Kids Research Express

Dams are classified by the type of material used in their construction and by their shape. Dams can be constructed from concrete, stone masonry, loose rock, earth, wood, metal, or a combination of these materials. Engineers build dams of different types, depending on the conditions of the riverbed, the geology of the surrounding terrain, the availability of construction materials, and the availability of workers. When more than one type of dam will suffice, engineers often opt to construct a type that they have built previously. A. Gravity Dams Gravity dams use only the force of gravity to resist water pressure—that is, they hold back the water by the sheer force of their weight pushing downward. To do this, gravity dams must consist of a mass so heavy that the water in a reservoir cannot push the dam downstream or tip it over. They are much thicker at the base than the top—a shape that reflects the distribution of the forces of the water against the dam. As water becomes deeper, it ex

Building a dam changes the ecology of the surrounding area. Among the most affected animals are fish that depend on free-flowing water to live. Some kinds of salmon, trout, and other fish species migrate downstream to spend part of their lives in the open ocean. As adults, they return upstream to lay their eggs in the gravel bottoms of the rivers where they were born. Large dams block the passage of such migratory fish. Some dams incorporate a fish pass to allow fish a chance to swim around the dam and reach upstream spawning grounds. Fish passes called fish ladders comprise a series of small pools arranged like stair steps. Each pool is slightly higher than the previous one. Fish ladders are based on the idea that a fish swimming upstream cannot leap over a dam that is more than about 5 meters high, but it can leap up a series of pools, each slightly higher than the one below it. Despite fish passes and other efforts to help fish bypass dams, the cumulative effect of multiple dams bui

The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km (6,000 mi) above the surface. Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consists of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon. Layers of the Atmosphere The layers of the atmosphere are the troposphere , the stratosphere , the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is

Earth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust, and the biosphere. See: Hydrosphere Crust Biosphere Changes to Earth’s Surface

. Soil Management is the basis of all scientific agriculture, which involves six essential practices: proper tillage; maintenance of a proper supply of organic matter in the soil; maintenance of a proper nutrient supply; control of soil pollution; maintenance of the correct soil acidity; and control of erosion. TILLAGE The purpose of tillage is to prepare the soil for growing crops. This preparation is traditionally accomplished by using a plow that cuts into the ground and turns over the soil. This removes or kills any weeds growing in the area, loosens and breaks up the surface layers of the soil, and provides a bed of soil that holds sufficient moisture to permit the planted seeds to germinate. Traditional tillage may harm the soil if used continuously over many years, especially if the fertile topsoil layer is thin. MAINTENANCE OF ORGANIC MATTER Organic matter is important in maintaining good physical conditions in the soil. It contains the entire soil reserve of nitrogen and signi

Healthy soil is indispensable for a healthy garden. Plants derive water, oxygen for their roots, and essential nutrients from the soil. Soil consists of two components: minerals from weathered rocks and organic matter from decayed organisms and animal wastes. The mineral content of the soil provides plants with nutrients, such as calcium, potassium, and phosphorus. Organic matter improves drainage and helps prevent waterlogged soils, reducing the occurrence of diseases such as root rot. Soil texture, the size of the individual soil particles, affects how fast water drains and how well plants absorb nutrients. The largest soil particles are grains of sand . Sand grains fit loosely together with large gaps between them, resembling marbles in a jar. The large pores let water (and the nutrients dissolved in it) drain out too quickly for most plants to absorb it. Clay particles, on the other hand, are very tiny, and they pack closely together, resembling tiny beads in a jar. The pores bet

Most scientists agree that if pollution and other environmental deterrents continue at their present rates, the result will be irreversible damage to the ecological cycles and balances in nature upon which all life depends. Scientists warn that fundamental, and perhaps drastic, changes in human behavior will be required to avert an ecological crisis. To safeguard the healthful environment that is essential to life, humans must learn that Earth does not have infinite resources. Earth’s limited resources must be conserved and, where possible, reused. Furthermore, humans must devise new strategies that mesh environmental progress with economic growth. The future growth of developing nations depends upon the development of sustainable conservation methods that protect the environment while also meeting the basic needs of citizens. Many nations have acted to control or reduce environmental problems. For example, Great Britain has largely succeeded in cleaning up the waters of the Thames and