found in tropical forests, the developing states de - manded access to Western environmental technology in return. At Founex, and again at Stockholm, the de- veloping countries of the south charged the north with using environmental issues to restrict their eco- nomic growth. A term coined at Founex, “ecodevelop- ment,” or development conducted in an ecologically sound manner, became the theme for the Stockholm Conference. Another dispute that surfaced at Stockholm was a result of the Cold War. The Western countries adopted the “Vienna formula,” which gave the Federal Repub- lic of Germany (West Germany) a seat at Stockholm and refused to seat the German Democratic Republic (East Germany). As a result, the Soviet Union and its Warsaw Pact bloc of countries refused to attend the conference. Provisions The Stockholm Conference was attended by 114 coun- tries, representing the larger part of the world’s popu- lation. Representaives adopted the Declaration of Twenty-six Principles on managing the global envi- ronment, which stated that to “defend and improve the human environment for present and future gen- erations has become an imperative goal for man- kind.” Representatives also adopted a Declaration on the Human Environment, which asserted the respon- sibility of humanity to preserve the Earth’s environ- ment, and an Action Plan for the Human Environ- ment, which took the form of recommendations for cooperation inenvironmental matters.The109-point Stockholm Action Plan was written to define the pa- rameters for future work and to mobilize a “common effort for the preservation and improvement of the human environment.” At the request of the conference, the United Na- tions was to carry out the recommendations. Thus, in December, 1972, the United Nations Environment Programme (UNEP) was created by the U.N. General Assembly as a focus and a coordinator for future envi- ronmental activities. Impact on Resource Use The Stockholm Conference was only a beginning to the work of preserving the environment, but it did formulate some basicprinciples, especially theimpor- tance ofpreserving theEarth’sgeneticresources.The implementation of those principles through action and specific agreements was left to the United Na - tions system,to international commissions,and tofur - ther conferences of concerned states. Colleen M. Driscoll See also: Biodiversity; Environmental movement; Ge- netic diversity;Montreal Protocol; UnitedNations En- vironment Programme. Stone and rock. See Aggregates Straddling Fish Stocks and Highly Migratory Fish Stocks Agreement Categories: Laws and conventions; government and resources Date: Signed December 4, 1995; in force December 11, 2001 To promote long-term conservation, the United Na- tions’ Straddling Fish Stocks and Highly Migratory Fish Stocks Agreement established principles for the conservation and management of fish stocks in heavily fished (and hence dwindling) species whose migratory patterns conflict with established international bor- ders of individual nations—that is, beyond zones of protected economic activity in so-called regional bodies of water. The protocol established that such manage- ment be basedon scientific information rather thanpo- litical or economic considerations, and that states must cooperate to ensure conservation of fish popula- tions on the high seas. Background Unlike land animals and birds trapped for economic use in which monitoring migratory patterns is fairly standard—as they are set by reliable parameters: ur- ban centers, food availability, weather, topography, competition for the food supply, and available water systems—tracking and monitoring deep-sea fish upon which the economies of coastal nations often rest poses a far more difficult challenge. These fish spe- cies—most prominently herring, mackerel, redfish, pollock, sharks, halibut, tuna, flounder, swordfish, and perch—freely migrate in open seas across the ex- clusive economic zones that extend up to interna - tional waters as defined by long-standing treaties and alliances. Because, as they feed and spawn, these fish 1156 • Straddling Fish Stocks and Highly Migratory Fish Stocks Agreement Global Resources do not respect such borders (hence the term “strad - dling”), the issue became a matter for the United Na- tions when, in the late 1980’s, fishing industries in a number of prominent coastal member-states—most notably in Scandinavia—began reporting precipitous declines in the available populations of such fish. Conservation was the only reasonable approach, but it would require an unprecedented level of agree- ment among nations that were essentially in eco- nomic competition for these fish. More problematic, given the fragile jurisdictional authority granted to the United Nations, the mechanism forenforcing any restrictions wouldnecessarily rest ultimatelyon a kind of honor system among the flag nations. The framework for an agreement to regulate the catch of migratory deep-sea species was formally in- troduced at the United Nations Conference on Envi- ronment and Development in Rio de Janeiro in 1992. Over the following several years, drafts of the agree- ment emerged from tireless shuttle diplomacy; these drafts found the traditional methods for restricting fishing activity, including having flag nations tax ves- sel licensing or restrict catching and trapping tech- niques, unsuitable and even counterproductive. These methods were seen as punitive to the fishing indus- try and as lacking the broad vision necessary to ad- dress a problem that industry experts argued was already pressing. Numerous U.N. commissions and independent scientific research findings argued the world’s fishing fleets were losing close to twenty bil- lion dollars annually because of depleted fish stocks and poorspecies management.TheUnited Nationsre- alized that any protocolfor addressing the problem of fish-stock management across flag state zones had to address the threat such depletion presented to the global community in order to encourage flag states to recognize that the sea was a shared resource and that cooperation was the only feasible way to proceed. Provisions The approach that emerged over the three years of the treaty’s evolution was holistic—that is, it addressed the widest possible issues of high-sea environmentalism, with a visionofthe ocean and theseasas a cooperative ecosystem that may be resilient but could not repair the damage from decades of careless fishing. The treaty, as it was proposed for ratification in 1995, promoted economic order in the oceans through the effective management and conservation of high-seas resources by establishing, among other things, detailedminimuminternational standards for the conservation and management of straddling fish stocks and highly migratory fish stocks. These stan- dards were to be reviewed regularly according to rele- vant migratory and trackingdata. The agreement reg- ulated the tonnage of dead discards (fish too small to harvest) asaway toaddress ocean pollutionand speci- fied an end to overfishing as a way to protect marine biodiversity. Thetreaty didnot detail exactlyhow such standards would be managed, nor did it address the thorny issue of international fishing conglomerates whose economic reach extended beyond single na- tions. However, the agreement recognized the right of states within the relevant region to board and in- spect vessels from other states suspected of violating conservation measures. Impact on Resource Use The Straddling Fish Stocks and HighlyMigratory Fish Stocks Agreement provided a groundbreaking frame- work that indicated the resolve of the signatory states to cooperate to ensure the industry’s continued exis- tence. Adopted in August, 1995, and opened for sig- nature in December, 1995, the treaty had been signed by fifty-nine member states andentitiesas of 2009. On November 11,2001,with the accessionof the islandof Malta, the thirty-nation requirement for ratification was met. One month later, the agreement went into effect. Joseph Dewey Further Reading Clover, Charles. The End of the Line: How Overfishing Is Changing the World and How We Eat. Berkeley: Uni- versity of California Press, 2008. Fujita, Rodney M. Heal the Ocean: Solutions for Saving Our Seas.Gabriola Island,B.C.: NewSociety,2003. Glover, Linda K., and Sylvia Earle, eds. Defying Ocean’s End: An Agenda for Action. Washington, D.C.: Island Press, 2004. Sloan, Stephen. Ocean Bankruptcy: World Fisheries on the Brink ofDisaster. Guilford,Conn.: LyonsPress, 2003. See also: Biodiversity; Ecozones and biogeographic realms; Exclusive economic zones; Fish and Wildlife Service, U.S.; Fisheries; International Union for Con- servation of Nature; United Nations Convention on the Law of the Sea. Global Resources Straddling Fish Stocks and Highly Migratory Fish Stocks Agreement • 1157 Strategic resources Category: Social, economic, and political issues Strategic resources are energy and mineral resources that are necessary for a nation’s industries and mili- tary defense and must be obtained from limited sources far from the nation. There is debate about how much risk such dependence creates for the United States, which considers oil, chromium, cobalt, and platinum metals to be among its strategic resources. Background A large and diverse economy such as that of the United States needs significant amounts of natural re- sources of many types. The United States is one of the wealthiest nations in terms of resource endowment, yet its sheer economic size requires it to depend on outside sourcesof resources tomeet many ofitsindus- trial and military defense needs. There is always con- cern by a country about its dependence on outside sources for resources that meet critical needs. Of the minerals the U.S. government considers critical for the economy and military defense, one-half require foreign imports to meet more than 50 percent of the country’s annual consumption. This is a startling sta- tistic, but it tends to overstate the severity of the situa- tion. Most of these twenty-four minerals are imported either from closely linked neighbors such as Canada, Mexico, and the Caribbean or from a number of other locations around the world. A few of these criti- cal minerals come from very limited and distant loca- tions that are vulnerable to supply disruptions, and the United States depends almost solely on these sources for the nation’s needs. These minerals are known as strategic resources. Examples of Strategic Resources What resources are considered strategic for a particu- lar country depend on, and change with, the coun- try’sneeds and resource availability.The UnitedStates’ strategic resource concerns grew out of the experi- ences of supply cutoffs during World War II and the development of key manufacturing and military de- fense industries that require specialized materials not available in the United States. Minerals that are typi- cally considered strategic for the United States in - clude chromium, cobalt, manganese, oil, the plati - num group metals (platinum, palladium, iridium, osmium, rhodium, and ruthenium), and titanium. Chromium. Chromium is the alloying material added to steel that creates stainless steel, which has critical applications in many industrial and military defense products. Chromite ore comes largely from South Africawith lesseramounts fromKazakhstan, In- dia, Zimbabwe, Finland, Iran, and Brazil. Cobalt. Cobalt is another alloying material used in steel. It is important in applications that face high temperatures and pressures, such as jet engines. Co- balt is also used in permanent magnets. A major source of cobalt is as a secondary product from cop- per mining is the Democratic Republic of the Congo and Zambia in central Africa. It is also available from nickel producers inCanada, Russia,andScandinavia. Manganese. Manganese is an additive to the steel- making process that removes sulfur—an important step in making high-quality steel. The African coun- tries of South Africa and Gabon are the important sources of this material. Platinum group metals. The key useof platinum group metals is as chemical catalysts. They are a vital part of the process used by the petroleum industry to turn crude oil into consumer products. The catalytic converter used on cars to clean the exhaust emissions requires platinum as well. South Africa and Russia are the major producers. Titanium. Titanium is a light but strong, corro- sion-resistant metal that is important in applications such as jet engines, air frames, and missile compo- nents. It comes from rutile and ilmenite ores that are recovered from the heavy sands found in Australia and from rutile ores inSouth Africaand Sierra Leone. Oil. The United States is a major producer of oil, and oil supplies are widely distributed worldwide. However, oil is absolutely vital to the U.S. economy and is required in huge quantities. The United States therefore is a large importer of oil. The U.S. supply of these strategic resources depends on regions—such as southern and central Africa, Russia and central Asia, and the Middle East—that are politically unsta- ble and often isolated from the United States. Much of the world’s excess supply of oil is controlled by a few members of the Organization of Petroleum Ex- porting Countries (OPEC). Risk and Strategic Resources The greatest fear about dependence on limited im - port possibilities for strategic resources is that this de - pendence will lead to significant economic, social, 1158 • Strategic resources Global Resources and military defense problems for a country because of its inability to handle a supply shortfall. There are two potential risks to be considered. The risk of most concern is a direct supply cutoff. Such a cutoff can oc- cur as a political strategy, as demonstrated by the oil embargo by some members of OPEC against the United States in 1973 and the grain embargoes by the United States against the former Soviet Union during the same era. A supply cutoff can also result from po- litical instability—as has happened with cobalt pro- duction from the Democratic Republic of the Congo. Another cutoff possibility is military action, as seen in World War II (resources such as tin from Southeast Asia) and the Gulf War period of 1990-1991 (Kuwaiti oil). A second risk is the use of the threat of a cutoff to impose a political agenda on the importing nation. Such attempts have been made by OPEC against de- pendent nations in Europe and Asia and by the United States against nations that fail to meet its standards of human rights and world citizenship. There is debate about how significant the risk is to the U.S.economy andmilitarydefense frompotential supply cutoffs of strategic resources. One side argues that the risk is minimal and can be adequately han- dled by responding to adjustments in the resource markets. There are several market responses to a po- tential supply disruption. One response is to develop new and more diverse sources for the strategic re- source. Another market response is to develop substi- tutes and alternative possibilities that replace the use of the strategic resource. Another possibility is for us- ers to keep astockpileof material as insurance against supply disruptions. Cobalt offers a good case study of how resource markets respond to a supply disruption and the riskof such a disruption. During the 1970’s, the central African country of Zaire (now the Democratic Republic of the Congo) produced more than 90 per- cent of the world’s supply of cobalt. The low cost of the cobalt had appar- ently offset any concerns there had been about possible supply disrup- tions. Stockpiles, which are expen- sive, were not held by users either. Political instability in the region led to supply cutoffs in 1976, 1978, and 1979. The immediate market effect was an increase in the price of cobalt from $3 or $5 per kilogram to $12 per kilogram. This higher price created the incentive to seek new (and more expensive) sources of cobalt. For example, sec- ondary production of cobalt from nickel mining be- came more important. The higher price also led to the reduction in the use of cobalt where possible. Nickel was substituted for cobalt in some uses, and new alloys were developed for permanent magnets that did not need cobalt. Even in the uses that re- mained, new cobalt alloys were developed that re- quired less cobalt than in the past. When cobalt sup- plies from Zaire were again disrupted by political unrest in the 1990’s, the market impact was small. The major weaknessof these market adjustmentsis that they happen in response to rising prices after the supply disruption has already occurred. Therefore, any problems created by the supply disruption have already occurred as well. In the case of cobalt, the im- pact of the disruption was controlled adequately. In the case of the oil embargo of 1973, the disruption had impacts on inflation and unemployment that af- fected the overall U.S. economy for years. Private companies are concerned solely with profit-maximiz- ing and their competition with other companies, and they seek the lowest possible cost for their materials. Preventive measures such as stockpiling are avoided because of the additional expense. This weakness has led to the counterargument that market responses are not sufficient in themselves because there is a significant externality aspect (benefits or costs to ev- eryone) totherisk ofstrategic resource supplydisrup- tion, andprivate usersof theresource grossly underes- Global Resources Strategic resources • 1159 The U.S. government considers cobalt, pictured, to be a strategic resource. timate the problem because they only consider their own needs and goals. This argument urges that direct government involvement is necessary to develop and implement policiesthat reduce theproblems facedby society with a cutoff of a strategic resource, because only thegovernment considers thetotal welfareof the country. Government Policies A government has a wide variety of policies available to it to counter the risks associated with strategic re- sources. The most extreme policy is that of “autarky.” Under autarky, a country uses only its own internal re- sources and does not depend on any foreign sources. There is a heavy economic cost to complete self-reli- ance. Japan has made extensive use of cheap foreign raw materials andhasa strong economy,while the for- mer SovietUnion followeda policy ofautarky andhad a weak economy, especially for consumer products. Less extreme policies to promote the use of domestic resources over foreign ones include subsidizing do- mestic production to make them more competitive and making government-owned lands more open to resource development. Alternatively, military force can be used to protect the availability of supplies of strategic resources. This policy requires the mainte- nance of an adequate military force and the willing- ness to use it (as in the Gulf War of 1991 and the Iraq War beginning in 2003). The government can also try to plan the efficient use of a strategic resource and to encourage conservation. The U.S. government tries to do this with oil consumption through a variety of educational pro- grams designed to show and to en- courage the efficient use of energy. Additionally, there has been legisla- tive debate about the possible use of a tax on oil imports to discourage its use. The primary policy followed uniquely by the U.S. government is, however, the use of a national strate- gic resource stockpile. The Strategic and Critical Mate- rials Stockpiling Act of 1946 was passed to create a national stockpile of materials deemed necessary for the nation’s defense. The goal was to maintain a three-year supply of im- ported materials required to meet both military defense and domestic needs (based on a World War II scenario). Two acts, the Strategic and Critical Materials Stockpiling Revi- sion Act of 1979 and the National Materials and Min- erals Policy, Research, and Development Act of 1980, unified the stockpiling programs and their manage- ment. A separate strategic petroleum reserve is also held. A national stockpile of this size is very expensive and difficult to create and maintain, and the United States is the only country to have done so at an exten- sive level. The actual use of the stockpile for its stated strategic purpose has been limited and not overly effective. It has had more of an economic impact on world resource markets and trade. For example, during the 1980’s, the U.S. government sold large quantities of excess tin from the stockpile (a strategic resource of an earlier period) that had been held since the end of World War II. Tin producers asked for relief (with little success) from the sales because of the depressing impact it had on the price and need for tin. The U.S. government’s response to the supply dis- ruption of cobalt also makes an interesting case study. Prior to the supply cutoffs of the late 1970’s, the U.S. government’s policy toward cobalt was to sell it from the national stockpile as excess material. The policy was reversed in 1976, and new goals were set. By the early 1980’s, only one-half of the stockpile goal had been achieved, and the crisis had already passed be- cause of market adjustments and better political con - ditions in Zaire. A second response by the govern - ment was to consider subsidizing domestic mining of 1160 • Strategic resources Global Resources Data from U.S. Census Bureau.Source: 100 94 78 62 Percentage 12010080604020 Cobalt Manganese Platinum group Chromium U.S. Imports of Strategic Resources, 2007 cobalt from available low-grade ores. It would have been necessary for the government to guarantee a price wellabove theexistingmarket priceand at levels comparable to prices during the supply disruptions. The subsidy was not approved. The government also has a policy of placing no tariffs (import taxes) on co- balt in recognition of its importance as a strategic re- source. This only makes imports more attractive and does nothing to encourage less dependence and more domestic production. Government policies are onlyas good astheir plan- ning and implementation. Often, other political con- siderations and conflicting goals drive the decisions. In theory, the government can design policies to re- duce successfully the risk of dependence on strategic resources, but in observed practice, the actual effec- tiveness of policies are questionable. The negative im- pact of the interference of the government in re- source markets, in fact, may more than offset any risk reduction that is achieved with its policies. Interest and concern about strategic resources rise and fall with the needs and conditions of the time. The 1970’s, for example, was a time of high interest and concern because of the oil crises of 1973 and 1979, the rising prices of other critical resources, and the attempts by developing, resource-producing na- tions to form cartels to control resource markets and prices. Asadditional resource sources were developed and the cartel efforts failed, the interest and concern declined. Similarly, the precipitous rise in oil prices during 2007 and 2008,accompanied by gasoline prices nearing $5 dollars per gallon, led some to call on the Bush administration toease the “painat the pump” by releasing some of the nation’s strategic oil reserves. Government efforts to reduce the risk of depend- ing on strategic resources have been sporadic and sometimes poorly executed, but market adjustments have overcome problems that have arisen from using strategic resources. The uses of strategic resources have so far been flexible enough to adjust to sudden shortfalls without dire consequences. Even the prob- lems encountered duringthe oilcrises (likelongwaits at gasstations)could be blamedmore on government price controls and oil rationing than on the supply disruptions themselves. A country should be aware of its dependence on strategic resources and not over- look the possible consequences, but the benefits to the United Statesof using strategic resources have his - torically outweighed any risk they presented. Gary A. Campbell Further Reading Anderson, Ewan W., and Liam D. Anderson. Strategic Minerals: Resource Geopolitics andGlobal Geo-Economics. New York: Wiley, 1998. Bamberger, Robert. Strategic Petroleum Reserve. New York: Nova Science, 2006. Beaubouef, Bruce A. The Strategic Petroleum Reserve: U.S. Energy Security and Oil Politics, 1975-2005. Col- lege Station: Texas A&M University Press, 2007. Hargreaves, D.,and S.Fromson. World Index ofStrategic Minerals: Production, Exploitation,and Risk. NewYork: Facts On File, 1983. Maull, Hanns W. Energy, Minerals, and Western Security. Baltimore: Johns Hopkins University Press, 1984. Perkins, Patricia E. World Metal Markets: The United States Strategic Stockpile and Global Market Influence. Westport, Conn.: Praeger, 1997. Ridgeway, James.It’s Allfor Sale: TheControl of GlobalRe- sources.Durham,N.C.: DukeUniversity Press,2004. Singer, Clifford. Energy and International War: From Babylon to Baghdad and Beyond. Hackensack, N.J.: World Scientific, 2008. See also: Chromium; Cobalt; Manganese; Oil em- bargo and energy crises of 1973 and 1979; Organiza- tion of Arab Petroleum Exporting Countries; Peak oil; Platinum and the platinum group metals; Tita- nium; United States. Streams and rivers Category: Ecological resources Rivers, streams, creeks, brooks, and rills are long, nar- row bodies of water that flow downslope in a channel under the influence of gravity. Although the list is ar- ranged in general order of size, the term “stream” is accepted as the scientific term for any amount of sur- face water flow. Streams are an integral part of the hydrologic cycle, as they transport water and sediment from the land to the sea. Background The vast majorityof all of thewater in the worldis con- tained intheoceans (97.2 percent) and icesheetsand glaciers (2.15 percent). This means that 99.35 per - cent of the world’s water is either salty or frozen. Of the remaining 0.65 percent, groundwater accounts Global Resources Streams and rivers • 1161 for 0.63 percent, followed by freshwater lakes (0.009 percent), saline lakes and inland seas (0.008 per- cent), soil water (0.005 percent), the atmosphere (0.001 percent), and stream channels (0.0001 per- cent). It is obvious that streamflow makes up the smallest amount ofwateron Earth, yet it hasbeencrit- ical for the development of human societies. Streams have been associated with the develop- ment of civilization from the beginning of human his- tory. The elaborate irrigation systems that developed in Egypt, China, and the Tigris-Euphrates valley in Iraq and portions of surrounding countries started thousands of years ago. Streams have also been used for navigation, municipal water supplies, wastewater disposal, and in more modern times, power-plant cooling, hydropower, and firefighting. Major cities developed on or near streams: London on the Thames; Albanyand NewYork onthe Hudson;Shang- hai on the Huangpu, a tributary of the Chang (also known as the Yangtze); New Orleans and St. Louis on the Mississippi; Montreal on theSt. Lawrence; Vienna and Budapest on the Danube; and Cairo on the Nile. This brief list could include many other cities, but the important fact is that most of the world’s major settle- ments have developed on or very close to a source of water, and most of these sources are streams. The Development of Streams Precipitation events result in several types of natural response. The first response is simply that the precipi- tation can evaporate before it even reaches the Earth’s surface. This generally occurs in drier climatic re- gimes. The second response occurs when some of the precipitation infiltrates into the soil and forms the groundwater component of thehydrologic cycle. The third response is usually associated with storm events that could easily exceed the infiltration capacity of an area and thereby result in overland flow. This type of flow would travel downslope and eventually reach an area where this surficial downward movement of water coalesces and starts to erode the land surface to a point where a permanent channel starts forming. Over time, streams become perennial (meaning they flow continuously throughout the year) when they have cut deeply enough so that the surface of the stream is below the adjoining water table, thereby allowing groundwater to supply the stream. This movement of groundwater is called “base flow,” and can easily ac - count for one-half or more of the total water flowing in a stream, particularly in coastal plain formations such as those along the eastern coast of the United States. Thus, surface runoff consists of both overland flow and groundwater, even though they move at very different rates (groundwater is much slower). The ultimate sink for most of the streams in the world as they move from higher to lower elevations is the oceans. Steep gradients, rapids, and waterfalls are generally associated with streams in the uppermost portions of their watersheds.Over time, these streams develop a progressively gentler slope as theirregulari- ties in the channel are removed and the profile be- comes smooth. With time, the stream widens its bed and valley as downward cutting of the channel is re- placed by lateral cutting. Stream sinuosity increases as meanders develop.A depositional feature ofa stream, described as a floodplain, widenswithtime and down- stream progression. Streams often build deltas when they empty into the sea when circumstances permit. Deltas form when the velocity of the water moving in the channel slows down as the stream enters a much larger body of water. The particles settle out and form a variety of deltaic types. Well-known deltas include the Nileand theMississippi. Whereconditions arenot favorable for deltaic formation, as in the case of the two largest streams in the world (the Amazon and Congo rivers), there is no delta. Some drier areas of the world have streams that are located in interior basins without any outlet to empty into theoceans.These endoreicwatershedsrepresent internal drainage basins. Examples include the Great Basin in Nevada and portions of surrounding states, Lake Chad in sub-Saharan north-central Africa, the Dead Sea in Israel and Jordan, and the Aral and Cas- pian sea basins in central Asia. Runoff Measurement The amount of precipitation that falls on the Earth’s surface, minus the amount that evapotranspires from vegetation back to the atmosphere, either enters the soil to become a component of groundwater or runs over the Earth’s surface as overland flow to eventually wind up in streams. The measurement of all of the myriad components of surface and groundwater run- off would be a daunting task at the least. However, it is not necessary to determine the flow rates of all of these upslopecontingentsof runoff. The simpler pro- cedure is to measure the flow at various points along the stream inorderto determine the runoff from part of or the entire watershed. The earliest attempts to at least partly measure 1162 • Streams and rivers Global Resources streamflow began about five thousand years ago on the Nile in Egypt. The height of the river could be measured by reference to staff gauges called Nilo- meters that were fixed to the banks of the Nile. These permanent structures provided a measure of river stage that was then used to alert downstream farmers of the riseor fall of floodwavescoming down the Nile. Staff gauges are still used to measure river, lake, and canal levels. In order to determine the volume of water that goespasta given point onastream during a certain period of time, it is necessary to measure the volume and velocity of water at that point. This flow is calculated by multiplying the area of the cross section in square meters by average velocity in meters per sec- ond to yield discharge in cubic meters per second. The first permanent streamgauging station in the United States was established in 1889 on the upper Rio Grande in New Mexico by the U.S. Geological Survey (USGS), an agency of the federal government that, in conjunction with states and municipalities, is responsible for maintaining gauging stations. The to- tal number of gauging stations in the USGS network was onceaboutseven thousand forthe whole country, but funding cuts have been reducing the overall total over the years. Regrettably, some stations that were dropped had records of continuous daily measure- ment in excess of thirty years, thereby representing a loss of valuable hydrologic data. Watershed Area The total amount of land that collects all of the sur- face and groundwater within an area is called a water- shed (or drainage basin). Watersheds vary enormously in size, from a tiny lot that drains into a pond to the Amazon River basin, with a drainage area of 6.16 mil- lion square kilometers. The second and third largest watersheds in area in the world are the Congo in cen- tral Africa (3.83 million square kilometers) and the Mississippi (3.26 million square kilometers). Another characteristic of watersheds is their varia- tion in shape. Although many of them are pear-shaped (reflecting a preponderance of horizontal rock strata), others are elongated, indicating a landscape of ridges (resistant formationscomposed of igneousrocks such as schist or gneiss) and valleys (weaker formations composed of shale). This diversity in the underlying rock formations has an important bearing on slopes, infiltration, and runoff characteristics of all water - sheds in addition to the obvious effects of climatic variations. Stream Discharge and Length Given the huge variation in precipitation, tempera- ture, and landscapes in different regions on the Earth, it is obvious that average stream discharge would also vary enormously. Given its large size and favorable lo- cation in the wet equatorial belt, with average annual precipitation in excess of 200 centimeters, the Ama- zon leads the list with an average discharge of 175,100 cubic meters per second. This astounding amount of flow accounts for about 20 percent of the total dis- charge of all of the world’s rivers, even though the area of the Amazon watershed is only about 2 percent of the total land area in the world. The second and third largest average discharges in the world are the Congo, with 40,000 cubic meters per second, and the Mississippi, with 18,400 cubic meters per second. Note that the averagedischarge of the Amazonis nearly ten times that of the Mississippi, even though the area of the Amazon is somewhat less than twice the size of the Mississippi. The difference is attributed to climatic factors in the watersheds. The four longest streams in the world are the Nile (6,648 kilometers), the Amazon (6,436 kilometers), the Chang (6,300 kilometers), and the Mississippi (5,970 kilometers). Although the Nile is the longest river in the world, it is ranked thirty-third in terms of average discharge, as it flows for a considerable dis- tance through the deserts of northern Sudan and Egypt before it reaches the Mediterranean. Drainage Density and Topographic Texture The variation in rock formation and soil type within each watershed governs the number of streams per unit area that canform in differentlandscapes.Drain- age density is determined simply by dividing the total length of streams within a selected region by the area of the region. For example, an area underlain by very resistant beds of hard sandstone or granite and cov- ered by heavy tree cover in a humid environment would experience minimal development of streams, resulting in a very low drainage density and conse- quent coarse texture. Another example, also in a hu- mid environment with tree cover, would be an area underlain by weaker and therefore less resistant shale rock that would have more streams in the same size area and therefore have a somewhat higher drainage density number that would be considered to be me- dium texture. High drainage density (fine texture) would be expected to develop in landscapes under - lain by weak sedimentary rocks in conjunction with Global Resources Streams and rivers • 1163 limited coverage of vegetation. Instances of very high drainage density and resulting ultrafine texture can be found in the badlands of South Dakota, where the underlying earth materials are very weak clays and shales in conjunction with limited vegetative cover. Those areas that have coarse texture (low drainage density) have much fewer streams than those water- sheds with finer texture that are more easily eroded with many more streams per unitarea. The difference in drainage density can therefore range up to two or- ders of magnitude. Stream Ecosystems Free-flowing streams have natural pulses of seasonal floods that maintain a dynamic equilibrium between the biological and physical aspects of aquatic ecosys- tems. The flora and fauna of stream ecosystems are well adapted to the natural variations in streamflow. For example, water levels and flooding events in the eastern United Statesare generallyhighest during the late winter-early spring period when evapotranspi- ration is low and soil moisture is at or close to its field capacity (the amount of water that is retained in the soil orrock againstgravitational forces).This does not mean that flooding events cannot occur during other parts of the year, such as the summer and fall. For example, although long- term recordsclearly indicatethat the usual time for floods to occur in the Mississippi is in the spring, major floods have also occurred in mid- summer. Another example is pro- vided bythe long-term recordsof the Connecticut River at Hartford, which show expected floods during March and April but also a second period during the fall, when heavy rain- storms and hurricanes can occur. The Geologic Work of Streams Erosion, transportation, and deposi- tion represent the three closely re- lated activities of streams. Stream ero- sion refers to the removal of earth material from the channel. The same process will occur in both alluvial and bedrock channels, although dif- ferent removal rates will apply based on resistance to erosion (alluvium is much easier to erode). As the particles are eroded, they are transported by the stream in one of the following modes: in solu- tion, in suspension, or as bed load. Salts from min- eral alteration constitute a very common form of ma- terial in solution in all streams. The dissolved matter (salt) cannot be seen, as it is mostly in the form of chemical ions. Stream turbulence allows clay and silt particles to be carried as suspended load. Highly visi- ble soil particles that have been eroded from the ex- tensively cultivated fields in the Midwest and wind up in the Mississippi have given the river the nick- name “the Big Muddy.” Sand, gravel, and cobbles are much larger than clay and silt particles and there- fore move as bed load close to the channel floor. The suspended load is usually the greatest of the three forms of stream transport. For example, it is es- timated that the Mississippi transports about 90 per- cent of its total load in suspended form. This high proportion is attributed to the semiarid area of the Missouri River basin, a major tributary of the Missis- sippi, which also includes the easily eroded Badlands of South Dakota. Deposition, the third activity of streams, can occur on the streambed, in the floodplain, oron the bottom of a water body—such as an ocean—into which the 1164 • Streams and rivers Global Resources Characteristics of Selected Major Drainage Basins River Outflow Length Area Average Annual Suspended Load Amazon 180.0 6,300 5,800 360 Congo 39.0 4,700 3,700 — Chang 22.0 5,800 1,900 500 Mississippi 18.0 6,000 3,300 296 Irrawaddy 14.0 2,300 430 300 Brahmaputra 12.0 2,900 670 730 Ganges 12.0 2,500 960 1,450 Mekong 11.0 4,200 800 170 Nile 2.8 6,700 3,000 110 Colorado 0.2 2,300 640 140 Ching 0.06 320 57 410 Note: Rivers are ordered by outflow; outflow is multiplied by 1,000 cumecs (cubic meters of water per second); length is measured in kilometers; area is measured in square kilometers multiplied by 1,000; average annual suspended load is measured in millions of metric tons. stream empties. For example, the mouth of the Ama - zon is 161 kilometers wide as it flows into the Atlantic. The enormous amount of sediment that it carries forms large islands at the mouth or is simply depos- ited on the adjoining continental shelf. Stream “capacity” is defined as the maximum amount of suspended load and bed load that can be transported in a stream at a specified discharge. An increase in discharge results in a substantial increase in suspended load, as the faster the water is moving, the greater the turbulence and consequent ability to keep material in suspension. The same effect oc- curs with bed load: As the stream velocity increases, the ability to move material along the bottom of the stream increases by three to four orders of magni- tude as compared to the velocity. Thus, if the velocity of a stream is doubled in a flood situation, bed-load movement can increase by a factor of eight to sixteen times. There is an enormous range in sediment load transport among the major rivers in the world.For ex- ample, the average annual sediment yield varies from a low of 4 metric tons per square kilometer for the Yenisey River in Siberia to a high of 2,600 metric tons per square kilometer for the Huang (also known as the Yellow) River in China. This enormous difference is causedby thelarge soil-erosionrate inthe cultivated and easily eroded silt soils of the Huang River basin as compared to the mostly forested and uncultivated Yenisey River watershed. Another major example is the Mississippi River, where most of the annual sedi- ment yield is attributed to the Missouri River, which flows through subhumid and semiarid grasslands, large portions of which have been cultivated. Proper design of reservoirs should include esti- mates of the sediment load carried by an incoming stream. The sediment that gets trapped behind the dam of a reservoir will eventually eliminate its storage capacity. For example, several inland reservoirs that were used to supply water to Santa Barbara on the coast in California filled with sediment in only a few decades and had to be abandoned. Even major reser- voirs in semiarid to arid regions, where vegetative cover on watershed slopes is limited, will experience reduced lifetimesas a consequenceof continuingsed- iment input. Exotic Streams Ancient civilizations that developed in arid regions, such as Egypt or Mesopotamia in what is now Iraq, based their elaborate irrigation systems on streams that originated in upstream mountainous areas that have a water surplus. The Blue Nile originates in Lake Tana in the Ethiopian highlands, which receive heavy summer rains. The White Nilebegins in Lake Victoria in east-central Africa, which experiences heavy pre- cipitation throughout the year. The Blue and White Niles join at Khartoum in Sudan and flow north to Egypt through an arid region. The Tigris and Eu- phrates rivers start in the highlands of east-central Turkey, flow through Syria and Iraq, and empty into the PersianGulf.Another classic exampleof an exotic stream is the Colorado River, which begins in the Rocky Mountains of Colorado and Wyoming and flows through Utah, New Mexico, Arizona, Nevada, California, and Mexico before emptying into the Gulf of California. Large portions of this basin, especially the lowerpart, receivelimited precipitation inan area that hasexperienced considerablepopulation growth and commensurate demand for water. Anthropogenic Effects on River Systems It is well known that natural factors occur that may have substantial effects on streamflow. Some obvious examples are wet and dry periods that can occur dur- ing anyyearand other climatologicalevents such asEl Niño that may have return cycles of a decade or more. However, these are natural events that may easily be superseded byhumanintervention, as exemplifiedby such activities as extensive diversion for agricultural irrigation, interbasin transfers of large volumes of water for public water supply, and the development of a large system of dams. Dams can have a major impact on streamflow, as the water can be released below a dam without re- gard for natural cycles; instead, the release may be ruled by a particular need for water or electricity on cycles that may change on an hourly basis. As a re- sult, stream channels below a dam can experience sharp changes based on annual flow, flood peaks, and the sediment load in the stream. These changes can range from sand accumulation in one channel to veg- etation moving into another section of the channel to heavy bank erosion in still another part of the stream channel. Dam-building has a long history. For example, dams were built upstream of Cairo on the Nile by the Egyptians some five thousand years ago. The Chinese built a dam with a height of 27.4 meters on the Abang Xi River some twelve centuries ago that is still usedfor Global Resources Streams and rivers • 1165 . flow rates of all of these upslopecontingentsof runoff. The simpler pro- cedure is to measure the flow at various points along the stream inorderto determine the runoff from part of or the entire. of 1160 • Strategic resources Global Resources Data from U.S. Census Bureau.Source: 100 94 78 62 Percentage 12010080604020 Cobalt Manganese Platinum group Chromium U.S. Imports of Strategic Resources, . nation’s needs. These minerals are known as strategic resources. Examples of Strategic Resources What resources are considered strategic for a particu- lar country depend on, and change with, the