Ground Water Atlas of the United States: Segment 13, Alaska, Hawaii, Puerto Rico, and the U.S. Virgin Islands

Hydrologic Atlas 730-N

, , , , and



Alaska is the largest State in the Nation and has an area of about 586,400 square miles, or about one-fifth the area of the conterminous United States. The State is geologically and topographically diverse and is characterized by wild, scenic beauty. Alaska contains abundant natural resources, including ground water and surface water of chemical quality that is generally suitable for most uses.

The central part of Alaska is drained by the Yukon River and its tributaries, the largest of which are the Porcupine, the Tanana, and the Koyukuk Rivers. The Yukon River originates in northwestern Canada and, like the Kuskokwim River, which drains a large part of southwestern Alaska , discharges into the Bering Sea. The Noatak River in northwestern Alaska discharges into the Chukchi Sea. Major rivers in southern Alaska include the Susitna and the Matanuska Rivers, which discharge into Cook Inlet, and the Copper River, which discharges into the Gulf of Alaska . North of the Brooks Range, the Colville and the Sagavanirktok Rivers and numerous smaller streams discharge into the Arctic Ocean.

In 1990, Alaska had a population of about 552,000 and, thus , is one of the least populated States in the Nation. Most of the population is concentrated in the cities of Anchorage, Fairbanks, and Juneau, all of which are located in lowland areas. The mountains, the frozen Arctic desert, the interior plateaus, and the areas covered with glaciers lack major population centers. Large parts of Alaska are uninhabited and much of the State is public land. Ground-water development has not occurred over most of these remote areas.

The Hawaiian islands are the exposed parts of the Hawaiian Ridge, which is a large volcanic mountain range on the sea floor. Most of the Hawaiian Ridge is below sea level (fig. 31) . The State of Hawaii consists of a group of 132 islands, reefs, and shoals that extend for more than 1 ,500 miles from southeast to northwest across the central Pacific Ocean between about 155 and 1 79 degrees west longitude and about 19 to 28 degrees north latitude. The main inhabited islands are at the southeastern end of the group (fig. 31 ); not all the small islands, reefs, and shoals included in the State are shown.

The Hawaiian islands are geologically youngest in the southeast and oldest in the northwest. This report discusses only the eight largest islands near the southeastern end of the group; these eight main islands account for practically all of the 6,426-square-mile land area of the State. The eight islands and their approximate size, in square miles, from southeast to northwest are Hawaii, 4 ,021; Maui, 728; Kahoolawe, 45; Lanai, 141; Molokai, 259; Oahu, 603; Kauai, 553; and Niihau, 71. The total resident population in 1995 was 1, 179,198, of which about 75 percent were on the island of Oahu. Honolulu, which is on Oahu, is the largest and most developed city and had a population of 369,485 in 1995. In addition to the resident population, a visitor population of about 150,000 has typically been present at any given time during the 1990's. Many of these visitors stay in Honolulu.

The State Land Use Commission is responsible for classifying the lands of the State into one of four categories called districts: conservation, agricultural, urban, or rural (fig. 32). In 1995, conservation, agricultural, urban, and rural districts accounted for about 48, 47, 5, and 0.2 percent of the land area in the State, respectively. Conservation districts include areas necessary for protecting the State's watersheds and water resources and are typically located in high-altitude, high-rainfall areas. Much of the urban development in Hawaii is in the lowland coastal areas of each island. Agricultural irrigation can place large demands on the water resources; prior to the 1990's, one of the largest uses of water was for sugarcane irrigation. The five largest islands (Hawaii, Maui, Molokai, Oahu, and Kauai) have extensive areas of mountainous land where urbanization and large-scale agricultural operations are not feasible.

The island of Hawaii is the largest island of the State (fig. 33) and has the highest altitude at 13,796 feet. Maui is about 10,000 feet above sea level in its eastern part and about 5,800 feet above sea level in its western part; a broad lowland area separates the two parts. Kahoolawe is the smallest of the eight major islands and is only about 1 ,500 feet above sea level in its eastern, highest part. Lanai is about 3,400 feet above sea level in its highest part, but much of the island is less than 1,000 feet above sea level. Molokai is mountainous in its eastern half, where it rises to about 5,000 feet above sea level, but most of the island is less than 1,000 feet above sea level. Oahu has a mountainous ridge along its eastern side and another mountainous area along the western side, where it rises to about 4,000 feet above sea level; however, most of Oahu is less than 1,000 feet above sea level. Kauai is about 5,200 feet above sea level in its central part, but from the base of the mountains shoreward, large areas of the island are less than 1,000 feet above sea level in the southern, eastern, and northern parts. Niihau is mostly less than 1 ,000 feet above sea level, except for a narrow ridge about 1 ,300 feet above sea level along its northeastern side. The topography of each island has a profound effect on development and climate.

Puerto Rico and the Virgin Islands of the United States (fig. 66) are located about 1, 100 miles east-southeast of Miami, Fla. The islands are part of the Greater Antilles island chain which, along with the Lesser Antilles island chain, separates the Caribbean Sea from the Atlantic Ocean. Puerto Rico and the Virgin Islands lie approximately 100 miles south of the Puerto Rico Trench, a depression of the ocean floor that reaches depths of 30,249 feet , the deepest known part of the Atlantic Ocean.

The main island of Puerto Rico is a rectangular-shaped island that extends approximately 110 miles from east to west and 40 miles from north to south (fig. 67). Puerto Rico and its three principal offshore islands- Vieques, Culebra, and Mona-have an overall area of about 3,471 square miles.

The Virgin Islands are about 50 miles east of Puerto Rico and consist of about 80 islands and cays. In this report, the Virgin Islands include only the three island territories of the United States-St. Croix, St. Thomas, and St. John. The three island territories also are the largest and most important of the Virgin Islands chain , with respective areas of approximately 84, 32, and 19 square miles.

The population of Puerto Rico was estimated to be about 3.4 million in 1985; approximately two-thirds of the people lived in cities, and the remaining one-third lived in rural areas. A population of about 104,000 inhabited the Virgin Islands in 1985; only about 30 percent of these people lived in urban areas. In Puerto Rico during 1985, ground water was the source of supply for approximately 16 percent of the population. Only a small percentage of the urban inhabitants of the Virgin Islands use ground water for water supply; however, ground water is an important supplemental source for rural inhabitants. Most of the population of Puerto Rico obtains its water supply from surface-water reservoirs. In the Virgin Islands, seawater desalination plants on St. Croix and St. Thomas are the principal sources of water supply for most urban areas; however, by law, all residences, hotels, and most public buildings are required to have cisterns supplied from rooftop precipitation collectors. Cruz Bay on St. John is primarily supplied by ground water. Culebra Island is primarily supplied by pipeline from Puerto Rico. Mona Island has no known source of water supply and is uninhabited.

Puerto Rico and the three principal U.S. Virgin Islands are mountainous with central highland areas that rise to a maximum altitude of about 4,400 feet above sea level in Puerto Rico and about 1,100, 1,560, and 1,280 feet above sea level on St. Croix, St. Thomas, and St. John, respectively. In Puerto Rico , the Cordillera Central, the Sierra de Luquillo, and the Sierra de Cayey generally are oriented east-west and dominate the mountainous southern two-thirds of the island (fig. 67). An area of gently dipping limestone that has been deeply dissected by dissolution forms a wide band of karst topography along most of the north coast. Flat-lying coastal plains and alluvial valleys compose a discontinuous belt around much of the periphery of the island. The coastal plain is especially prominent along part of the south coast where coalescing fan deltas were deposited by adjacent streams to form a broad, continuous plain.

In St. Thomas, flat, low-lying areas are limited to the Charlotte Amalie area and a few narrow beaches. St. John is similar to St. Thomas but has even less flat land; in St. John, flat land is limited mostly to the Cruz Bay and Coral Bay areas. The northwestern and eastern parts of St. Croix are formed by low mountains and rugged hills; however, the central and southwestern parts of the island are low-lying to gently rolling.

Drainage on each of the islands characteristically consists of short, deeply incised streams that have steep gradients in the upper reaches. Drainage generally is radial from the central highlands to the sea. Few of the streams along the southern coast of Puerto Rico , its offshore islands, or the U.S. Virgin Islands are perennial, but flow only after major precipitation and during sustained wet periods.

Precipitation in Puerto Rico and the Virgin Islands is highly variable, both seasonally and areally. Seasonally, a dry period begins in December and ends in March or April; it usually is followed by a period of intensive rainfall in April and May. A period of diminished rainfall in June and July is followed by the wet season that extends from August through November during which about 50 percent of annual rainfall occurs (fig. 68).

Areally, the orographic effect of the steep topography of highland areas causes average annual precipitation to vary almost in direct relation to altitude, but it also is affected by prevailing wind direction. Average annual precipitation in Puerto Rico ranges from less than 40 inches on the southern coastal plain to greater than 200 inches in the mountains (fig. 69A). Annual precipitation averages about 75 inches on the windward north coast of Puerto Rico compared to about 30 inches on the lee side of the island along the southwestern coast. Annual precipitation averages from less than 30 to greater than 55 inches in the Virgin Islands (fig. 698).

Runoff in Puerto Rico and the Virgin Islands also varies seasonally and areally in response to fluctuations in precipitation. Average annual runoff (1951-80) in Puerto Rico ranges from about 20 inches on the northern and southern coasts to greater than 100 inches in the mountainous rain forest of the Sierra de Luquillo (fig. 70). In general, runoff is greatest during two periods that coincide with periods when precipitation is greatest-in August through November and April through May. Most streams have little flow during dry periods except for the larger streams on the north, west, and south coasts of Puerto Rico that originate in the igneous and volcanic rocks of the interior. The northern and southern coastal streams especially are perennial in coastal areas where they are underlain by limestone and thick alluvium, and water from the limestone and alluvial aquifers discharges to the streams as base flow.

Most of the precipitation is returned to the atmosphere by evapotranspiration-evaporation from the land and water surfaces and transpiration by plants. Average annual evaporation in Puerto Rico is estimated from pan evaporation to range from about 64 inches in the coastal areas to about 50 inches in the interior.

Some of the water from precipitation is stored in reservoirs on the land surface. There are 11 surface-water reservoirs in Puerto Rico with capacities in excess of 5,000 acre-feet. One acre-foot is the volume of water that would cover 1 acre to a depth of 1 foot. The water in some of these impoundments is used for hydroelectric power generation and irrigation water supplies. However, six of the island's reservoirs are used principally for public water supply. The impoundments also increase evaporation because they increase the area of standing water. This water loss is especially significant in the south coast area, where pan evaporation is almost twice the annual rainfall.

Some water from precipitation enters aquifers as groundwater recharge. Water that is stored in the aquifers might be released later either by withdrawal by wells, by evapotranspiration, or as seepage to streams.

Study Area

Additional publication details

Publication type:
Publication Subtype:
USGS Numbered Series
Ground Water Atlas of the United States: Segment 13, Alaska, Hawaii, Puerto Rico, and the U.S. Virgin Islands
Series title:
Hydrologic Atlas
Series number:
Revised 1999
Year Published:
U.S. Geological Survey
36 p.
Larger Work Title:
Ground Water Atlas of the United States
First page:
Last page:
United States
Alaska, Hawaii, Puerto Rico, Virgin Islands of the United States
Online Only (Y/N):
Additional Online Files (Y/N):