DRAINAGE OF WET LANDS OF SOUTHERN" LOUISIANA. 17 



slight elevation of the land, above sea level all of the streams are very sluggish 

 in character. Their surface slopes are alvv^ays very slight and are due entirely 

 to the piling up of the vs^ater in the interior until sufficient head is created to 

 force the w^ater out to the Gulf. At times of high tide in the Gulf and small 

 precipitation in the interior, the current is often reversed in many of the 

 streams, and salt water then flows many miles inland. However, at such times 

 the water in the channels is so low that the tide rarely causes a stage suffi- 

 cient to flood any of the adjoining land. (This condition should not be con- 

 fused with tidal overflow, which will be discussed later.) The fluctuation in 

 water level, due to direct precipitation, in the various bayous and interior 

 lakes is never very great and depends quite as much on the direction of the 

 prevailing winds as on the amount of precipitation. Bayou La Fourche is one 

 of the largest and longest natural drainage channels in this section, extending 

 about 120 miles into the interior. A gauge has been maintained for more than 

 three years at a point about 70 miles inland from the Gulf, and back 1 mile 

 from the bayou on a short canal which connects with the general water level 

 in the swamps. The extreme variation of the water surface observed at this 

 gauge was 3.4 feet. At the time of the lowest gauging the salt water had 

 reached this point, so that it was approximately sea level. This low stage was 

 caused by a combination of the following conditions : A prolonged and record- 

 breaking drought, only 1.12 inches of precipitation having fallen in the pre- 

 ceding two months ; warm weather giving high evaporation ; and northerly 

 winds followed by southerly winds, which later caused a gradual rise by 

 bringing in the salt water. This low stage was 1.3 feet below mean tide for 

 this point. During the winter of 1911-12 the stage reached its greatest height 

 for the three years beginning June, 1909. It was then 3.4 feet above low tide, 

 which is approximately sea level, and 2.1 feet above mean tide. This high 

 stage was caused by a combination of the following conditions: Heavy and 

 continual precipitation, 9.7 inches having fallen in the preceding 40 days, cold 

 weather and small evaporation, and continued southerly winds. 



At points farther inland the fluctuation in water level is proportionately 

 greater. The situation on Bayou La Fourche is mentioned because it Is 

 typical of all the long sluggish bayous that carry away the drainage water. 

 Most of the interior watercourses are connected with each other by cross 

 bayous and canals so that they are all somewhat similar in their action. The 

 drainage areas are very poorly defined, and no doubt lap somewhat, as some of 

 the connecting canals and bayous often reverse direction of currents, according 

 to the stages of water in the various parts of the system; for this reason it 

 is practically impossible to measure the run-off from these drainage areas. It 

 is probable that the natural run-off is very low, due to small slopes and the 

 rank vegetation on all the land, only about 10 per cent along the bayous being 

 under cultivation. The bayous of sedimentation are quite free from growth of 

 vegetation, many of them having a considerable boat traffic which tends to 

 keep them cleared out and in good condition as drainage channels. Those of 

 tidal erosion are apt to be overgrown with water hyacinths, but owing to their 

 greater depth these are also quite efficient channels. As shown in figure 1, 

 many parts of this section discharge their drainage water almost directly into 

 the Gulf, or into large interior lakes that undergo very little fluctuation in 

 water surface. Thus these areas are relieved of all drainage water due to 

 direct precipitation without great rise of water in the carrying channels. In 

 the interior portions, such as that contiguous to the upper part of Bayou La 

 Fourche, there are often rises of water level of several feet iu the main drain- 

 25102°— Bull. 71—14 3 



