin contains perennial and ephemeral 

surface drainages. Streams originating in the Front Range 

enter the west side of the Denver Basin and are generally 

perennial because they receive water primarily from snowmelt runoff in the mountains. Perennial streams entering the 

western side of the basin include the main stem of the South 

Platte River and its major tributaries of Plum, Bear, Clear, 

Coal, and Boulder Creeks. The South Platte River flows 

east out of the mountains through Waterton Canyon and into 

Chatfield Reservoir, crosses the western part of the Denver 

Basin flowing north through Denver, and then flows northeast toward Greeley. Streams that originate on the semiarid 

plains within the Denver Basin are generally ephemeral and 

intermittent because they receive water primarily from local 

precipitation runoff and groundwater discharge from the 

Denver Basin aquifers. Drainage within the Denver Basin east 

of the mountains is radial to the north, east, and south away 

from the Palmer Divide. North of the Palmer Divide, streams 

drain toward the South Platte River, and south of the Palmer 

Divide, streams drain toward the Arkansas River. Ephemeral 

and intermittent streams that originate within the Denver Basin 

include the South Platte River tributaries of Cherry, Running/

Box Elder, Kiowa, Comanche, and Bijou Creeks. Ephemeral 

and intermittent tributaries that originate in the Denver Basin 

and drain south toward the Arkansas River include Monument, 

Black Squirrel, Adobe, and Big Sandy Creeks.

The Denver Basin aquifer system is composed of Upper 

Cretaceous- to Tertiary-age sandstone bedrock aquifers that 

occur in the uppermost layers of the structural Denver Basin 

above the Cretaceous Pierre Shale confining layer. From oldest to youngest, the four primary units are the Laramie–Fox 

Hills, Arapahoe, Denver, and Dawson aquifers. The Dawson 

and Arapahoe are further differentiated into upper and lower 

units in parts of the basin. The synclinal structure of the 

Denver Basin causes the bedrock aquifer units to crop out in a 

ring pattern where the oldest rocks of the Laramie–Fox Hills 

aquifer crop out around the outer margins of the basin and the 

youngest rocks of the Dawson aquifer crop out in the center 

of the basin. Confined groundwater conditions generally are 
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 83

found in the bedrock aquifers where they are overlain by 

younger units, and unconfined groundwater conditions are 

generally found in outcrop areas. Alluvial sand, gravel, and 

clay deposits overlie the bedrock formations primarily along 

stream channels, and these materials form an unconfined 

alluvial aquifer where saturated.

The Laramie–Fox Hills aquifer is the oldest, deepest, 

and most extensive of the bedrock aquifers underlying the 

area, and its extent defines the extent of the Denver Basin 

aquifer system. The Laramie–Fox Hills aquifer is composed 

of poorly to moderately consolidated sandstone units of the 

upper Cretaceous Fox Hills Sandstone and the lower Laramie 

Formation, and a base-altitude map of the Laramie–Fox 

Hills aquifer reflects the bowl shape and dipping margins 

of the Denver Basin geologic structure. The deepest part of 

the Laramie–Fox Hills aquifer base, and therefore also the 

deepest part of the Denver Basin aquifer system, occurs in 

Douglas County just north of Parker, Colorado, at an altitude 

of approximately 3,410 ft. Depth to the base of the Laramie–

Fox Hills aquifer near the structural center of the basin is 

approximately 2,200 to 2,300 ft below land surface, and the 

total thickness of the aquifer system reaches a maximum of 

approximately 3,200 ft beneath the topographic high of the 

Palmer Divide.

The Cretaceous upper Laramie Formation, composed 

of gray to black shale, coal seams, and minor amounts of siltstone and sandstone, overlies the Laramie–Fox Hills aquifer 

and forms a confining unit that separates the Arapahoe and 

Laramie–Fox Hills aquifers. Base elevation contours of the 

Laramie confining unit reflect the bowl shape and dipping 

margins of the Denver Basin similar to the base configuration 

of the Laramie–Fox Hills aquifer. The Laramie confining unit 

forms a wedge-shaped layer with a mountain-front thickness 

of as much as 700 ft that thins to less than 100 ft on the eastern 

margin of the basin.

The Arapahoe aquifer consists of a 400- to 600-ft-thick 

sequence of interbedded conglomerate, sandstone, siltstone, and shale of Cretaceous age. In the northern one-third 

of the basin, shale is more prevalent, and the Arapahoe is 

divided into upper and lower aquifers with an intervening 

shale confining unit. Predominant conglomerate and coarsegrained sandstone units in the lower Arapahoe aquifer represent erosion from the nearby Front Range Precambrian and 

Paleozoic rocks and deposition at the mountain front during 

the Laramide Orogeny. The coarse-grained sandstones of the 

lower Arapahoe aquifer are thickest and the most conductive 

in Douglas and El Paso Counties where they form a productive 

bedrock aquifer that is heavily used for municipal and domestic water supply. The base altitude ranges from 6,200 ft along 

the southwest basin margin to approximately 4,100 ft at the 

basin center just north of Parker, Colorado. Depth to the base 

of the lower Arapahoe aquifer near the structural center of the 

basin is approximately 1,700 ft below land surface, and the 

maximum depth to the base of the aquifer is approximately 

2,600 ft below land surface at the Palmer Divide.

The Denver aquifer and confining units consist of a 

600- to 1,200-ft-thick sequence of interbedded shale, claystone, siltstone, sandstone, coal, and volcanic ash deposits of 

Cretaceous to Tertiary age. The Arapahoe, Denver, and lower 

Dawson aquifers represent the synorogenic deposition of sediments in the Denver Basin during Laramide uplift of the Rocky 

Mountain Front Range. The prominent Wildcat Mountain alluvial fan mapped in the lower Arapahoe aquifer is also present 

in the Denver and lower Dawson portions of the sequence. The 

Cretaceous/Tertiary boundary occurs in the upper part of the 

Denver aquifer sequence and is closely established by vertebrate fossils. Away from the Wildcat Mountain alluvial fan, the 

Denver sequence is in general finer grained than the underlying 

Arapahoe aquifer sequence and is composed of isolated channel sandstones contained in extensive fine-grained deposits. 

Despite poor correlations between Denver aquifer sandstones, 

the base-altitude maps of the Denver aquifer and confining units 

depict the regional structure of the Denver Basin and provide reasonable representations of layering within the Denver 

sequence. Altitude of the Denver sequence base ranges from 

approximately 4,600 to 6,300 ft, and the maximum depth of the 

sequence is approximately 2,100 ft below land surface beneath 

the Palmer Divide. The greatest silt-plus-sand thickness in the 

Denver aquifer (400 to 600 ft) occurs along the western basin 

margin in southern Douglas County. The Denver confining units 

are thin (mean thicknesses of 50 ft) compared to the thickness 

of the Denver aquifer (mean thickness of 550 ft).

The Dawson aquifer sequence consists of a 100- to 

1,100-ft-thick interbedded sequence of Tertiary-age, fluvial 

conglomerate, sandstone, siltstone, and shale. In the northern 

two-thirds of the Dawson extent, a clay and shale confining 

layer is present, and the aquifer is separated into upper and 

lower aquifers. The lower Dawson aquifer forms the upper part 

of the D1 sequence defined by Raynolds (2002) and represents continued Laramide synorogenic fluvial deposition in an 

environment similar to that which existed during deposition 

of the Denver aquifer sequence. The base altitude of the lower 

Dawson aquifer ranges from 5,400 ft at the northern edge of 

its extent to 6,800 ft on the southern edge of its extent and is 

1,100 feet below land surface at the Palmer Divide. The greatest 

silt-plus-sand thickness (200 to 375 ft) is located in the westcentral part of the basin along the Douglas–El Paso County 

line similar to the Denver and Arapahoe aquifers. The Dawson 

confining unit consists of a shale-rich layer at the top of the 

lower Dawson sequence. The upper Dawson aquifer consists 

of arkosic, coarse-grained, poorly consolidated sandstones with 

some interbedded overbank mudstone deposits in a distribution 

pattern quite different from that of the underlying sediments. 

Sandstones of the upper Dawson aquifer are more uniform and 

less variable in composition than those from the underlying 

layers and were likely derived from the Pikes Peak area to 

the south rather than from the Front Range source area to the 

west. Paleochannels at the base of the lower Dawson aquifer are evident from the base-altitude configuration map, and 

one such paleochannel aligns with the present-day course of 

Cherry Creek.
84 Groundwater Availability of the Denver Basin Aquifer System, Colorado

Quaternary alluvial, colluvial, and eolian deposits overlie 

of the Denver Basin bedrock aquifers, primarily along stream 

channels, forming an extensive and transmissive, unconfined 

alluvial aquifer where interconnected and saturated. Alluvial 

deposits along the main channel of the South Platte River and 

tributaries draining directly from the Front Range tend to be 

coarser grained than alluvial deposits along ephemeral tributaries draining from the Palmer Divide because the mountain 

streams receive coarser-grained source materials from steeper 

terrain than streams draining the sedimentary rocks of the 

Denver Basin. In general, the base of the alluvial aquifer slopes 

outward in a radial fashion away from the Palmer Divide, 

reflecting paleo and present-day topography. Alluvial deposit 

thickness ranges from 0 to approximately 300 ft, and the thickest part of the South Platte alluvial aquifer (more than 270 ft) is 

located in the downstream part of the aquifer near Wiggins.

Aquifer-property values for hydraulic conductivity, 

storage coefficient, and specific yield were compiled from 

previous Denver Basin studies. A summary of hydraulicconductivity values from pumping tests for the bedrock 

aquifers indicates the Arapahoe aquifer is the most permeable 

of the bedrock units (median of 1.8 ft/d; geometric mean of 

1.6 ft/d) followed by the Dawson aquifer (median of 0.80 ft/d; 

geometric mean of 0.7 ft/d), the Denver (median of 0.45 ft/d; 

geometric mean of 0.42 ft/d), and the Laramie–Fox Hills 

aquifers (median of 0.40 ft/d; geometric mean of 0.40 ft/d). 

Hydraulic-conductivity values for the alluvial aquifer are 

many orders of magnitude greater than those for the bedrock 

aquifers ranging from about 18 ft/d to about 2,560 ft/d with 

a median of about 480 ft/d based on pumping-test results. 

The difference in hydraulic conductivity between alluvial 

and bedrock aquifers is an important part of the Denver Basin 

conceptual model because it affects the water budget and 

water movement in the basin. Specific-storage values for this 

study were estimated during calibration of the groundwater 

flow model. A specific-storage value of 5.2×10–7/ft was estimated for aquifers, and a value of 1.7×10–6 /ft was estimated 

for confining units. Estimates of specific-yield values were 

compiled from previous studies, and the resulting mean values 

varied somewhat from values used for water administration. 

Mean specific-yield values of 15.2 percent for the Dawson 

aquifers, 13.3 percent for the Denver aquifer, 17.8 percent for 

the Arapahoe aquifers, and 18.6 percent for the Laramie–Fox 

Hills aquifer are reported by the current study and are used in 

the groundwater flow simulations.

Predevelopment groundwater availability discussions 

describe conditions prior to 1880 and are based on previous studies as well as interpretations and results from the 

steady-state stress period of the calibrated groundwater flow 

model. Predevelopment conditions are described in terms of 

groundwater budgets, hydraulic head, and groundwater flow 

directions. Groundwater inflow components are sources of 

water to the aquifer, and for predevelopment conditions in 

the Denver Basin, inflow was from precipitation plus irrigation recharge, streams, and interaquifer flow between hydrogeologic units. Because agricultural irrigation, primarily by 

surface-water diversions, began in the South Platte River 

valley prior to 1880, irrigation recharge was included as the 

predevelopment recharge estimates. Predevelopment outflow 

components of the water budget were discharge to evapotranspiration, streams, springs, and interaquifer flow. Discharge to 

pumping wells was assumed negligible before 1880 and was 

not included in the predevelopment groundwater budget. For 

some groundwater budget components such as streams, there 

are both inflows and outflows, and the difference between 

inflows and outflows is referred to as the “net.” For example, 

net recharge is calculated as the difference between inflow 

from recharge and outflow to evapotranspiration. Groundwater 

budgets are presented for the entire system (sum of all model 

layers), the alluvial aquifer (model layer 1), and the bedrock 

units (model layers 2 through 12) and are discussed in terms of 

individual groundwater budget components.

Precipitation plus irrigation recharge was the primary 

inflow component of the predevelopment groundwater 

budget for the entire system (630 ft3/s, 98 percent of inflow), 

the alluvial aquifer (454 ft3/s, 90 percent of inflow), and the 

bedrock aquifers (176 ft3/s, 94 percent of inflow) and is an 

important source of water in this semiarid region. However, 

much of the precipitation goes to surface-water runoff and 

evapotranspiration with only a small part of precipitation 

providing recharge to the bedrock aquifers. Simulated predevelopment net recharge (recharge minus evapotranspiration) 

to the alluvial aquifer was 147 ft3/s (84 percent of net inflow 

to the alluvial aquifer), and simulated predevelopment net 

recharge to the bedrock units was 41 ft3/s (100 percent of net 

inflow to the bedrock units). In areas where bedrock crops 

out with little to no overlying alluvium, deep infiltration of 

precipitation provides direct recharge to the bedrock aquifers. 

Inflow from streams and interaction between the alluvial 

aquifer and the bedrock units accounted for small percentages 

of groundwater inflow.

Evapotranspiration and streams were the primary outflow 

components of the predevelopment water budget. Simulated 

evapotranspiration was 442 ft3/s (68 percent of outflow) for 

the entire system, 307 ft3/s (61 percent of outflow) for the 

alluvial aquifer, and 135 ft3/s (72 percent of outflow) for the 

bedrock units. Streams accounted for 182 ft3/s (28 percent 

of the outflow) from the entire system, 164 ft3/s (33 percent 

of outflow) for the alluvial aquifer, and 18 ft3/s (10 percent 

of outflow) for the bedrock units. Streams provided inflow 

to and outflow from the system, but the net flow was from 

aquifers to streams. Simulated net outflow to streams from 

the alluvial aquifer was 151 ft3/s (88 percent of net outflow) 

and 14 ft3/s (34 percent of net outflow) from bedrock aquifers. 

In general, ephemeral streams that originate in the basin gain 

flow in upland areas near the Palmer Divide where alluvium is 

thin or absent and bedrock groundwater discharges directly to 

streams. In downstream parts of the basin where the alluvial 

aquifer is present, ephemeral streams can lose water to the 

alluvial aquifer as topographic gradients flatten, and streambeds can be dry.
Introduction, Conceptual Model, Framework, and Groundwater Availability of the Denver Basin Aquifer System 85

In areas overlain by alluvial aquifers, groundwater 

moves between the alluvial aquifer and bedrock units. Simulated outflow from the bedrock units to the alluvial aquifer for 

predevelopment conditions was 34 ft3/s (7 percent of alluvial aquifer inflow), and simulated outflow from the alluvial 

aquifer to the bedrock units was 8 ft3/s (4 percent of bedrock 

inflow), resulting in a net outflow from the bedrock units to 

the alluvial aquifer of 26 ft3/s (64 percent of net outflow from 

the bedrock units and 16 percent of net inflow to the alluvial 

aquifer). In the center of the basin, downward movement of 

ground water from overlying bedrock layers is an important source of recharge to deeper confined bedrock aquifers. 

Although the rate of vertical movement is small compared to 

lateral movement in the bedrock aquifers, the large area over 

which downward movement occurs results in large volumes of 

water moving between aquifers. For predevelopment conditions, interaquifer flow between all bedrock units is downward 

with flow to underlying aquifers. The greatest predevelopment 

downward flow is from the Denver aquifer (about 14 ft3/s net 

flow) and Denver lower confining unit (about 16 ft3/s net flow) 

into the Arapahoe aquifers because the Denver lower confining 

unit is generally thin (less than 50 ft in thickness) over much 

of its extent.

The quantity of groundwater in storage can be calculated 

as the product of the specific yield of an aquifer and aquifer 

volume plus the confined storage available from aquifer and 

water compressibility. In the Denver Basin, the quantity of 

groundwater in unconfined bedrock-aquifer storage was previously estimated by multiplying constant specific-yield values 

defined for water administration times the volume of aquifer 

materials, and the result is an estimate of 295 million acre-ft 

of groundwater in storage. While estimates of groundwater in 

storage are speculative, digital representation of the aquifer 

framework does facilitate storage calculations for the Denver 

Basin. This study estimated the volume of unconfined bedrock-aquifer storage as the product of aquifer area, simulated 

predevelopment (pre-1880) saturated thickness, and the spatial 

distribution of specific yield derived for each aquifer. The calculation, which accounts for spatial heterogeneity of specific 

yield as a function of the silt-plus-sand aquifer fraction, results 

in an estimate of 247 million acre-ft of groundwater in unconfined storage, which is about 18 percent less than the previous 

administrative estimate.

The Denver Basin aquifers are confined over much of 

their area where overlain by less- permeable claystone and 

shale confining units. Unconfined, or water-table, conditions 

occur in the near-surface parts of the Denver Basin bedrock 

aquifers and in the alluvial aquifer. The extent, depth, thickness and aquifer properties of the geologic units; climate 

and the resulting effects on the rate and location of recharge; 

and the location and altitude of surface drainages control 

the predevelopment distribution of hydraulic heads in the 

Denver Basin aquifers. Potentiometric-surface maps for 

predevelopment conditions for the seven aquifers show areas 

of high hydraulic head near the recharge area of the Palmer 

Divide and areas of low hydraulic head near discharge areas. 

Groundwater flows downgradient from the Palmer Divide 

toward streams in all directions. Groundwater moves laterally 

through the permeable sandstones and alluvial materials from 

areas of recharge toward areas of discharge on both a local 

and regional scale. On a local scale, water moves from upland 

recharge areas through the near-surface aquifer to discharge 

areas in nearby stream valleys. On a regional scale, bedrock 

groundwater moves laterally and downward from the topographic high of the Palmer Divide in the south-central part of 

the basin and ultimately discharges to downgradient alluvial 

aquifers or streams. Groundwater also moves vertically down 

to underlying bedrock aquifers. The lower potentiometric 

altitude in the underlying bedrock aquifers promotes downward water movement, and although the low vertical hydraulic 

conductivities limit the rate of downward water movement, 

this vertical leakage is an important source of recharge to 

underlying bedrock aquifers.

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