8 LAKES OF NORTH AMERICA. 



In the case of mature streams that have cut down the seaward portion 

 of their valleys nearly to base-level, that is approximately to the level of the 

 ocean, and where rivers rising in mountainous regions flow across low 

 plains, it frequently happens that the more energetic tributaries towards 

 their head Avaters bring in more detritus than the gently flowing trunk 

 streams are able to carry, and deposition takes place on their Ijottoms and 

 over their flood plains. When the main stream is flooded and inundates 

 its valley, its load is deposited most abundantly on the immediate borders 

 of its channel, and builds up lateral embankments or levees. When this 

 happens, the lateral tributaries joining the main stream in its lower course, 

 may not be able to fill up their valleys as rapidly as the borders of the 

 main -river are raised, and are consequently ponded. ^Nlany shallow lakes 

 have been formed in this manner along the borders of the large rivers 

 flowing to the Gulf of Mexico. The most conspicuous examples occur 

 along the banks of Red river, Louisiana, where lateral lakes, as has been 

 pointed out by Davis, are arranged along the side of its levees like the 

 leaves on a twig. 



In the maturity and old age of rivers, when they meander in broad 

 curves through a wide flood plain, as in the case of the lower ^Mississippi, 

 the loops are frequently cut off, as shown on Plate 1, and crescent-shaped 

 or " ox-bow " lakes are left. Examples of lakes of this character on a 

 small scale may be seen along the border of many sluggish brooks which 

 traverse deeply filled valleys. 



In the formation of low-grade deltas, like those now in process of con- 

 struction at the mouths of the Mississippi, Nile, Ganges, etc., the waters 

 break through the levees of the main stream during floods, and form 

 branching channels or " distributaries," which in their turn bifurcate in a 

 similar manner, and build up their channels and inundated borders. In 

 such instances low areas are frequently surrounded by embankments, and 

 left as basins containing shallow lakes. ]\Iany examples of this occur- 

 rence are found on the broad delta of the jNIississippi. Of these Lake 

 Pontchartrain is the largest at the present time. Lake Borgne, in the 

 same region, is another example, not yet completed. The delta lands of 

 the Rhine, in Holland, and of other rivers in northern Germany, contain 

 many lakes and swamps of the type here considered. The celebrated 

 Zuyder Zee was formed in part as a delta basin and in part by the con- 

 struction of natural embankments adjacent. to a low shore. ^Miniature 

 illustrations of this method of forming basins may be seen on the deltas 

 of many small streams, built in lakes and ponds. 



