U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3148

December 2006

Ground Water in the Anchorage Area, Alaska

Meeting the Challenges of Ground-Water Sustainability

Throughout the city of Anchorage, ground water is pumped from 

hydrologic units consisting of unconsolidated surficial deposits and metamorphic 

bedrock underlying hillside areas. The 

surficial deposits of gravel, sand, silt, 

and clay range in thickness from several 

feet to more than 1,500 feet below land 

surface (Barnwell and others, 1972). 

These water-bearing materials constitute 

two surficial aquifers overlying a bedrock 

aquifer and extend from the foothills of 

the Chugach Mountains to Cook Inlet and 

from the Elmendorf and Fort Richardson 

military bases to Potters Marsh. 

Present and projected water use

Anchorage Water and Wastewater 

Utility (AWWU) provides locally obtained 

surface water and ground water to satisfy 

most of the city’s public water demand. 

The remaining water is supplied primarily from private domestic wells. In 2002, 

AWWU delivered on average 27.6 million 

gallons per day (Mgal/d) of water to more 

than 52,600 customers, equating to nearly 

127 gallons per person per day (AWWU, 

2005a). About 83 percent of the water 

supplied by AWWU in 2002 was surface 

water obtained from Eklutna Lake (79 

percent) and Ship Creek (4 percent). The 

remaining 17 percent (4.7 Mgal/d) was 

obtained from ground water pumped from 

local aquifers (AWWU, 2005b). An additional estimated1

 non-AWWU production 

(4 Mgal/d) pumped from private domestic 

wells contributed to a total of about 8.7 

Mgal/d of ground water pumped from 

local aquifers in 2002.

As the demand for water increases 

in proportion to the expected 35-percent 

Ground water is an important component of Anchorage’s water supply. During the 

1970s and early 80s when ground water extracted from aquifers near Ship Creek was 

the principal source of supply, area-wide declines in ground-water levels resulted 

in near record low streamflows in Ship Creek. Since the importation of Eklutna Lake 

water in the late 1980s, ground-water use has been reduced and ground water has 

contributed 14– 30 percent of the annual supply. As Anchorage grows, given the 

current constraints on the Eklutna Lake water availability, the increasing demand 

for water could place an increasing reliance on local ground-water resources. The 

sustainability of Anchorage’s ground-water resources challenges stakeholders to 

develop a comprehensive water-resources management strategy.

A growing municipality

The city of Anchorage, part of the 

municipality of Anchorage, covers about 

200 square miles and extends from Ship 

Creek south of Eagle River, AK, to Potters 

Marsh just west of Girdwood, AK. About 

280,000 people, more than 42 percent of 

Alaska’s population, live within the city of 

Anchorage, which accounted for about 44 

percent of the State’s population growth 

in the 1990s (State of Alaska, 2004). 

Forecasts predict that Anchorage’s population will grow about 35 percent by 2020 

(MOA, 2001; Williams, 2004).

1

Based on 1972 estimate (USCOE, 1979) 

adjusted for growth.

LAND USE

Elmendorf and Fort Richardson Military Bases 

USGS Gaging

Station 15276000

C h u g a c h M o u n t a i n s

Potters

Marsh

1970 2000

Markon, 2003

Urban

Vegetated

Wetlands

Lakes/Ponds

Coastal

Roads

Barren

Ship Creek

62.45%

2.12%

0.87%

22.8%

9.67%

1.24%

0.84%

2.17%

46.79%

42.83%

5.39%

0.56%

1.26%

1.01%

Cook

Inlet

Eklutna Lake

City of Anchorage

Girdwood

Eagle River

ALASKA

Municipality of Anchorage

0 20 MI

Urban growth has changed land-use patterns significantly in the city of Anchorage since 

1970. Wetlands and vegetated areas in the city have been replaced by urban development.
growth in population, by 2020, AWWU 

will need to supply an estimated 37 Mgal/d 

of water. If ground-water use increased 

at the same rate as population, ground 

water would constitute about 6.3 Mgal/d 

of AWWU water deliveries, and domestic pumpage would contribute to a total 

of about 11.7 Mgal/d of ground water 

pumped from local aquifers. 

Surface-water availability

Anchorage has several good-quality sources of water. Eklutna Lake alone 

is capable of providing more than 100 

Mgal/d, but is currently limited by the 

35 Mgal/d capacity of the Eklutna Water 

Treatment Facility (AWWU, 2005a). 

Average flows in Ship Creek (USGS 

gaging station 15276000) as measured at 

the foothills of the Chugach Mountains 

below the Fort Richardson military base 

diversion dam were about 93 Mgal/d from 

1947–2003 (USGS, 2005). As potential 

sources these quantities far exceed the 

projected 2020 demand for water. 

Increasing the supply from Ekultna’s 

Water Treatment Facility would require 

substantial capital investment and 

would consume water currently used for 

electricity production. The water treatment facility could meet the expected 

2020 demand; however, by 2030–40, the 

expected demand likely would exceed its 

capacity. AWWU’s Ship Creek supply 

primarily is limited by the 10.5 Mgal/d 

annual water right and limits on withdrawals of 3 Mgal/d or less during lowflow winter periods. Because of variable 

flows and water quality of Ship Creek, 

its use has been minimized to maintain 

flows for aquatic and riparian habitat and 

to mitigate fecal coliform contamination 

(ADEC, 2004).

Ground-water availability

An estimated 75 Mgal/d of water 

on average recharges Anchorage-area 

aquifers (Barnwell and others, 1972). 

Ground water flows toward Cook Inlet 

from recharge areas in the Chugach Mountains and upland areas of the coastal plain 

deposits. Much of the flow discharges 

directly into Cook Inlet. Along the way, 

ground water is recharged by infiltration 

of streamflow in the upper reaches of the 

major watersheds, such as Ship Creek, and 

to a lesser degree throughout Anchorage 

by infiltration of precipitation through the 

surficial deposits. In the lowland areas of 

the coastal plain, ground water discharges 

into streams, springs, seeps, and wetlands, where it runs off, evaporates, or is 

transpired by plants (Barnwell and others, 

1972; Patrick and others, 1989).

Under natural conditions, over the 

long term, a flow system is in a dynamic 

equilibrium, or steady state—the average discharge approximates the average 

recharge. The equilibrium is dynamic 

because recharge fluctuates seasonally and 

annually. Major climate shifts, changes in 

landscape owing to natural events, such 

as earthquakes or human alteration, and 

varying ground-water pumping rates can 

disrupt the equilibrium for years until a 

new equilibrium is established. By 1969, 

pumping from high-capacity wells lowered water levels more than 50 feet in the 

lower part of Ship Creek basin, resulting 

in reduced Ship Creek streamflow, and by 

10 feet or more over an area of 40 square 

miles (Miller and Whitehead, 1999). However, ground-water levels in some areas of 

Anchorage have recovered to predevelopment levels owing to reduced extraction. 

Poor water quality can render 

ground water unusable for drinking. 

Human activities can introduce undesirable chemicals into aquifers and increase 

concentrations of the nutrients nitrogen 

and phosphorous. Regionally, the quality 

of Anchorage-area ground water generally is good (Glass, 2001). In isolated 

areas in Anchorage, however, oil and 

fuel spills and waste-disposal sites have 

released benzene, xylenes, arsenic, chromium, fluorescein, and sulfate into the 

ground water (ADEC, 2005). Leachate 

from septic systems, a landfill, and other 

disposal sites have introduced coliform 

bacteria and higher concentrations of iron, 

manganese, dissolved organic carbon, and 

chloride in local ground water (Munter, 

1987; Munter, and Maynard, 1987).

no data

Ship Creek (Mgal/d)

Eklutna Lake (Mgal/d)

Wells (Mgal/d)

Surface water

Ground water

WATER USE

POPULATION

‘65‘60‘29 ‘39 ‘59 ‘70 ‘75 ‘80 ‘85 ‘90 ‘95 ‘00

Year

0

10

20

30

WATER USE*,

IN MILLION GALLONS PER DAY

(Mgal/d)

100

0

200

Municipality of Anchorage 300

City of Anchorage

POPULATION, IN THOUSANDS

*AWWU, 2005a and 2005b and USGS, 1987

Principal recharge areaPrincipal recharge area

City of Anchorage Cook Inlet

Metamorphic

NORTH

SOUTH

Sedimentary

bedrock

Clay/silt

Sand/

gravel

Confined

Bootlegger Cove Formation

Unconfined

Modified from

Miller and Whitehead, 1999

Chugach Mountains

Both unconfined and confined groundwater discharges into Cook Inlet. 

Most ground-water recharge occurs through

alluvial deposits near the Chugach foothills.

Ground water is pumped

from the surficial aquifers

and the bedrock aquifer.

Campbell Cr.

Ship Cr.

Water use in Anchorage keeps pace with a growing population.
Ground-water sustainability

A sustainable ground-water supply largely is a subjective quantity that 

depends on the “acceptable” consequences and knowledge of the realized 

or expected outcomes of ground-water 

management practices. Whether the 

consequences and outcomes are “acceptable” is best determined by stakeholders. 

But choosing the best or most desirable 

outcomes requires knowledge of the 

hydrogeologic system and its responses 

to imposed stresses, such as ground-water 

pumping.

The development of water resources 

imposes stresses on flow systems. Poorly 

planned, ground-water pumping can 

reduce streamflow that sustains riparian 

ecosystems, which occurred along Ship 

Creek in the 1970s and 1980s. Additionally, high rates of ground-water extraction 

can reduce outflow to coastal areas causing landward migration of saline ground 

water—saltwater intrusion.

Overuse can deplete a ground-water 

resource and potentially cause adverse 

consequences. Maximizing the sustainable use of the ground-water resource in 

the context of conjunctive-use management can be beneficial. In Anchorage, 

for example, it may be more economical to substitute local ground water for 

some Eklutna Lake water. But, depending on the location, timing, and amount 

of ground-water pumping, increased 

extractions may be environmentally and 

economically unacceptable owing to: 

Consequences associated with 

reduced flows to streams, wetlands, 

and Cook Inlet; 

Costs associated with constructing 

new wells, rehabilitating old ones, and 

extracting deeper ground water; and 

Costs incurred for purifying poorquality ground water.

Optimal use of a ground-water 

resource refers to sets of choices or 

management alternatives that achieve 

sustainability of the resource while 

maximizing the overall acceptability 

of any environmental, economic, or 

social consequences. For Anchorage, the 

choices involve the conjunctive use of 

ground water and surface water—how 

much of each to use, when, and subject to 

what constraints. What constitutes “maximizing acceptability of consequences”? 

These are subjective decisions made by 

stakeholders, but, ideally, these decisions 

are based on objective scientific evaluation of alternative outcomes.

Meeting the challenges of 

ground-water sustainability

The ability to address issues concerning future water-resources management in Anchorage depends on many 

factors including an understanding of 

the hydrologic systems. The ability to 

simulate or predict the short- and longterm responses of the system to imposed 

stresses, hypothetical or real, is improved 

by monitoring natural and anthropogenic 

induced variations in the flow system. 

As Anchorage grows and the demand for 

water reaches the capacity of the surface1.

2.

3.

water resources, good-quality ground 

water again will become an important 

source of water. Effective management of 

these water resources will require: 

Further knowledge of the hydrogeology, sustainability, and interconnectivity of Anchorage’s ground-water 

and surface-water systems; 

Understanding the consequences of 

ground-water depletion; and 

A set of effective water-resources 

management objectives and constraints and optimal conjunctive-use 

alternatives.

Hydrologic data collection and 

analysis can lead to improved understanding of the ground-water flow system. The 

development of a comprehensive hydrogeologic database that reflects long-term 

natural and anthropogenic stresses can be 

used to assess future sustainable management strategies (Alley and others, 1999). 

These data would include lithologic, 

geophysical, and hydraulic properties of 

hydrogeologic units, ground-water levels, 

and water-level maps for the principal 

aquifers, ground-water quality, and 

streamflow characteristics indicative of 

ground- and surface-water interaction.

Numerical ground-water flow 

models, such as MODLFOW (Harbaugh, 

2005), are important tools that increase 

an understanding of ground- and surface-water systems. Models constrained 

by available hydrologic data can simulate periods of historical ground-water 

development and a means to forecast 

the consequences associated with future 

water-use requirements (Reilly and Harbaugh, 2004). 

1.

2.

3. City of Anchorage

0 2 4 MI

0 4 KM2

<10

10 to 20

20–30

30–40

40–50

>50

High-capacity well 

Chugach Mountains

Cook Inlet

Net water-level decline

in the confined aquifer

between 1955 and

March 21, 1969, in feet 

Miller and Whitehead, 1999

Ship Cr.

Campbell Cr.

Chester Cr.

Inflow

Outflow

Streams 59%

5%

63% Streams

22%

15%

36% Precipitation

Evapotranspiration

Ground Water

Ground Water

PRE-DEVELOPMENT WATER BUDGET

Water budget

Mgal/d discharged to local streams 

flowing to Cook Inlet (Patrick and others, 

1989).

A water budget defines the amount of 

inflow and outflow in a hydrologic unit. 

Prior to urban development, about an 

average 275 Mgal/d of water moved 

through the surface- and ground-water 

systems in the city of Anchorage. 

Streamflow and precipitation accounted 

for 95 percent and ground-water inflow 

from adjacent bedrock and other outlying areas contributed 5 percent. Stream 

discharge and ground-water flow to 

Cook Inlet accounted for 85 percent 

of the outflow and evapotranspiration 

accounted for the remaining 15 percent. 

Of the estimated 75 Mgal/d of pre-development recharge to local aquifers, 15 
 Implementing a strategy or policy 

for sustainable use of ground-water 

resources is a complex task. Stakeholders 

in developed ground-water basins have 

used simulation-optimization modeling 

techniques to determine optimal alternatives related to conjunctive-use management problems (Ahlfeld and Mulligan, 

2000; Ahlfeld and others, 2005) by coupling stakeholder-defined water-resources 

objectives and constraints with a physically-based model of the ground-water 

flow system. Simulation-optimization 

modeling could facilitate the sustainable 

use of the combined ground- and surfacewater resources in the Anchorage area. 

Edward H. Moran, 

USGS, Anchorage, Alaska

 Devin L. Galloway 

USGS, Sacramento, California

References 

Alaska Department of Environmental Conservation 

[ADEC], 2004, Total maximum daily load (TMDL) for 

fecal coliform in the waters of Ship Creek in Anchorage, Alaska, available online at http://www.dec.state.

ak.us/water/tmdl/pdfs/final%20fc%20tmdl%20ship%20

creek%205-12.pdf [accessed December 20, 2006], 45 p.

Alaska Department of Environmental Conservation 

[ADEC], 2005, Contaminate-sites program database 

searches, available online at http://www.dec.state.ak.us/

spar/csp/db_search.htm [accessed December 20, 2006]. 

Ahlfeld, D.P., Barlow, P.M., and Mulligan, A.E., 2005, 

GWM—A ground-water management process for the 

U.S. Geological Survey modular ground-water model 

(MODFLOW–2000): U.S. Geological Survey OpenFile Report 2005–1072, 124 p. 

Ahlfeld, D.P., and Mulligan, A.E., 2000, Optimal management of flow in groundwater systems: San Diego, CA, 

Academic Press, 185 p.

Alley, W.M., Reilly, T.E., and Franke, O.L., 1999, Sustainability of ground-water resources: U.S. Geological Survey Circular 1186, 79 p, available online at http://pubs.

usgs.gov/circ/circ1186/ [accessed December 20, 2006]. 

Anchorage Water and Wastewater Utility [AWWU], 2005a, 

AWWU general information, available online at http://

www.awwu.biz/website/AboutAWWU/GeneralInfo_

frame.htm [accessed December 20, 2006]. 

Anchorage Water and Wastewater Utility [AWWU] , 

2005b, USGS Water Production Report 2000–2004. 

Barnwell, W.W., George, R.S., Dearborn, L.L., Weeks, J.B., 

and Zenone, Chester, 1972, Water for Anchorage—An 

atlas of the water resources of the Anchorage Area, 

Alaska: City of Anchorage and the Greater Anchorage 

Area Borough, 77 p. 

Glass, R.L., 2001, Ground-water quality in the Cook 

Inlet Basin, Alaska, 1999: U.S. Geological Survey, 

Water- Resources Investigations Report 01-4208, 58 p, 

available online at http://pubs.usgs.gov/wri/wri01-4208/

[accessed December 20, 2006].

Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. 

Geological Survey modular ground-water model—the 

ground-water flow process: U. S. Geological Survey 

Techniques and Methods 6-A16, variously paginated, available online at http://water.usgs.gov/nrp/

gwsoftware/modflow2005/modflow2005.html [accessed 

December 20, 2006].

Markon, C.J., 2003, A temporal study of urban development for the Municipality of Anchorage, Alaska: 

Geocarto International, v. 18, no. 3, 33 p.

Miller, J.A., and Whitehead, R.L., 1999, Ground Water 

Atlas of the United States: Alaska, Hawaii, Puerto Rico 

and the Virgin Islands, U.S. Geological Survey, HA 

730-N, available online at http://capp.water.usgs.gov/

gwa/ch_n/index.html [accessed December 20, 2006].

Municipality of Anchorage [MOA], 2001, Anchorage 

2020—Anchorage Bowl comprehensive plan: Planning 

Department, 121 p. 

Munter, J.A., 1987, Review of a consultant’s report on 

septic system contamination at Anchorage, Alaska, with 

interpretation of data: Alaska Division of Geological 

and Geophysical Surveys, public-data file 87-14, 15 p. 

Munter, J.A. and Maynard, D. L., 1987, Data from Alaska 

inventory of contaminated aquifers: Alaska Division of 

Geological and Geophysical Surveys, public-data file 

87-23, 76 p.

Patrick, L.D., Brabets, T.P., and Glass, R.L., 1989, Simulation of ground-water flow at Anchorage, Alaska, 

1955–83: U.S. Geological Survey Water-Resources 

Investigations Report 88-4139, 41 p, available online 

at http://pubs.er.usgs.gov/usgspubs/wri/wri884139

[accessed December 20, 2006]. 

Reilly, T.E., and Harbaugh, A.W., 2004, Guidelines for 

evaluating ground-water flow models: U.S. Geological 

Survey Scientific Investigations Report 2004-5038, 

30 p, available online at http://pubs.usgs.gov/

sir/2004/5038/ [accessed December 20, 2006]. 

State of Alaska, 2004, Borough and census area estimates—Table 2.1, Population of Alaska by labor 

market area, borough and census area, 1990–2004: 

Department of Labor and Workforce Development, available online at http://almis.labor.state.

ak.us/?PAGEID=67&SUBID=115 [accessed December 

20, 2006]. 

U.S. Corp of Engineers [USCOE], 1979, Water Supply 

Summary Metropolitan Anchorage Urban Study: 

Volume 2, Alaska District Corp of Engineers and the 

Municipality of Anchorage. 

U.S. Geological Survey [USGS], 1987, Pumpage data from 

public supply wells at Anchorage, Alaska, 1957–1985: 

U.S. Geological Survey Open-File Report 86-542, 48 

p. + 1 oversize map, available online at http://pubs.

er.usgs.gov/usgspubs/ofr/ofr86542 [accessed December 

20, 2006]. 

U.S. Geological Survey [USGS], 2005, Calendar streamflow statistics for Alaska, available online at http://nwis.

waterdata.usgs.gov/ak/nwis/annual/calendar_year/

?site_no=15276000 [accessed December 20, 2006]. 

Williams, J. G., 2004, Population, Alaska Department 

of Labor and Workforce Development, Research and 

Analysis Section, available online at http://www.labor.

state.ak.us/research/trends/mar04pop.pdf [accessed 

December 20, 2006]. 

For more information, please contact: 

U.S. Geological Survey

4230 University Drive, Suite 201 

Anchorage, AK 99508-4664 

Telephone 907-786-7000 

 or vist website URL: http://ak.water.usgs.gov

Graphic design: David R. Jones

Printed on recycled paper

Eklutna Lake lies in the Chugach Mountain 

Range and receives much of its recharge from 

Eklutna Glacier.

Simulated effects of ground-water withdrawals

on the Anchorage ground-water flow system 

0 8

Years

Drawdown, in feet

below land surface

16 24

Pumping

32

Shallow Aquifer

Deep Aquifer

0

8

16

24

Recovery

Simulated drawdown and recovery in the aquifer 

system at a location between Ship Creek and 

Chester Creek, southeast of the city of Anchorage

Some of the potential effects of sustained 

pumping on the predevelopment groundwater system were simulated using a 

numerical ground-water flow model (MODFLOW) calibrated to pre-1955 conditions 

(Patrick and others, 1989). The model simulated about 21 years of pumping at a rate 

of about 19 Mgal/d from wells distributed 

throughout the basin, which was based on 

pumping distributions estimated during the 

1970s-80s. Although the simulated effects 

of pumping are hypothetical and uncalibrated, the model demonstrated decreased 

ground-water discharge to streams and 

Cook Inlet, increased recharge from 

streams to the shallow aquifer, increased 

leakage of water from the shallow aquifer 

to the deep aquifer, and 20-30 ft drawdowns of pre-pumping ground-water 

levels. Simulated ground-water levels 

recovered to near pre-pumping conditions in 7 years following the cessation of 

simulated pumping. The ability to calibrate 

the historical response of the flow system 

to ground-water development during the 

period 1955-84 was limited by the available 

hydrogeologic data.