The principal rivers are the Mississippi, which flows nearly 600 m. through and along the border of the State, the Red river, the Ouachita, Sabine and Pearl ; all except the last are navigable at all stages of the water. There are many bayous, several of which are of great importance, both for navigation and for drain age. They may be characterized as secondary outlets of the rivers or flood distributaries. Among them are Bayou Teche, Bayou Plaquemine, Atchafalaya bayou, Bayou Lafourche and Bayou Boeuf. Almost all secondary water-courses, particularly if they have sluggish currents, are known as bayous. The alluvial portion of the State, especially below the mouth of the Red river, is an intricate network of these bayous, which, before their closure by a levee system, served partially, in time of flood, to carry off the escaping surplus of river waters.
The alluvial region of the State is mainly protected against overflow from the Mississippi river by approximately 75o m. of levee on that river within the State, and by levees on the Mis sissippi river, Cypress and Amos bayous in Arkansas, forming part of the general system which extends up the Mississippi to the highlands above the junction of tie Ohio river. The State and Federal Government co-operated in the construction and main tenance of this system, but the latter did not give national aid (except the grant of swamp lands in 1850) until 1879, when it began acting through a board of engineers, known as the Mis sissippi River Commission. For about a century and a half before that time, levee building had been undertaken in a more or less spasmodic and tentative way, first by riparian interests, and finally by the State, acting through levee districts, advised by a board of engineers. The system of Mississippi river levees, within the State, was built almost entirely after 1866 and represents an expenditure of over $6o,000,000 for primary construction alone. Some of the levees, especially those in swampy regions where out let bayous were closed, are of extraordinary solidity and dimen sions, being 3o to 5o ft. high, or even more, across former streams or bayous, with bases of 1 o ft. or more to one of height. Levee systems on the interior rivers and bayous, aggregating hundreds of miles, are built and maintained exclusively by the State.
The unprecedented heights of the flood in 1922 led the Mis sissippi River Commission to recommend the raising of the levees along that stream an additional three feet. To carry out this policy Congress authorized the expenditure of $6o,000,000 on the entire Mississippi system. More than half of this sum had been expended when the deluge of April and May 1927 demonstrated that even the three-ft. margin contemplated would not be suf
ficient. U.S. army engineers believe that adequate flood protec tion will result from the $325,000,000 Mississippi Valley Flood Control act approved by President Coolidge on May 15, 1928. The lakes are mainly in three classes. First come the coast lagoons, many of which are merely land-locked salt-water bays, the waters of which rise and fall with the tides. Of this class are Pontchartrain, Borgne, Maurepas and Sabine. These are simply parts of the sea which have escaped the filling-in process carried on by the great river and the lesser streams. A second class, called "ox-bow" lakes, large in number but small in area, includes ordi nary cut-off meanders along the Mississippi and Red rivers. A third class, those upon the Red river and its branches, are caused mainly by the partial stoppage of the water above Shreveport by the "raft," a mass of drift such as frequently gathers in western rivers, which for a distance of 45 m. almost completely closed the channel until it was broken up by government engineers. These lakes are much larger at flood season than at other times and have been much reduced in size by the cutting of a channel through the raft.
Climate.—The climate is semi-tropical and exceptionally equable over large areas. In the south and south-east the equable temperature is largely the effect of the network of bays, bayous and lakes, and throughout the State the climate is materially in fluenced by the prevailing southerly winds from the Gulf of Mexico. Some daily variation in the temperature of adjoining localities is caused by a dark soil in the one and a light soil in the other, but the differences of mean annual temperature are almost wholly due to differences of latitude and elevation. The mean annual temperature ranges from 70° F at Port Eads, in the extreme south-east, to 65° F at Lake Providence, in the north east. The mean temperature of July, the hottest month, is com paratively uniform over the State, varying only from 81° to 83° ; the mean for January, the coldest month, varies from 46° in the extreme north to 56° in the extreme south. Even in the coldest localities eight or nine months are wholly free from frost, and in the coast parishes frost occurs only a few days in each year. Rainfall is usually heavy in the south-east, but it decreases toward the north-west. As much as 85.6 in. has fallen within a year at New Orleans, but in this locality the average for a year is about 57.4 in. ; at Shreveport the average is 46 in., and for the entire State it is 55 inches. Summer is usually the season of heaviest rainfall.