The Colorado River Runs Dry | Science | Smithsonian Contact Privacy Policy Terms of Use The Colorado River Runs Dry Dams, irrigation and now climate change have drastically reduced the once-mighty river. Is it a sign of things to come? Reservoirs along the river may never rise to previous levels. Utah's Lake Powell has a "bathtub ring" that rises at least 70 feet above the water. (Peter McBride) Smithsonian Magazine | Subscribe October 2010 From its source high in the Rocky Mountains, the Colorado River channels water south nearly 1,500 miles, over falls, through deserts and canyons, to the lush wetlands of a vast delta in Mexico and into the Gulf of California. From This Story [×] CLOSE In a potash mine near Moab, Utah, the water is dyed blue to speed evaporation. (Peter McBride) Reservoirs along the river may never rise to previous levels. Utah's Lake Powell has a "bathtub ring" that rises at least 70 feet above the water. (Peter McBride) South of Kremmling, Colorado, an oxbow reflects a harsh sun.

(Peter McBride) Pistachio trees are irrigated in Arizona. (Peter McBride) A boat casts a forlorn shadow in a dry river channel 25 miles from the river's historical end at the Gulf of California. (Peter McBride) A Cocopah Indian surveys what was once the tribe's traditional fishing grounds. (Peter McBride) Jonathan Waterman, entering the delta, paddled the length of the Colorado River. (Peter McBride) Like many Western communities, Las Vegas has limited watering and restricted the planting of grasses. (Peter McBride) Greater Phoenix continues to expand into the desert. The population of the American West is expected to grow, putting added pressures on dwindling water resources. (Peter McBride) Photographer Peter McBride, who lives near Basalt, Colorado, has taken pictures in 50 nations on six continents for magazine, books and films, but he relished the chance to turn his camera on the river that fed his childhood home, a Colorado cattle ranch. (Peter McBride) The Grand Canyon as seen above Phantom Ranch, looking toward the Little Colorado. (Peter McBride) An aerial view of Lake Mead and the Hoover dam.

(Peter McBride) Once the largest dam in the world, the Hoover dam continues to provide power and water to Las Vegas. Behind the dam, Lake Mead shows the dramatic drop in water levels—now 130 feet lower than it was in 2000. (Peter McBride) The river has been running especially low for the past decade, as drought has gripped the Southwest and demand for water continues. (Peter McBride) Enforced catch and release measures mean this rainbow trout is headed back to the upper stretches of the river. (Peter McBride) In the Imperial Valley, feedlots are still abundant and utilize a large supply of water. The dwindling Colorado is a growing concern for ranchers and farmers in the region. (Peter McBride) Horse farms and other businesses depend on the river for survival. 70 percent or more of the Colorado is siphoned off to irrigate 3.5 million acres of cropland. (Peter McBride) With over 150 golf courses and a continuing expansion into the desert, greater Phoenix continues to be a large consumer of water. (Peter McBride) The Colorado River winds through Rocky Mountain National Park.

(Peter McBride) The Colorado meanders for nearly 1,500 miles across seven states. Over the past century, the river has been diverted for drinking water, industry and agriculture. (Guilbert Gates) The Cahaba: A River of Riches That is, it did so for six million years. Then, beginning in the 1920s, Western states began divvying up the Colorado’s water, building dams and diverting the flow hundreds of miles, to Los Angeles, San Diego, Phoenix and other fast-growing cities. The river now serves 30 million people in seven U.S. states and Mexico, with 70 percent or more of its water siphoned off to irrigate 3.5 million acres of cropland. The damming and diverting of the Colorado, the nation’s seventh-longest river, may be seen by some as a triumph of engineering and by others as a crime against nature, but there are ominous new twists. The river has been running especially low for the past decade, as drought has gripped the Southwest. It still tumbles through the Grand Canyon, much to the delight of rafters and other visitors.

And boaters still roar across Nevada and Arizona’s Lake Mead, 110 miles long and formed by the Hoover Dam. But at the lake’s edge they can see lines in the rock walls, distinct as bathtub rings, showing the water level far lower than it once was—some 130 feet lower, as it happens, since 2000. Water resource officials say some of the reservoirs fed by the river will never be full again. Climate change will likely decrease the river’s flow by 5 to 20 percent in the next 40 years, says geoscientist Brad Udall, director of the University of Colorado Western Water Assessment. Less precipitation in the Rocky Mountains will yield less water to begin with. Droughts will last longer. Higher overall air temperatures will mean more water lost to evaporation. “You’re going to see earlier runoff and lower flows later in the year,” so water will be more scarce during the growing season, says Udall. Other regions—the Mediterranean, southern Africa, parts of South America and Asia—also face fresh-water shortages, perhaps outright crises.

In the Andes Mountains of South America, glaciers are melting so quickly that millions of people in Peru, Bolivia and Ecuador are expected to lose a major source of fresh water by 2020. In southwestern Australia, which is in the midst of its worst drought in 750 years, fresh water is so scarce the city of Perth is building plants to remove the salt from seawater. More than one billion people around the world now live in water-stressed regions, according to the World Health Organization, a number that is expected to double by 2050, when an estimated nine billion people will inhabit the planet. “There’s not enough fresh water to handle nine billion people at current consumption levels,” says Patricia Mulroy, a board member of the Colorado-based Water Research Foundation, which promotes the development of safe, affordable drinking water worldwide. People need a “fundamental, cultural attitude change about water supply in the Southwest,” she adds. “It’s not abundant, it’s not reliable, it’s not going to always be there.” Mulroy is also general manager of the Southern Nevada Water Authority, which serves two million people in greater Las Vegas.

The city is one of the largest in the Colorado River basin, but its share of the river is relatively small; when officials allocated the Colorado’s water to different states in 1922, no one expected so many people to be living in the Nevada desert. So Nevadans have gotten used to coping with limitations. They can’t water their yards or wash their cars whenever they like; communities follow strict watering schedules. The water authority pays homeowners to replace water-gulping lawns with rocks and drought-tolerant plants. Golf courses adhere to water restrictions. Almost all wastewater is reused or returned to the Colorado River. Single Page Previous Page 1 of 2 Next 1 2 Like this article? Deciding About the Colorado River Delta Deciding About the Colorado River Delta Rejuvenated Wetlands Raise New Issues About Where Flood Flow Should go River Report, Water Education Foundation , Spring 1999, pp.1,4-9,11 By S. Joshua Newcom* It starts as a trickle in the mountains of Colorado - snow and ice melting to the demand of rising temperatures.

Tumbling thousands of feet, the alpine water melds into a succession of creeks, springs and rivers, forming the waterway known as the Colorado River. Before the creation of state and international boundaries, the unchecked Colorado wound its way through the southwestern corner of North America and into what is now known as Mexico. There, a combination of nutrient-rich water and silt from the river created widespread wetlands - at times extending from southern California where the Salton Sea is today to the northern tip of the Gulf of California (gulf). Bordered by mountains and desert, the historic delta comprised over 2.5 million acres of wetlands and provided habitat for an estimated 400 species of plants and wildlife. Along its shores, some 20,000 Cocopah Indians made a life from the ecosystem by fishing, hunting and farming. The area became known as the Delta del Rio Colorado - the Colorado River Delta. Cienaga de Santa Clara With completion of Hoover Dam in 1935, the United States began damming the Colorado River - on a much larger scale than in the past - to meet the needs of burgeoning cities and farms in the West.

Nearly 30 years later, the last of the great dams on the Colorado River was completed when the gates of Glen Canyon Dam closed in 1963. Mexico, which at one time received the full flow of the Colorado, agreed to an annual delivery of 1.5 million acre-feet (plus up to 200,000 acre-feet in surplus years) of water under the Mexican Water Treaty of 1944, all of Mexico's river apportionment is typically consumed by agriculture and municipal industrial (M&I) uses in the Mexicali and San Luis valleys. Thus, in a normal water year, the last drop of the Colorado River evaporates in Mexican sands - about 1,450 miles from its birth and short of its natural termination in the gulf. Increased public interest in environmental issues over the years has substantially boosted attention to the delta from governments, non-governmental organizations (NGOs) and environmental groups on both sides of the border. According to U.S. fish & Wildlife Service (USFWS) figures, over half of the wetlands in the United States have disappeared since 1780.

And according to the California resources Agency, California, a major stopover for birds along the Pacific Flyway, has only 10 percent of the wetlands that existed before European settlement. The short-fall is this rich habitat has led some to see Mexico's delta as a potential saving grace. Nearly two decades of heavy rain and snowfall, as well as the filling of Lake Powell, have boosted river flows into Mexico. The Colorado River has reached the gulf five times since 1983, most recently in 1998. Wetlands in the region have benefited from the flood flows that have regenerated vegetation and fish and wildlife populations. Sections of the delta and the northern gulf support several endangered species like the totoaba, a fish species once used commercially and for sport fishing; the vaquita, the world's smallest propoise and rarest ocean mammal; the desert pupfish; and endangered species of birds such as the southwestern willow flycatcher and the Yuma clapper rail. Questions now abound about if and how to maintain and enhance these vital wetland ecosystems, but a solution - like anything on the Colorado River - is far from easy.

In an act of protest, two environmental groups within the United States have bowed out of a steering committee for the Lower Colorado River Multi-Species Conservation Program (LCRMSCP) claiming the program falls short of addressing environmental needs in the Mexican delta. The LCRMSCP is an environmental management plan for the Lower Colorado River being implemented by the Bureau of Reclamation (Bureau) in cooperation with the USFWS, the Lower Basin states, Tribes and National park Service. (See Winter '98 River Report.) The Colorado River flows to the Gulf of California only in extremely wet years. "Eighty percent of the top Lower Colorado River habitat is in Mexico," said Tom Larousek, a conservation associate with American Rivers (one of the letter's signatories), an organization that listed the Colorado River Delta as one of its top 10 endangered rivers in North America in 1998. "It is a biological reservoir of sorts and if you enhance that habitat, you enhance the recovery of species all the way up the Colorado River into the U.S." But state interests and the United States government remain cautious about supplying more water to Mexico.

"We don't have any jurisdiction over how Colorado River water is used once it crosses over the border into Mexico," said Robert Johnson, regional director for the Bureau's Lower Colorado Region. This issue of River Report examines the complexities surrounding the Colorado River Delta, its ecological importance, the issues that impact it and what is being done internationally to preserve and enhance its existence. Brief history of the delta Before humans began to build cities, farm western soil and dam the Colorado, the delta was as untamed and wild as the river that fed it. During a boating journey down the Colorado River Delta for sunset Magazine in 1923, journalist Lewis Freeman remarked: "At the time of which I write, indeed, it is probably that no place in the world with the exception of the removed regions of Africa harbored so great a variety of wildlife." Nearly 80 years later, the delta is a very different place, primarily due to agricultural development and changes to both water quantity and water quality resulting from controlled river runoff. Prior to the completion of Glen canyon Dam, between 4 and 6 millions acre-feet of Colorado River water still inundated Mexico's wetlands in a normal water year. Today, flows only reach the delta in very wet years.

El Niño created a succession of these wet years on the Colorado River system from 1983-87 and once again, waters flooded the delta, scrubbing floodplains, spreading tree and plant seeds and soaking lands that had not been touched directly by the river's water in nearly two decades. Since the turn of the century, about 150,000 acres of wetlands in Mexico have been converted into agricultural fields. Levees built by the Mexican government to prevent flooding of farmlands follow the Colorado River from Morelos Dam to the Gulf of California. The levees channel these flows and have essentially created a revitalization of natural wetlands in parts of the delta during abundant water years. In 1993 the government of Mexico declared 2.3 million acres of water and land in the Northern Gulf and parts of the delta as the Reserva de la Biosfera del Alto Golfo de California y Delta del Rio Colorado (Biosphere Reserve) - a region of habitat managed and protected by the Mexican government. The reserve, 80 percent of which is aquatic, includes portions of the Rio Hardy wetlands and the Cienega de Santa Clara (Cienega) in the deltas.

Activities in the core zone - about 400,000 acres - are limited to research, small-scale shellfish harvesting and low-impact ecotourism. Some in the United States view this protection of fish and wildlife habitat as an indication of Mexico's commitment to ecosystem preservation. "Part of the Reserve's core zone extends into about 72,000 acres of the delta," said José Campoy, director of the reserve, " and one of the major concerns of the reserve is obtaining more fresh-water flows." Studies In the past year, the delta has been the subject of two reports: "Importance of United States' Water Flows to the Colorado River Delta and the Northern Gulf of California, Mexico" (Glenn and Valdes-Casillas, October 1998) and "Wetland Management & Restoration in the Colorado River Delta: The First Steps," (North American Wetlands Conservation Council, November 1998). The reports are collaborative efforts between academic institutions and NGOs in Mexico and the United States.

The studies make two strong assertions about what could be done to improve the region: The most valuable native riparian habitat is in the 150,000-acre core of the delta, and The delta can be sustained with relatively marginal annual instream flows and periodic flooding. In all, the reports conclude that the Mexican delta contains nearly twice the riparian habitat and marshland found along the adjoining stretch of river in the United States from Davis to Morelos dams including cottonwood-willow forest, flood plains and emergent wetlands. Cienega De Santa Clara One wetland in the delta is an anomaly to the rest, existing regardless of instream flows from the Colorado River: The Cienega de Santa Clara (Cienega). In the 1970s, the Cienega comprised less than 500 acres, the result of natural artesian springs, tailwater (agricultural runoff) from surrounding agricultural lands in Riito and occasional high tides or storm surges from the gulf.

Today, the Cienega has expanded to cover approximately 15,000 acres of dense vegetation, due almost entirely to agricultural return flows from the Wellton-Mohawk Irrigation and Drainage District (WMIDD) across the border in the United States. WMIDD diverts approximately 400,000 acre-feet of Colorado River water annually to irrigate crops. Already salty Colorado River water, combined with the saline soil of WMIDD farmlands, creates a saline groundwater supply. As the underlying groundwater basin fills with water from the irrigated fields, salts are pushed to the surface. To keep the salts below the root zone, pumps remove about 120,000 acre-feet of brackish groundwater annually. Initially, the saline groundwater pumped from WMID farmlands was blended with Colorado river water going to Mexico. but Minute 242, negotiated in 1973 (an addition to the '44 Treaty), curbed the amount of salts allowed in credited water deliveries over the border. To desalinate the Wellton-Mohawk drainage water before its delivery to Mexico, the Bureau constructed the world's largest reverse osmosis water treatment facility in Yuma, Ariz.

At a construction cost of $250 million, the Yuma Desalting Plant is designed to treat up to 96,000 acre-feet of water annually. During construction of the treatment plant, a $45 million drainage canal was constructed in 1977 to dispose of the brackish WMIDD groundwater until it could be treated by the plant. The bypass drain transports the salty drainage water 50-miles south into the Cienega. (This water does not meet international salinity requirements and is not counted towards Mexico's Colorado River apportionment.) To date, the Yuma Desalting Plant has never been fully online. It operated at one-third capacity of nearly a year, but a flood on the Gila river in 1993 washed out a section of the drainage canal, halting operation. In the meantime, the Yuma Desalting Plant is being kept in "ready reserve" at a cost of $6 million a year. the possibility of operating the plant looms closer as water use on the Colorado continues to grow.

"There is recognition that operation of the desalting plant will decrease the quantity and increase the salinity of the water traveling to the Cienega," said William Rinne, are manager for the Bureau's Lower Colorado Region. "I would expect the Cienega to become more limited in terms of what it could sustain," he said. Rinne said that bypass drain would still be used to discharge the salty byproduct of the desalting plant to the Cienega. Additionally, the reports conclude that riparian habitat mitigation can be sustained with a relatively nominal amount of water annually - perhaps less than 1 percent of normal river flow, augmented by treated wastewater streams and occasional flood releases. "Because the primary water needs in the delta are intermittent, it doesn't require huge amounts of the river's annual flow,," said Edward Glenn, a professor in the Department of Soil, water and Environment at the University of Arizona and one of the lead scientists in the studies.

"Around a half-million acre-feet of water every three to four years would probably take care of riparian habitat for the region." Using satellite photos, aerial videography and photos, ground surveys and extensive on-site work including vegetative analysis and mapping, water quality analysis and community outreach, the group of researchers compared high-flow years in the delta with low flow years. The studies estimate that the delta's core can be flooded with about 256,000 acre-feet of water, a process important to germination for the delta's vegetation, according to Glenn. This number was achieved by examining the records of flows through Morelos Dam and a comparison of delta vegetation during those corresponding flows. The studies showed trees were able to continue growing during three or four successive years of receiving no water, thus concluding that overbank flooding of the delta core is needed every four years to sustain existing vegetation and that more frequent floods could expand the scope of the riparian zone. Approximately 32,000 acre-feet of inflows annually to the delta are sufficient to maintain the cottonwood-willow gallery forest, a habitat capable of supporting a wide variety of birds and wildlife.

Although the Bureau is interested in information about the ecology of the delta, officials remain cautious about the flow requirement findings in the reports. "At present, it is difficult to evaluate the accuracy of the estimates because there is not a good understanding of how the data was generated and the assumptions made about conditions which existed within the river corridor preceding these flow events," said William Rinne, area manager for the Bureau's Lower Colorado Region. To help solidify the answers, a joint effort is underway between USFWS and universities in the U.S. and Mexico to conduct field studies of wildlife and vegetative counts in the delta. "The delta is one of the most important migratory routes in all of North America," said Charlie Sanchez, associate regional director for international affairs with the USFWS. "We have to leave a legacy in the protection of these resources." Sanchez said he is working with affiliates in Mexico and the United States to conduct population assessments of species like the Yuma clapper rail and the desert pupfish,to assess the riparian values of delta wetlands areas and to map those areas. So far, Sanchez said Interior has spent around $909,000 to initiate the studies.

Glenn said some of the information will be incorporated into a third study planned for publication later this year that makes specific recommendations for managing habitats in the delta region. Policy There are those who would like the United States to be required to release Colorado River flows for the environment, including the delta. But this idea has sparked considerable debate among policy makers because of legal constraints and the lack of additional water. "Water and power agencies in the U.S. want all Colorado River water for use in the U.S. and refuse to allocated any for conservation purposes," said David Hogan, river program coordinator for the Southwest Center for Biological Diversity, an environmental group based in Tucson, Ariz. The Southwest Center, along with Defenders of Wildlife, pulled out of a process to develop a LCRMSCP - the management plan for fish, wildlife and habitat on the U.S. portion of the Lower Colorado River - because the plan would not address the Mexican delta. The Yuma clapper rail is one of the delta's endangered species. In March 1999, a letter was addressed to Interior Secretary Bruce Babbitt from the Southwest Center and eight other organizations expressed their dismay over what they consider to be shortcomings of the program.

"Conservation planning that stops at geopolitical borders without regard for ecosystem boundaries and sound conservation biology principles is domed to fail," they wrote. The letter also condemns a resolution passed by the Colorado River Board of California stating its desire to limit the LCRMSCP to the United States. Salton Sea "We must also come up with a solution about how to approach Salton Sea restoration in a way that isn't detrimental to the Delta, " said Jason Morrison, a senior associate with the Pacific Institute. The February 1999 Pacific Institute study, entitled "Haven or Hazard: The Ecology and Future of the Salton Sea" recommends that restoration plans for the 35-mile long Salton Sea be coordinated with restoration efforts in the delta. Part of the same historical basin, the report denounces proposed restoration alternative that would divert Colorado River flood water away from the delta to the Salton Sea or pump in potentially harmful brine water from the Salton Sea somewhere near the delta. But not everyone agrees with the Institute's recommendations. "In a perfect world, barring political, practical and economic considerations, the ecology of all these places would get comprehensively studied and managed," said Tom Kirk, executive director of the Salton Sea Authority.

"We do not live in a perfect world." Kirk went on to say that due to time constraints under the Salton Sea Reclamation Act of 1998, scientists working on the sea must make recommendation to Congress by January 1, 2000. "It was the board's opinion that with the limited time and money available to us, what we need to do now is focus on developing the LCRMSCP within the U.S.," said Gerald Zimmerman, executive director of the Colorado River Board of California. Currently in development, the 50-year LCRMSCP is expected to be ready for implementation by 20001. "There is the adaptive management component of the LCRMSCP that encourages the exchange of information between U.S. and Mexican officials and could allow for expansion into the delta once the LCRMSCP is implemented," he said. Under a separate effort, a coalition of U.S. government departments and agencies, and their Mexican counterparts, have entered into discussions with the Mexican government via the International Boundary and Water Commission (IBWC). The IBWC is the arm of the U.S. government that handles issues associated with water deliveries to Mexico such as the availability of water and when such water can be delivered.

"Discussion of restoring the delta was added to our agenda about a year ago," said John Bernal, commissioner for the United States Section of the IBWS. Bernal said the IBWC has facilitated one major meeting and several technical -level working sessions with various U.S. and Mexico federal and state agencies to determine what still needs to be gathered. "We are addressing the challenge of assuring conditions in the delta do not deteriorate any further and that preferably valuable resources are sustained," he said. In May 1997 - the same year work began in earnest on developing the LCRMSCP - Babbitt and Julia Carabias Lillo, Mexicos secretary of Environment, Natural Resources and Fisheries, signed a Letter of Intent in which their "two agencies plan to expand existing cooperative activities in the conservation of contiguous natural protected areas in the border zone, and to consider new opportunities for cooperation in the protection of natural protected areas in the United States-Mexico border." The letter was the latest in a series of agreements between the countries intended to augment binational protection and enhancement of the border environment through an exchange of scientific information, community outreach programs and joint studies.

But for the Southwest Centers Hogan, the time for increased water deliveries to Mexico is now. He said that center could sue to try and get committed river flows for the delta by proving the United States is responsible for endangered species protection in Mexico. "Its a simple issue of encouraging the U.S. to take responsibility for the effect of exploiting the river," he said. Supplying the delta If it were determined that more water should be given for the delta, how to increase those allocations to Mexico without reopening the Law of the River is considered by many to be a major undertaking and one that could involve massive litigation. The 1922 Colorado river Compact only allocated water to seven U.S. states and, despite subsequent state, federal and international agreements, treaties, court decisions and legislation, the environment is only recognized as a limited "user" of the Colorado River. (Water was allocated under the 1968 Colorado River Basin Project Act for a number of wildlife refuges within the basin.) Stakeholders Over 200,000 people live in the delta area (not including Mexicali), members of what remains of the Cocopah Indian tribe near El Mayor, a tributary to the Hardy River.

Part of the equation for restoring the delta is to involve these communities in discussions about the delta. Goals of an outreach program organized and funded by the North American Wetlands conservation Council, a coalition supported by federal, academic and nongovernmental entities include: Identifying stakeholder groups; Explaining restoration possibilities and determining if the stakeholders are interested in participating; Bringing stakeholders, local government leaders, research centers, academics, NGOs and policy makers to the table and facilitate discussions about what everyone's needs from the delta are; and Conducting workshops and conferences to discuss restoration plans and implementation. "Part of the niche we are trying to fill in the absence of community leaders in modifying and changing the use of water in the delta," said Steve Cornelius, director of borderlands program for Tucson-based Sonoran institute, one of the participants in the program. "Managing the delta has to be beneficial to the communities using the water." Cornelius said he expects the process, which has been spearheaded in the Rio Hardy Wetland area, to take up to seven years to complete. "We need to understand what the various stakeholder positions are and they need good, solid, scientific information in order to design a position," he said.

Carlos Valdés-Cassillas, director of the Center for Conservation and Use of Natural Resources at the Monterey Institute in Guyamas agrees. "The best possibility for accomplishing a management plan for the delta is to look at a more effective use of water including reuse of agricultural run-off and additional instream flows during flood years," he said. "We must look to different institutions and water users to partake in the responsibility." Other groups taking part in the process include Pronatura Sonora, the University of Arizona, California State University, Dominguez Hills and the Environmental Defense Fund. A variety of programs and treaties have been implemented over the past 50 years to address border issues, but none has dealt specifically with the delta region. The Border XXI Program and North American Free Trade Agreement are two examples of major binational agreements that include environmental provisions, but neither has the authority to allocate more Colorado River water for the delta. For parties in the United States, a major concern of allocating greater flows to Mexico is a lack of certainty over how water intended for environment would be used. The Mexicali Valley relies on the Colorado and Gila rivers for nearly 100 percent of its water supply.

Preliminary calculations by the Pacific Institute in its 1996 report "The Sustainable Use of Water in the Colorado River Basin" indicate that the Mexicali Valley is suffering from a groundwater overdraft of roughly 96000 acre-feet annually. The overdraft could become even greater with the added lining of the All American Canal north of the border - a source of groundwater recharge for Mexico. Mexico's desire for more water has given rise to fears that increased flows to Mexico would be used to recharge goundwater overdraft or to irrigate fields in Mexico instead of as instream flows for the environment. One proposed method for earmarking additional flows to Mexico for instream purposes is amending the existing Law of the River. Some have used Minute 242, the addition to the 1944 Treaty with Mexico that improved the quality of water deliveries to Mexico, as an example of a potential way to permanently mitigate water for fish, wildlife and habitat in the delta. Similarly, some purport that a like measure could modify the Colorado River Basin Salinity Control Act of 1974 to ensure continued deliveries of equal or better quality water to the Cienega.

The Colorado River Delta in a Glance The Riparian Corridor (1) Bordered by farmland levees. Stretches about 70 miles from Morelos Dam to the junction with the Hardy River. Contains large stands of cottonwood and willow trees measuring 13-feet to 50-feet tall, from high water in the 1980s and saplings from recent floods in 1993 and 1979. The Rio Hardy Wetland (2) Maze of sloughs and old river channels and confluence of the Hardy and Colorado rivers. High salinity levels in area due to high tides from the gulf 35 miles away, geothermal wells and agricultural drainage. Some cottonwood and willow trees in northerly portion giving way to more salt-tolerant vegetation including, salt cedar, mesquite, areas of Palmers saltgrass and cattails. Once a natural earthen dam located 21 miles northwest of gulf created a natural 45,000 acre floodplain that grew to 160,000 acres with 80s upstream releases.

Wetlands shrunk to 3,000 acres after dam breached by floods. Ducks Unlimited studied rebuilding the natural dam, but determined frequency of flows too few and of poor quality. Laguna (3) Only during very wet years does water reach this salt flat. In majority of years, area looks like a desert plain. Cienega de Santa Clara (4) Largest and one of the most biologically rich delta wetland areas. Supported by brackish agricultural flows in the U.S. and Mexico and natural artesian springs. Supports large populations of endangered desert pupfish and Yuma clapper rails. El Doctor Wetlands (5) 1,700 acre wetland fed primarily by natural springs. Area biologically rich and believed to contain over 22 different types of plant species and endangered desert pupfish. El Indio Wetlands (6) Covers over 4,000 acres fed by return flows from surrounding San Luis farmlands. Agricultural flow once followed a drainage channel back to the river wetlands but around 1993, the drain was damaged and flows spilled outside the flood plain.

Typical regional coastal wetlands vegetation, primarily salt cedar, mesquite, Palmers grass and cattails. Intertidal Wetlands (7) Mudflat much of the time, though an estuary during we years. 13 mile-long section provides - at times - spawning habitat for numerous species, including corvina, totoaba and shrimp. Easily disrupted by a lack of fresh water, creating water too saline to support the propagation of many species. The Water Cycle summary, USGS Water Science School Water storage in oceans: Saline water existing in oceans and inland seas The ocean as a storehouse of water The water cycle sounds like it is describing how water moves above, on, and through the Earth ... and it does. But, in fact, much more water is "in storage" for long periods of time than is actually moving through the cycle. The storehouses for the vast majority of all water on Earth are the oceans.

It is estimated that of the 332,600,000 cubic miles (mi3) (1,386,000,000 cubic kilometers (km3)) of the world's water supply, about 321,000,000 mi3 (1,338,000,000 km3) is stored in oceans. That is about 96.5 percent. It is also estimated that the oceans supply about 90 percent of the evaporated water that goes into the water cycle. During colder climatic periods more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age glaciers covered almost one-third of Earth's land mass, with the result being that the oceans were about 400 feet (122 meters) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 18 feet (5.5. meters) higher than they are now.

About three million years ago the oceans could have been up to 165 feet (50 meters) higher. Oceans in movement If you have ever been seasick (we hope not), then you know how the ocean is never still. You might think that the water in the oceans moves around because of waves, which are driven by winds. But, actually, there are currents and "rivers" in the oceans that move massive amounts of water around the world. These movements have a great deal of influence on the water cycle. The Kuroshio Current, off the shores of Japan, is the largest current. It can travel between 25 and 75 miles (40 and 121 kilometers) a day, 1-3 miles (1.4-4.8 kilometers) per hour, and extends some 3,300 feet (1,000 meters) deep. The Gulf Stream is a well known stream of warm water in the Atlantic Ocean, moving water from the Gulf of Mexico across the Atlantic Ocean towards Great Britain. At a speed of 60 miles (97 kilometers) per day, the Gulf stream moves 100 times as much water as all the rivers on Earth.

Coming from warm climates, the Gulf Stream moves warmer water to the North Atlantic. More about water storage in the oceans. Evaporation: The process by which water is changed from liquid to a gas or vapor Evaporation and why it occurs Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in our atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration . Heat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point (212° F, 100° C) but evaporates much more slowly at the freezing point. Net evaporation occurs when the rate of evaporation exceeds the rate of condensation. A state of saturation exists when these two process rates are equal, at which point, the relative humidity of the air is 100 percent.

Condensation, the opposite of evaporation, occurs when saturated air is cooled below the dew point (the temperature to which air must be cooled at a constant pressure for it to become fully saturated with water), such as on the outside of a glass of ice water. In fact, the process of evaporation removes heat from the environment, which is why water evaporating from your skin cools you. Evaporation drives the water cycle Evaporation from the oceans is the primary mechanism supporting the surface-to-atmosphere portion of the water cycle. After all, the large surface area of the oceans (over 70 percent of the Earth's surface is covered by the oceans) provides the opportunity for such large-scale evaporation to occur. On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation. Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation.

Once evaporated, a water molecule spends about 10 days in the air. More about evaporation. Sublimation: The changing of snow or ice to water vapor without melting Sublimation describes the process of snow and ice changing into water vapor without first melting into water. Sublimation is a common way for snow to disappear in certain climates. It is not easy to actually see sublimation happen, at least not with ice. One way to see the results of sublimation is to hang a wet shirt outside on a below-freezing day. Eventually the ice in the shirt will disappear. Actually, the best way to visualize sublimation is to not use water at all, but to use carbon dioxide instead, as this picture shows."Dry ice" is solid, frozen carbon dioxide, which sublimates, or turns to gas, at the temperature -78.5 °C (-109.3°F). The fog you see in the picture is a mixture of cold carbon dioxide gas and cold, humid air, created as the dry ice sublimates. Sublimation occurs more readily when certain weather conditions are present, such as low relative humidity and dry winds.

It also occurs more at higher altitudes, where the air pressure is less than at lower altitudes. Energy, such as strong sunlight, is also needed. If I was to pick one place on Earth where sublimation happens a lot, I might choose the south side of Mt. Everest. Low temperatures, strong winds, intense sunlight, very low air pressure - just what is needed for sublimation to occur. More about sublimation. Evapotranspiration: The process by which water vapor is discharged to the atmosphere as a result of evaporation from the soil and transpiration by plants. Although some definitions of evapotranspiration include evaporation from surface-water bodies, such as lakes and even the ocean, on this Web site, evapotranspiration is defined as the water lost to the atmosphere from the ground surface and the transpiration of groundwater by plants through their leaves. Transpiration: The release of water from plant leaves Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves.

It is estimated that about 10 percent of the moisture found in the atmosphere is released by plants through transpiration. Plant transpiration is an invisible process—since the water is evaporating from the leaf surfaces, you don't just go out and see the leaves "breathing". During a growing season, a leaf will transpire many times more water than its own weight. A large oak tree can transpire 40,000 gallons (151,000 liters) per year. Atmospheric factors affecting transpiration The amount of water that plants transpire varies greatly geographically and over time. There are a number of factors that determine transpiration rates: Temperature:Transpiration rates go up as the temperature goes up, especially during the growing season, when the air is warmer. Relative humidity: As the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air. Wind and air movement: Increased movement of the air around a plant will result in a higher transpiration rate.

Soil-moisture availability: When moisture is lacking, plants can begin to senesce (premature ageing, which can result in leaf loss) and transpire less water. Type of plant: Plants transpire water at different rates. Some plants which grow in arid regions, such as cacti and succulents, conserve precious water by transpiring less water than other plants. More about evapotranspiration. Water storage in the atmosphere: Water stored in the atmosphere as vapor, such as clouds and humidity The atmosphere is full of water The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. There is always water in the atmosphere. Clouds are, of course, the most visible manifestation of atmospheric water, but even clear air contains water—water in particles that are too small to be seen. One estimate of the volume of water in the atmosphere at any one time is about 3,100 cubic miles (mi3) or 12,900 cubic kilometers (km3).

That may sound like a lot, but it is only about 0.001 percent of the total Earth's water volume. If all of the water in the atmosphere rained down at once, it would only cover the ground to a depth of 2.5 centimeters, about 1 inch. More about water stored in the atmosphere. Condensation: The process by which water is changed from vapor to liquid Condensation is the process in which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation. You don't have to look at something as far away as a cloud to notice condensation, though. Condensation is responsible for ground-level fog, for your glasses fogging up when you go from a cold room to the outdoors on a hot, humid day, for the water that drips off the outside of your glass of iced tea, and for the water on the inside of your home windows on a cold day.

Condensation in the air Even though clouds are absent in a crystal clear blue sky, water is still present in the form of water vapor and droplets which are too small to be seen. Depending on meteorological conditions, water molecules will combine with tiny particles of dust, salt, and smoke in the air to form cloud droplets, which grow and develop into clouds, a form of water we can see. Cloud droplets can vary greatly in size, from 10 microns (millionths of a meter) to 1 millimeter (mm), and even as large as 5 mm. This process occurs higher in the sky where the air is cooler and more condensation occurs relative to evaporation. As water droplets combine (also known as coalescence) with each other, and grow in size, clouds not only develop, but precipitation may also occur. Precipitation is essentially water cloud in its liquid or solid form falling form the base of a cloud. This seems to happen too often during picnics or when large groups of people gather at swimming pools. You might ask ... why is it colder higher up? As we said, clouds form in the atmosphere because air containing water vapor rises and cools.

The key to this process is that air near the Earth's surface is warmed by solar radiation. But, do you know why the atmosphere cools above the Earth's surface? Generally, air pressure, is the reason. Air has mass (and, because of gravity on Earth, weight) and at sea level the weight of a column of air pressing down on your head is about 14 ½ pounds (6.6 kilograms) per square inch. The pressure (weight), called barometric pressure, that results is a consequence of the density of the air above. At higher altitudes, there is less air above, and, thus, less air pressure pressing down. The barometric pressure is lower, and lower barometric pressure is associated with fewer molecules per unit volume. Therefore, the air at higher altitudes is less dense. Since fewer air molecules exist in a certain volume of air, there are fewer molecules colliding with each other, and as a result, there will be less heat produced. This means cooler air. Do you find this confusing? Just think, clouds form all day long without having to understand any of this. More about condensation.

Precipitation: The discharge of water, in liquid or solid state, out of the atmosphere, generally upon a land or water surface Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain. How do raindrops form? The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud you can see some parts disappearing (evaporating) while other parts are growing (condensation). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds. For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Water droplets may grow as a result of additional condensation of water vapor when the particles collide.

If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop. Precipitation rates vary geographically and over time Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40-50 inches (102-127 centimeters (cm)) per year. Summer thunderstorms may deliver an inch or more of rain on one suburb while leaving another area dry a few miles away. But, the rain amount that Georgia gets in one month is often more than Las Vegas, Nevada observes all year. The world's record for average-annual rainfall belongs to Mt. Waialeale, Hawaii, where it averages about 450 inches (1,140 cm) per year. A remarkable 642 inches (1,630 cm) was reported there during one twelve-month period (that's almost 2 inches (5 cm) every day!).

Is this the world record for the most rain in a year? No, that was recorded at Cherrapunji, India, where it rained 905 inches (2,300 cm) in 1861. Contrast those excessive precipitation amounts to Arica, Chile, where no rain fell for 14 years The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts? On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters). More about precipitation. Water storage in ice and snow: Freshwater stored in frozen form, generally in glaciers, icefields, and snowfields ice caps around the world Although the water cycle sounds like it is describing the movement of water, in fact, much more water is in storage at any one time than is actually moving through the cycle.

By storage, we mean water that is locked up in its present state for a relatively long period of time, such as in ice caps and glaciers. The vast majority, almost 90 percent, of Earth's ice mass is in Antarctica, while the Greenland ice cap contains 10 percent of the total global ice-mass. The Greenland ice cap is an interesting part of the water cycle. The ice cap became so large over time (about 600,000 cubic miles (mi3) or 2.5 million cubic kilometers (km3)) because more snow fell than melted. Over the millenia, as the snow got deeper, it compressed and became ice. The ice cap averages about 5,000 feet (1,500 meters) in thickness, but can be as thick as 14,000 feet (4,300 meters). The ice is so heavy that the land below it has been pressed down into the shape of a bowl.

In many places, glaciers on Greenland reach to the sea, and one estimate is that as much as 125 mi3 (517 km3) of ice "calves" into the ocean each year—one of Greenland's contributions to the global water cycle. Ocean-bound icebergs travel with the currents, melting along the way. Some icebergs have been seen, in much smaller form, as far south as the island of Bermuda. Ice and glaciers come and go The climate, on a global scale, is always changing, although usually not at a rate fast enough for people to notice. There have been many warm periods, such as when the dinosaurs lived (about 100 million years ago) and many cold periods, such as the last ice age of about 20,000 years ago. During the last ice age much of the northern hemisphere was covered in ice and glaciers, and, as this map from the University of Arizona shows, they covered nearly all of Canada, much of northern Asia and Europe, and extended well into the United States. Some glacier and ice cap facts Glacial ice covers 10 - 11 percent of all land.

According to the National Snow and Ice Data Center (NSIDC) , if all glaciers melted today the seas would rise about 230 feet (70 meters). During the last ice age (when glaciers covered more land area than today) the sea level was about 400 feet (122 meters) lower than it is today. At that time, glaciers covered almost one-third of the land. During the last warm spell, 125,000 years ago, the seas were about 18 feet (5.5 meters) higher than they are today. About three million years ago the seas could have been up to 165 feet (50.3 meters) higher. More about water storage in ice and snow. Snowmelt runoff to streams: The movement of water as surface runoff from snow and ice to surface water If you live in Florida or on the French Riviera you might not wake up everyday wondering how melting snow contributes to the water cycle. But, in the world-wide scheme of the water cycle, runoff from snowmelt is a major component of the global movement of water. In the colder climates much of the springtime runoff and streamflow in rivers is attributable to melting snow and ice.

The effect of snowmelt on potential flooding, mainly during the spring, is something that causes concern for many people around the world. Besides flooding, rapid snowmelt can trigger landslides and debris flows. Contribution of snowmelt to streamflow A good way to visualize the contribution of snowmelt to streamflow in rivers is to look at the hydrograph below, which shows daily mean streamflow (average streamflow for each day) for four years for the North Fork American River at North Fork Dam in California ( real-time data streamflow ). The large peaks in the chart are mainly the result of melting snow, although storms can contribute runoff also. Compare the fact that minimum mean-daily streamflow during March of 2000 was 1,200 cubic feet per second (ft3), while during August streamflows ranged from 55-75 ft3. Note that runoff from snowmelt varies not only by season but also by year. Compare the high peaks of streamflows for the year 2000 with the much smaller streamflows for 2001. It looks like a major drought hit that area of California in 2001.

The lack of water stored as snowpack in the winter can affect the availability of water (for streamflow) in streams the rest of the year. This can have an effect on the amount of water in reservoirs located downstream, which in turn can affect water available for irrigation and the water supply for cities and towns. More about snowmelt runoff to streams. Surface runoff: Precipitation runoff which travels over the soil surface to the nearest stream channel Surface runoff is precipitation runoff over the landscape Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. That is "overly simplified" because rivers also gain and lose water to the ground. Still, it is true that much of the water in rivers comes directly from runoff from the land surface, which is defined as surface runoff. When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill.

Water will flow along channels as it moves into larger creeks, streams, and rivers. This picture gives a graphic example of how surface runoff (here flowing off a road) enters a small creek. The runoff in this case is flowing over bare soil and is depositing sediment into the river (not good for water quality). The runoff entering this creek is beginning its journey back to the ocean. As with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is evaporated, transpired, or soaks into groundwater. Surface runoff can also be diverted by humans for their own uses. More about surface runoff. Streamflow: The movement of water in a natural channel, such as a river The U.S. Geological Survey ( USGS ) uses the term "streamflow" to refer to the amount of water flowing in a river.

Although USGS usually uses the term "stream" when discussing flowing water bodies, in these pages we'll use "rivers" more often to describe flowing creeks, streams, and rivers, since that is probably what you are more familiar with. Importance of rivers Rivers are invaluable to not only people, but to life everywhere. Not only are rivers a great place for people (and their dogs) to play, but people use river water for drinking-water supplies and irrigation water, to produce electricity, to flush away wastes (hopefully, but not always, treated wastes), to transport merchandise, and to obtain food. Rivers are indeed major aquatic landscapes for all manners of plants and animals. Rivers even help keep the aquifers underground full of water by discharging water downward through their streambeds. And, we've already mentioned that the oceans stay full of water because rivers and runoff continually refreshes them. Watersheds and rivers When looking at the location of rivers and also the amount of streamflow in rivers, the key concept to know about is the river's "watershed". What is a watershed? Easy, if you are standing on the ground right now, just look down. You're standing, and everyone is standing, in a watershed.

A watershed is the area of land where all of the water that falls in it and drains off of it goes into the same place. Watersheds can be as small as a footprint in the mud or large enough to encompass all the land that drains water into the Mississippi River where it enters the Gulf of Mexico. Smaller watersheds are contained in bigger watersheds. It all depends of the outflow point—all of the land above that drains water that flows to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above" the river-outflow point Streamflow is always changing Streamflow is always changing, from day to day and even minute to minute. Of course, the main influence on streamflow is precipitation runoff in the watershed. Rainfall causes rivers to rise, and a river can even rise if it only rains very far up in the watershed—remember that water that falls in a watershed will eventually drain by the outflow point. The size of a river is highly dependent on the size of its watershed.

Large rivers have watersheds with lots of surface area; small rivers have smaller watersheds. Likewise, different size rivers react differently to storms and rainfall. Large rivers rise and fall slower and at a slower rate than small rivers. In a small watershed, a storm can cause 100 times as much water to flow by each minute as during baseflow periods, but the river will rise and fall possibly in a matter of minutes and hours. Large rivers may take days to rise and fall, and flooding can last for a number of days. After all, it can take days for all the water that fell hundreds of miles upstream to drain past an outflow point. More about streamflow. Freshwater storage: Freshwater existing on the Earth's surface One part of the water cycle that is obviously essential to all life on Earth is the freshwater existing on the land surface. Just ask your neighbor, a tomato plant, a trout, or that pesky mosquito. Surface water includes the streams (of all sizes, from large rivers to small creeks), ponds, lakes, reservoirs (man-made lakes), and freshwater wetlands.

The definition of freshwater is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt. The amount of water in our rivers and lakes is always changing due to inflows and outflows. Inflows to these water bodies will be from precipitation, overland runoff, groundwater seepage, or tributary inflows. Outflows from lakes and rivers include evaporation and discharge to groundwater. Humans get into the act also, as people make great use of diverted surface water for their needs. So, the amount and location of surface water changes over time and space, whether naturally or with human help. Certainly during the last ice age when glaciers and snowpacks covered much more land surface than today, life on Earth had to adapt to different hydrologic conditions than those which took place both before and after. And the layout of the landscape certainly was different before and after the last ice age, which influenced the topographical layout of many surface-water bodies today.

Glaciers are what made the Great Lakes not only "great," but also such a huge storehouse of freshwater Surface water keeps life going As this satellite picture of the Nile Delta in Egypt shows, life can even bloom in the desert if there is a supply of surface water (or groundwater) available. Water on the land surface really does sustain life, and this is as true today as it was millions of years ago. I'm sure dinosaurs held their meetings at the local watering hole 100 million years ago, just as antelopes in Africa do today. And, since groundwater is supplied by the downward percolation of surface water, even aquifers are happy for water on the Earth's surface. You might think that fish living in the saline oceans aren't affected by freshwater, but, without freshwater to replenish the oceans they would eventually evaporate and become too saline for even the fish to survive. Usable freshwater is relatively scarce Freshwater represents only about three percent of all water on Earth and freshwater lakes and swamps account for a mere 0.29 percent of the Earth's freshwater. Twenty percent of all fresh surface water is in one lake, Lake Baikal in Asia.

Another twenty percent is stored in the Great Lakes (Huron, Michigan, and Superior). Rivers hold only about 0.006 percent of total freshwater reserves. You can see that life on Earth survives on what is essentially only a "drop in the bucket" of Earth's total water supply! More about freshwater storage. Infiltration: The downward movement of water from the land surface into soil or porous rock Groundwater begins as precipitation Anywhere in the world, a portion of the water that falls as rain and snow infiltrates into the subsurface soil and rock. How much infiltrates depends greatly on a number of factors. Infiltration of precipitation falling on the ice cap of Greenland might be very small, whereas, as this picture of a stream disappearing into a cave in southern Georgia, USA shows, a stream can act as a direct funnel right into groundwater! Some water that infiltrates will remain in the shallow soil layer, where it will gradually move vertically and horizontally through the soil and subsurface material. Eventually it might enter a stream by seepage into the stream bank. Some of the water may infiltrate deeper, recharging groundwater aquifers.

If the aquifers are shallow or porous enough to allow water to move freely through it, people can drill wells into the aquifer and use the water for their purposes. Water may travel long distances or remain in groundwater storage for long periods before returning to the surface or seeping into other water bodies, such as streams and the oceans. In places where the water table (the top of the saturated zone) is close to the land surface and where the water can move through the aquifer at a high rate, aquifers can be replenished artificially Groundwater storage: Water existing for long periods below the Earth's surface Stored water as part of the water cycle Large amounts of water are stored in the ground. The water is still moving, possibly very slowly, and it is a part of the water cycle. Most of the water in the ground comes from precipitation that infiltrates downward from the land surface. The upper layer of the soil is the unsaturated zone, where water is present in varying amounts that change over time, but does not saturate the soil. Below this layer is the saturated zone, where all of the pores, cracks, and spaces between rock particles are saturated with water. The term groundwater is used to describe this area.

Another term for groundwater is "aquifer," although this term is usually used to describe water-bearing formations capable of yielding enough water to supply peoples' uses. Aquifers are a huge storehouse of Earth's water and people all over the world depend on groundwater in their daily lives. To find water, look under the table ... the water table I hope you appreciate my spending an hour in the blazing sun to dig this hole at the beach. It is a great way to illustrate the concept of how at a certain depth the ground, if it is permeable enough to hold water, is saturated with water. The top of the pool of water in this hole is the water table. The breaking waves of the ocean are just to the right of this hole, and the water level in the hole is the same as the level of the ocean. Of course, the water level here changes by the minute due to the movement of the tides, and as the tide goes up and down, the water level in the hole moves, too. In a way, this hole is like a dug well used to access groundwater, albeit saline in this case. But, if this was freshwater, people could grab a bucket an supply themselves with the water they need to live their daily lives.

You know that at the beach if you took a bucket and tried to empty this hole, it would refill immediately because the sand is so permeable that water flows easily through it, meaning our "well" is very "high-yielding" (too bad the water is saline). To access freshwater, people have to drill wells deep enough to tap into an aquifer. The well might have to be dozens or thousands of feet deep. But the concept is the same as our well at the beach—access the water in the saturated zone where the voids in the rock are full of water. More about groundwater storage. Groundwater discharge: The movement of water out of the ground There's more water than just what you can see You see water all around you every day as lakes, rivers, ice, rain and snow. There are also vast amounts of water that are unseen—water existing in the ground. And even though groundwater is unseen, it is moving below your feet right now. As part of the water cycle, groundwater is a major contributor to flow in many streams and rivers and has a strong influence on river and wetland habitats for plants and animals. People have been using groundwater for thousands of years and continue to use it today, largely for drinking water and irrigation.

Life on Earth depends on groundwater just as it does on surface water. Groundwater flows underground Some of the precipitation that falls onto the land infiltrates into the ground to become groundwater. Once in the ground, some of this water travels close to the land surface and emerges very quickly as discharge into streambeds, but, because of gravity, much of it continues to sink deeper into the ground. If the water meets the water table (below which the soil is saturated), it can move both vertically and horizontally. Water moving downward can also meet more dense and water-resistant non-porous rock and soil, which causes it to flow in a more horizontal fashion, generally towards streams, the ocean, or deeper into the ground. As this diagram shows, the direction and speed of groundwater movement is determined by the various characteristics of aquifers and confining layers (which water has a difficult time penetrating) in the ground. Water moving below ground depends on the permeability (how easy or difficult it is for water to move) and on the porosity (the amount of open space in the material) of the subsurface rock. If the rock has characteristics that allow water to move relatively freely through it, then groundwater can move significant distances in a number of days.

But groundwater can also sink into deep aquifers where it takes thousands of years to move back into the environment, or even go into deep groundwater storage, where it might stay for much longer periods. More about groundwater discharge. Spring: A place where a concentrated discharge of groundwater flows at the ground surface What is a spring? A spring is a water resource formed when the side of a hill, a valley bottom or other excavation intersects a flowing body of groundwater at or below the local water table. A spring is the result of an aquifer being filled to the point that the water overflows onto the land surface. They range in size from intermittent seeps, which flow only after much rain, to huge pools with a flow of hundreds of millions of liters per day. Springs may be formed in any sort of rock, but are more prevalent in limestone and dolomite, which fracture easily and can be dissolved by rainfall that becomes weakly acidic. As the rock dissolves and fractures, spaces can form that allow water to flow. If the flow is horizontal, it can reach the land surface, resulting in a spring. Spring water is not always clear Water from springs usually is remarkably clear.

Water from some springs, however, may be "tea-colored." This picture shows a natural spring in southwestern Colorado. Its red iron coloring and metals enrichment are caused by groundwater coming in contact with naturally occurring minerals present as a result of ancient volcanic activity in the area. In Florida, many surface waters contain natural tannic acids from organic material in subsurface rocks, and the color from these streams can appear in springs. If surface water enters the aquifer near a spring, the water can move quickly through the aquifer and discharge at the spring vent. The discharge of highly colored water from springs can indicate that water is flowing quickly through large channels within the aquifer without being filtered through the limestone. Thermal springs Thermal springs are ordinary springs except that the water is warm and, in some places, hot, such as in the bubbling mud springs in Yellowstone National Park, Wyoming. Many thermal springs occur in regions of recent volcanic activity and are fed by water heated by contact with hot rocks far below the surface. Even where there has been no recent volcanic action, rocks become warmer with increasing depth. In such areas water may migrate slowly to considerable depth, warming as it descends through rocks deep in the Earth.

If it then reaches a large crevice that offers a path of less resistance, it may rise more quickly than it descended. Water that does not have time to cool before it emerges forms a thermal spring. The famous Warm Springs of Georgia and Hot Springs of Arkansas are of this type. And, yes, warm springs can even coexist with icebergs, as these happy Greenlanders can tell you.

Question: Which 2000 mile-long river enters the sea at the top of the Gulf of California?
Answer: The Colorado