The Delaware, Susquehanna, and Potomac are all important rivers which rise in the CatskillAllegheny Plateau and find their way across the various mountain ridges of the Appalachian system by water gaps, and enter bays due to sinking of land and invasion of old valleys by the salt waters. In the plateau, where the strata are horizontal, the streams are dendritic, or finger-like, in arrangement. Within the mountain belt, the streams are in parts longitudinal, and in part transverse. By thus running between the ridges and cutting through them in water gaps, a rectangular or trellised drainage is formed which is widely found in the Appalachian region. The northern Appalachian waters thus flow mainly to the open Atlantic. In the south, however, or beyond the Potomac, the coastal rivers head in the eastern edge of the mountains, while the Kanawha and the Tennessee, with their branches, head far across the mountains and carry the waters to the Ohio River. The great core of the southern Appalachians, in western North Carolina, so convenient, it would seem, to the sea, is thus drained by a circuitous route into the Gulf of Mexico. The entire belt of Appalachian uplands has for the most part arrived at the stage of mature dissection, with abundant valleys, sunk from 500 to 1500 feet below the prevailing level of the uplands.

Many rivers of local importance rise on the eastern slope of the Blue Ridge, and cross the Piedmont Plateau and the Coastal Plain to the sea. Such are the James and other rivers of Virginia, the Roanoke, Cape Fear, Great Pedee Savannah, and Altamaha of the Carolinas and Georgia. The rivers of Florida are mainly small, but have important tidal courses. The other Gulf drainage is overshadowed in magnitude by the Mississippi. The greater streams aside from this are the Chattahoochee, Alabama, and Tombigbee on the east, and the Colorado (of Texas) and Brazos on the west, and the international Rio Grande, rising in Colorado and New Mexico. THE MISSISSIPPI RIVER. See MISSISSIPPI RIVER.

PACIFIC DRAINAGE. The Rocky Mountains in Colorado offer the most compact source of important drainage within the United States. To the south they send forth the Rio Grande. To the east flows the Arkansas, while to the north and east pass important branches of the Missouri. From the southwest issues the San Juan, while farther north the Grand, White, and Yampa rivers leave the State. All these join the Colorado River, and thus introduce us to the most southerly of rivers in the United States that reach the Pacific Ocean. Some of the chief facts concerning this river have been given in the description of the topography of the region. Its ultimate sources are in the Wind River Mountains of Wyoming. Thence it crosses the plateau of the southeastern part of that State and passes by a great gorge through the heart of the Uinta Mountain Range. It is here known as the Green River, but becomes the Colorado after receiving the Grand. Thence it threads its greater cañons and crosses the low desert region to the Gulf of California. Not far from its mouth it receives the Gila from the east. The only remaining master stream is the Columbia, with its widespread sources in the Rocky Mountains. These largely belong to its tributary, the Snake River,

which after passing the lava plateaus joins the trunk stream in southern Washington. The Columbia has an important tidal course before it enters the ocean, and this and the Saint Lawrence are the only rivers of the United States which receive great accessions to their waters from foreign territory.

The foregoing review has shown how various are the rivers of the United States in their physical features. The Hudson may be taken as the type of that great number of streams, especially on the Atlantic side, which have a tidal course. Within the glaciated district most rivers show serious inequalities in their beds, causing innumerable rapids and waterfalls. Such concentration of descent does not occur elsewhere, save in the mountainous belts. The greatness and destructiveness of floods depends upon rainfall, gradient, the porosity of the rocks or soil, and other features. Thus the Ohio has steep slopes, an impervious bed, and a moist climate, when compared with the Missouri taken as a whole. In its long course across the plains the latter loses by seepage, by evaporation, and by abstraction of water for irrigation.

The rivers exhibit great diversity in relation to human uses. The Merrimac and many other streams within the glacial belt are mainly useful for manufacturing or for water supply. The Mississippi is chiefly of value as a highway, while the Hudson combines transportation, water and ice supply, and the furnishing of power. The Colorado is thus far almost purely scenic in its relation to man, arousing interest by the origin and magnificence of its cañons. A new era of utilization of these American river waters is now being entered upon. See IRRIGATION.

CLIMATE. The greater part of the area of the United States proper has a climate pertaining to the temperate zone. The average annual temperatures vary from somewhat less than 50° on the northern border to 75° in the extreme southeast. The average temperature for July is about 60° on the northern border and for January it is about 20°. The whole country is exposed to much greater annual oscillations of temperature than occur in Europe. The average of the absolute maximum temperatures is as high as 115° to 120° in the drier portions of Texas and Arizona, and the average minimum falls as low as -40° in northern Minnesota. The climate of the United States is controlled very largely by its characteristic winds. In January these are from the west, but in July southeast winds prevail in the Southern States, which penetrate up the eastern slope of the Rocky Mountain range, but finally turn eastward with the westerly winds of the northern portion of this region.

The storm centres that pass across the country originate almost equally on the Pacific Ocean and on the Atlantic. Those that approach from the Pacific move southeastwardly into Kansas and Nebraska and then turn eastward over the lake region; those that come from the Atlantic move westward among the West Indies and turn northward toward the Lower Lake Region; a certain number originate on the southeastern slope of the Rocky Mountains and move northeastward to the lake region. The lake region is, therefore, on the average the stormiest portion of the continent, and indeed of the whole Northern Hemisphere. The passage of every storm

The

centre is accompanied by a shifting of the winds from warm southerly to cold northwesterly, and this, consequently, gives to the interior of the United States its extreme variability of climate, so that even the southern extremity of the country scarcely belongs to the tropical zone. cold air that flows in behind the storm centres is very dry and the sky is quite clear. Thus originate the so-called anti-cyclones or cold waves which follow each other rapidly, moving south eastwardly toward the Gulf of Mexico, while the areas of low pressure or cyclones move northeast over and beyond the lake region. The absolute annual range of temperature is therefore greatest in the interior of the country, being about 150° in the upper portion of the Missouri Valley and diminishing to 60° in the southern part of Florida and the northwestern part of Oregon. The suddenness with which the air temperature falls is an important consideration from many points of view. Thus changes of 20° in twentyfour hours occur far more frequently in the lake region than in the Ohio Valley, and there again oftener than on the southern coasts.

The average date of killing frosts in the spring and autumn determines the average length of the growing season for most of the important crops. The autumnal date is September 1st for the region from North Dakota to Lake Huron, and October 1st for the region from Colorado to Pennsylvania and northeastward along the New England coast. The latest spring frosts occur on May 15th from Idaho to Lake Superior and February 15th along the south Atlantic and east Gulf coasts. The growing season may be considered as the interval between the last frost of spring and the first of autumn, or it may also be defined as the season within which the average daily temperature does not fall below 40°. (See Maps accompanying article FROST.) From this point of view the growing season diminishes as one goes northward and amounts to about 120 days at the northern border of the United States. By a natural process of selection, stimulated by culture, plants that formerly required this length of time for maturity are now pushing northward beyond the borders into Canadian regions where the growing period is as much as twenty days shorter. Mr. Pennywitt of the Weather Bureau has published, in the Monthly Weather Review for February, 1901, three charts showing the dates on which the normal daily temperature at any place equals the annual mean at that place. There are two such dates for each station, dividing the year into the warmer half and the colder half. These are not quite the dates of frost, but generally come earlier than the frost in spring and later in autumn. His chart reveals the interesting fact that in general over the United States the warmer portion of the year is decidedly longer in duration than the colder season. This difference is greatest on the eastern slope of the Rocky Mountain range, and increases from thirty days in Texas and Kansas to forty-eight days on the northern border of Montana and probably increases somewhat as one goes north into Canada.

One of the most notable features in the climate of the United States is the great contrast between the Atlantic and Pacific coasts. This is due to the fact that the cold, dry air following each area of low pressure is confined in its

movement southward between the Rocky Mountains and the Appalachian ranges. Occasionally dry, cold air descends from the north or northeast upon The Great Valley' of California with disastrous consequences, but in general the Pacific coast is under the influence of south and west winds, which bring abundant winter rains to northern California, Oregon, and Washington without severe cold weather. On the Atlantic coast the winter rains and snows are followed by very cold weather. The contrast between the mean temperatures on the immediate coast of the United States in January and July are shown in the following table. The temperatures here given are as read off on charts of isotherms, and referto locations on the land at sea level near the shore. These temperaturès are controlled largely by the prevailing winds and the adjacent oceans:

Of course a similar great contrast prevails between the temperatures on the east side of Asia and west side of Europe, but nothing like this occurs in the Southern Hemisphere, owing to the presence of the great southern ocean and the small area and low altitude of the lands.

Another striking peculiarity in the climate of the United States is the great contrasts of dryness and rainfall. The heaviest precipitation and also a fairly uniform humidity prevail on the coast of Oregon and Washington; next to this are the humidity and rainfall of the coasts of Florida and the adjacent States. Throughout the Mississippi Valley and along the Atlantic coast from Cape Hatteras to Nova Scotia there are great fluctuations in moisture and evaporation. On the average the driest portion of the continent extends from southern California and Arizona eastward over the eastern slopes of the Rocky Mountain region. Not only is the rainfall here a minimum, but the humidity of the atmosphere is also very small and the proportion of the clear sky very large. Yuma, in Arizona, Santa Fe, in New Mexico, and Pueblo, in Colorado, have mean annual relative humidities of 43, 45, and 46 per cent, respectively. The amount of evaporation from the soil is correspondingly large in these regions, and artificial irrigation is quite essential for successful agriculture. Even the summits of the mountains of this portion of the continent show but little permanent snow, and that in sheltered spots. What would remain on the ground in an ordinary, moist climate is rapidly evaporated in this dry air. See map showing average annual relative humidity in the