multitude of purposes. Roughly, this basic water supply is divided into two categories-surface water and groundwater.

SURFACE WATER

Surface water is water that is flowing in our streams or rivers, resting in our lakes and ponds, or flowing into the sea. Its origin lies in the water that falls from the atmosphere, together with that which flows from the ground under certain circumstances. The water precipitated upon the surface of the earth from the atmosphere can be in the form of rain, snow, sleet, fog, or dew. Depending upon the character of the soil, this precipitated moisture is partly absorbed by the soil, partly evaporated or transpired by plant growth with the remainder caught in surface depressions or flowing over the surface to natural stream beds where it continues on its way to the sea or into the crevices of the earth. In olden days, it was thought that the vast underground water storage reservoirs were tied by surface streams. This is only partly true. In many cases where geological conditions permit, the groundwater sources feed the stream instead. It is true that the underlying beds of some surface streams are composed of sand and gravel and other materials deposited through the ages by sedimentation or glacial action. In these cases, water from the stream sometimes trickles down by gravity through the stream bottom into the underlying sands or gravel. When this happens, the water in the bottom gravel generally flows in the same direction as the stream itself. In other cases, it may be held in storage by natural barriers in the path of its flow. These underlying sands and gravel, generally referred to as "alluvium," are discussed in the section covering groundwater. In many cases, riverbeds become completely dry while the flow through the alluvium continues. This occurs in many cases in the western sections of the United States and on the Pacific Coast of North America.

GROUNDWATER

Groundwater is that part of the water or moisture that has precipitated from the atmosphere upon the surface of the earth and has been absorbed by the soil and collected below a certain level called the "water line." The waterline is of utmost importance and interest. The uppermost part of the surface of the earth is composed of layers of materials of a varying nature. There is a topsoil capable of sustaining plant growth. This topsoil is composed largely of minute particles of rock mixed with decayed vegetable matter 7-2 or other material. A layer of material generally referred to as "soil" underlies the topsoil. Soil is composed of minute particles of rock mixed with various materials, sometimes of vegetable or animal origin, but often containing nothing more than materials of mineral origin. The depth of the soil bed is not fixed and may vary from a few inches to several feet.

Under the soil layer is the top layer of rock, which is decomposed in some measure and which at a deeper level becomes more solid. Ultimately this rock becomes solid, as it was in the original cooling process. That part of the crust of the earth between this solid rock and the surface of the earth is of interest in discussing groundwater. Again, the depth of this outer layer is a variable because in many locations the virgin rock appears at the surface with no overlying decomposed rock or soil. Certainly, groundwater could not be found at such locations.

Now, consider the layer of decomposed rock which is between the uppermost layer of soil and the solid or virgin rock itself. Here, during the ages, many things have happened. The action of the elements, atmospheric conditions, earthquakes and upheavals, volcanic action and chemical reactions, as well as pressure conditions and other influences, have caused this layer to become anything from a semisolid rock to a conglomeration of layers of various materials. These layers of materials are referred to as "strata." The layers normally follow the contour of the surface of the earth; however, in some cases, they outcrop at the surface and slant downward. These strata may be composed of sand, gravel, broken stone of all sizes and character, minerals of all kinds, and even layers of solid rock. Some of the softer materials are shales, chalk, clays, and gypsum. The harder materials consist of limestone, granite, quartzite, flint, silica, dolomite, and other minerals. The types of material depend on the geographical location and the conditions under which the top most layers of rock were formed. In the formation process, because of earth movement and other influences, these varying strata were bent, folded, and broken in such fashion that it is not possible to chart their exact course through the upper part of the crust of the earth. Their presence and their position relative to each other are important to the storage and production of groundwater.

Depending upon the composition of these various strata, they either absorb the water which falls from the sky or flows at a level above them, or they reject this water and form a bed upon which the water flows in one direction or the other. The capacity of the material