water is essential to every living thing all those that need a continuous supply of water in order to live in food making and in other life processes plans water derived from rain roughly one third of all rainfall runs off and just dreams rivers and lakes and these constitute our chief surface water supplies about one third soaks into the ground and becomes our underground water life here is greatly restricted rainfall is unequally distributed with the result that we find native forests in the west and east grass in the central states and spires of desert plants in the southwest to exist man requires a continuous supply of water springs furnish and accessible a limited source underground water may also be tapped by wells such as this old it done well and sweep many large cities in the summer located here by dots depend upon underground water tapped by modern wells for other studies of the surface water taken from rever constitutes the only adequate and available source some said is take their water from nearby lakes surface waters are also made available to certain cities by impounding the headwaters of rivers by dams america's largest city depends mainly on watershed for its water supply new york city needs one billion gallons of water daily an amount about equal to the flow in this river originally used became inadequate and about a century ago the city started to develop the croton watershed fifty miles away the water was carried by an aqueduct here crossing the harlem river the with associated dykes and wearies to form a reservoir later a second watershed was developed on the northern slope of the catskills by constructing a dam to impound its water from this reservoir a tunnel eighteen miles long passes under the dividing mountains and discharges its water into a creek which flows into the reservoir on the south slope from this catskills system water enters an aqueduct and continues on through this aqueduct for about seventy miles to an intermediate reservoir through the greater portion of this distance the aqueduct is a cut and cover construction about seventeen feet in diameter grade tunnels carry the water through obstructing mountains this enormous problem required extensive collaborative work by gian just engineers and other experts in tunnel construction cutting through solid rock hundreds of feet underground as shown here  one of the many problems in constructing an a because of the difference in elevation gravity provides sufficient pressure to transport the water from the catskill reservoirs to the canseco intermediate reservoir the great bulk of this water remains in this reservoir for about one month from here it flows to a small equalizing reservoir which takes care of our lead demand fluctuations with increasing population and water consumption new york city has been compelled to provide the delaware system consisting of three separate watersheds still farther away this has necessitated the construction of a new aqueduct that finally connects the original system represented here is the watershed system utilized by america's largest city just as important as an adequate water supply is a safe water supply municipal sewage treatment plants reduce pollution caused by sewage and industrial waste time forward fever is one disease commonly carried by water in one city the installation of sand filters in the practice of chlorination was followed by a marked decrease in the number of times for fever cases until today this disease is very rare where scientific controls are employed the laboratory occupies a key position in safeguarding the city water supply water samples are collected daily one test known as a premonition test indicates the presence of contamination for them or the water examined for microscopic organisms many of these organisms impart specific odors and tastes to the water but certain bacteria cannot easily be detected under the microscope therefore a  bacterium has now multiplied into a colony that is easily detected these colonies can now be counted or examined under a microscope for identification the water is also examined chemical a for the presence of foreign substances that may affect its palatability or if used in industry in the home has the presence of ammonia nitrogen indicates organic decay this forms of flaky precipitate which settles out carrying with it much of the suspended material and bacteria since alum makes water acid and most of lime is added to render the water slightly many cities use filter beds of sand to remove the solid matter this site well eight feet deep indicates the clearness of the water after treatment cities more and more mix activated carbon into the water remove color odor and taste a aeration is another common method of removing odors and increasing palatability of drinking water the final step in water treatment involves adding chlorine this completes the sterilization by destroying all the harmful bacteria and other organisms in new york city water already made pure and safe and is the city through two huge tunnels real through solid rock about two hundred feet underground risers bringing the water up near to the surface the water flows from the risers to connecting mains them into  finally through service connections into each building on the street the higher elevation of the reservoir causes water to flow into the city by gravity however some consumption points are higher than the source and pumps must be used many high buildings have their own pumps to maintain a sufficient water pressure to the top the water that's provided has many uses it flows through thousands of safe drinking fountains it is used in the cooking of many of our food and laundering our clothing and in other ways water  cleanliness and health water is used in generating steam for heating building water aids in defending the city's health but washing our streets and by carrying away sewage clean water plays an important role in healthful recreation water also is an essential part of the composition of beautiful fountains the control of this stuff the fires requires water sometimes in great amounts waters without which man cannot live must be supplied in abundance and with safety to a sure healthful living in a modern city