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sluggishly in winding courses and in the lower counties open out and become estuaries. Here also the streams are navigable in their lower courses, but owing to the slight velocity they split up rapidly in places, and on many streams the head of navigation is several miles farther down stream than it was a half century ago. As a result of the general flatness of the country there are no water-power sites in this section. In this area also a smaller proportion of the rainfall finds it way into the streams, as the loose, porous soil, most of which is cultivated, absorbs the water very quickly.

As there are no natural lakes of importance in the state there is no regularity in the stream flow, such as would exist if there were storage reservoirs on the headwaters of the streams. The flow of the streams varies according to the rainfall. Hence in cleared areas that are highly cultivated the rainwater runs off quickly and the streams rise rapidly at times of large precipitation. In wooded areas the water is held back and reaches the streams more gradually.

SURFACE WATERS OF THE COASTAL PLAIN

The surface waters of the coastal district are relatively unimportant as sources of potable water supply when compared with the underground waters. At no points are they employed to any extent for either public or private uses. This is due not so much to the generally small size of the streams above the head of tide as to their unpotable quality. Not only are they quickly rendered roily by the rush of the fine materials found along their banks, but their sluggish currents render the waters unwholesome in other respects. Furthermore, vegetation is widely found in the waters, rendering them disagreeable for domestic purposes, and few are not contaminated by sewage.

The streams of northern Newcastle and Cecil counties, and those of western Anne Arundel and Prince George's counties, especially when their sources are in the highlands of the Piedmont Plateau, are better adapted for potable uses than those elsewhere in the area. Some of the streams reaching the margin of the coastal district in Baltimore and Harford counties may be similarly employed. Even these streams, how

ever, are inferior to those found in the higher areas of the state. Practically all of the streams therefore of Eastern and Southern Maryland may be classed as undesirable for potable uses.

SURFACE WATERS OF CENTRAL MARYLAND

Streams of Central Maryland are much more important as sources of · potable water supply than those of the Coastal Plain, and are extensively used for municipal and corporate purposes, at the same time spring waters are also widely employed for private uses. Except where man has been responsible for their contamination, these waters are almost universally of good quality and adapted to potable uses. Great care has to be exercised in using surface waters anywhere to see that sanitary regulations are enforced, since a contaminated supply of this character may and frequently has spread disease over wide areas.

The Potomac, Susquehanna, Jones Falls, and Gunpowder rivers, and Brandywine Creek, as well as one or two other streams have been used as sources of public water supply in this district, and although they have not been as fully protected from contamination in all instances as is wise, the supplies on the whole are excellent, especially when scientifically filtered. Baltimore, Washington, Wilmington, and Havre de Grace use the surface waters of the larger streams mentioned, while Hagerstown, Frederick, and Perryville derive their supplies from small streams and springs.

The region under discussion has many unused supplies which, with proper safeguards against contamination and filtration, might be rendered available, among the larger streams being the Patapsco, Patuxent, Monocacy, Antietam, and Conococheague rivers.

SURFACE WATERS OF WESTERN MARYLAND

The surface waters of Western Maryland are the chief sources of potable water supply for that area. The streams flowing in narrow valleys can be readily empounded, and as they flow for the most part in regions of few inhabitants can be readily protected from contamination. The numerous springs occurring on the lower slopes and in the valleys are fed from the higher ridges and are admirable sources of local water supply. Cumber

land, the largest city in this district, obtains its water supply from Evitts Creek. The North Branch of the Potomac above Westernport, the Youghiogheny, Tonoloway, and Sideling creeks afford fine supplies of unutilized potable waters.

UNDERGROUND WATERS

Under the head of underground waters are included supplies obtained from springs, from the old-fashioned, relatively shallow, dug wells, and from the generally deeper drilled wells that utilize artesian waters.

MOVEMENT OF UNDERGROUND WATERS

The underground waters rarely move in definite channels of appreciable size in any way resembling surface streams, and this occurs only in regions where thick limestone beds contain caverns. Such cavern streams of any size are practically unknown in this area, and for the world at large they may be said to constitute an extremely small proportion of the world's water supply.

Supplies which form the source from which practically all wells draw their water are to be regarded as coming from more or less saturated parts of porous beds, such as sand and sandstones, or beds with minute systems of joints, such as those common in crystalline rocks. In all such situations the underground water whether influenced by gravity or capillarity moves in the minute interstices of the rock or sand bed, and this movement may best be conceived as a slow seeping in which the rate of movement is a few feet per day, rather than a few miles as in surface streams.

WATER TABLE

The effect of the constant percolation of water into the ground is to saturate certain layers, and this saturated zone, or frequently its upper surface, is known as the water table. The position of the water table depends on the topography, the varying porosity of different rocks, and the amount of rainfall. Where precipitation is heavy the water table is near the surface, except on very steep slopes, while in arid regions or during dry seasons it retires to greater or less depths. The relation of the

water table to the topography is shown in the accompanying diagram

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FIG. 76.-DIAGRAMMATIC SECTION SHOWING THE RELATION OF WATER TABLE TO SURFACE IRREGULARITIES.

VARIATIONS OF PRESSURE OR HEAD

Water pressures or head are very unequal even in the same bed or zone of saturation. They also vary greatly in different beds in the same region. Pressures depend upon a number of variable factors. These include: (1) The position of the water table, which varies with the factors already enumerated under that head; (2) the character of the water-containing beds, whether coarse- or fine-grained, and hence offering less or more resistance to the underground movement of the water; (3) changes in the adjacent beds as regards their coarseness of grain, and the effects of faults, joints, or other natural breaks. In artesian wells a fourth and most

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FIG. 77.-DIAGRAM SHOWING THE LOSS IN HEAD BY FRICTION AND LEAKAGE.

important factor is the character and elevation of the source of the underground water for the particular bed penetrated. The principles governing the loss of head, or what is known as the hydraulic grade, are illustrated in the accompanying diagram (Fig. 77).

CAUSES PRODUCING ARTESIAN WELL'S

The term artesian well has been employed in different senses in the past; and frequently in Maryland the term is used for only those wells which overflow at the surface. This is an incorrect conception. In this report, as well as in the publications of the U. S. Geological Survey, by an artesian well is meant any well which conforms to the hydrostatic principle by means of which confined underground waters tend to rise because of the pressure of the water column due to the ascending of the watercontaining beds to higher elevations. The presence or absence of a flow is dependent entirely on whether this pressure is sufficient to lift the water above the surface of the ground or not. It is customary in works on hydrology to compute these heads with reference to a general datum plane, usually that of mean sea level, but in the present report the more readily understandable datum is the surface, consequently throughout the

F

D

B

E

FIG. 78.-DIAGRAM ILLUSTRATING ARTESIAN CONDITIONS IN THE COASTAL PLAIN.

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text heads are given as + or with respect to the surface of the country where wells are located, although in the maps the contours are drawn with respect to depths below tide.

Artesian wells are possible where a porous stratum capable of saturation reaches the surface in a region where the rainfall is sufficient. This stratum should be confined between two impervious layers. The ideal conditions for obtaining an artesian well are indicated in the accompanying diagram (Fig. 78), which is practically an ideal vertical section from northwest to southeast across the Coastal Plain region of Maryland. The actual conditions are much more variable and complex.

Referring to the first diagram, there is a porous sand or sandstone with clay or shale both above and below it. Rain falling on the surface-marked catchment area sinks into the sand and gradually moves down the slope toward the southeast. If the position of a drilled well is enough lower than the catchment area so that the mouth of the well is below the

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