150 : PHYSICAL GEOGRAPHY.

 

Of the rivers flowing into the Mexican Gulf,
the Appalachicola, Alabama, Mississippi, Sabine,
Trinity, Brazos, Colorado, and the Rio Grande are
the most important.

The Mississippi, taking its origin in the head-waters of
the Missouri—which is the true parent stream—is the long-

est river in the world, its length being 4490 miles. Its _

tributaries are, in general, navigable for great distances,
and thus afford ready communication with different parts
of the basin. The important tributaries of the Mississippi
on the west are the Minnesota, the Missouri, the Arkansas,
and the Red. On the east, the Wisconsin, the Illinois, and
the Ohio.

The Pacific System.—The principal rivers emp-
tying into the Pacific Ocean are the Columbia,
Sacramento, San Joaquin, and the Colorado.

393. Inland Drainage—The rivers and lakes
of the Great Basin have no outlet to the ocean,
and therefore form true steppe systems. Great
Salt and Humboldt lakes are the principal lakes,
and the Humboldt and the Reese, the principal
rivers.

There are two regions in the United States below the
mean level of the sea:

(1.) In the southern part of California, in Soda Valley,
200 feet below the sea.

(2.) Death Valley in Eastern California. These regions
are extremely arid. ;

394, Lake-Systems.—The most important lake-
system of the United States lies in the northern
part. It includes, among numerous others, five
of the largest fresh-water lakes in the world:
Superior, Michigan, Huron, Erie, and Ontario.
From their immense extent, they resemble great
inland seas.

Numerous fluviatile or river lakes occur near the borders
of the middle and lower courses of the Mississippi and its

tributaries. They are nearly all found in the States west
of the Mississippi.

039400

CEEA PE Ree

Meteorology.

395. Climate——The United States, exclusive of
Alaska, lies entirely within the limits of the mathe-
matical north temperate zone.

Physical Zones.—<As regards the actual distri-
bution of heat, the United States lies between
the annual isothermal lines of 40° and 77° Fahr.
Its territory therefore embraces two zones of phys-
ical climate, the physical north temperate and the
physical torrid zones. The isotherm of 70°, the
boundary of the physical torrid zone, passes

 

through Florida, Louisiana, Texas, and Arizona.
All of the country south of this line lies in the
physical torrid zone; all north of it, in the
physical north temperate zone.

The Territory of Alaska lies in the north frigid
and north temperate zones.

896. Mean Arinual Isotherms.—The following pecu-
liarities in the mean annual distribution of heat, will be
seen from a study of the mean annual isotherms on the
map (page 145):

The isotherm of 40°, the lowest mean annual tempera-
ture, is found in a few elevated districts in New England,
in the elevated districts around Lake Superior, and in. the
higher plateaus of the Rocky Mountains.

The isotherm of 45°, east of the Mississippi, runs slightly
north of the 44° of N. lat., and, except in New York, Ver-
mont, and New Hampshire, is nearly parallel with it. In
the Dakotas it bends toward the north-west, reaching
the northern boundary of the United States in Mon-
tana, when it bends suddenly toward the south-east, until
it reaches Central Colorado, where, nearly parallel with
its southward deflection, it again. turns abruptly to the
north. :

The isotherm of 50° is nearly parallel with the 41° N.
lat., until it reaches Colorado, when it bends sharply south
to the 36° lat.; then it extends in a nearly direct line
toward the north-west, entering the Pacific coast some-
what to the north of Washington. ;

The isotherm of 55° enters the Atlantic coast at lat. 40°;
it then extends south-west to Tennessee; thence north-
ward to Kentucky, from which its course is nearly due
west to Indian Territory, when it bends southward to
about lat. 34° in New Mexico. From this point it extends
in a nearly direct north-westerly course to lat. 41° in
Northern California, when it bends sharply to the south,
entering the Pacific Ocean at about lat. 36°.

The isotherm of 52.5°, traced only on the western half
of the map, starts from the north-western part of New
Mexico, and runs in a nearly direct north-westerly course
to North-eastern Nevada, when ‘it divides, the northern
branch extending to the north-western extremity of
Washington, and the southern entering the Pacific in
the neighborhood of San Francisco.

The isotherm of 60° enters the Atlantic coast at Norfolk,
Virginia; it then trends south-west to Northern Georgia,
from which its course is nearly due west until it reaches
Indian Territory, when it runs south-west to New Mexico,
Here it divides into two branches: the northern extends
in irregular curves to lat. 40° in California, when it bends
sharply to the south-east, entering the Pacific at about lat.
34°, The southern branch enters the Gulf of California
at about lat. 25°.

The isotherm of 70° extends through Northern Florida,
Southern Louisiana, and Texas.

The isotherm of ‘75° extends through Southern Florida,

397. Climatic Contrasts—There is a marked
contrast between. the climate of the eastern’ and
western coasts of the United States.
coast 1s colder than the western.

The difference in temperature is greater in the
north ; the mean annual temperature of the coast,
between New Jersey and Maine, is from 52° to

The eastern