THE CONTROL OF FLOOD WATER LN SOUTHERN 

 CALIFORNIA. 



Edw. N. Munns 

 Forest Examiner, U. S. Forest Service. 



The control of floods has received more definite consideration in 

 southern CaHfornia than perhaps any other portion of the country, and 

 control work used abroad, tackling the trouble at its source, has been 

 applied here on a scale sufficiently large to show its worth. This 

 thorough consideration of the water problem is due to the floods and 

 the large quantities of silt and debris brought from the mountains by 

 the storm waters during the rainy season, causing much loss to the 

 fertile valley fields. Much of this material comes from the small 

 tributary canyons at the headwaters of the streams, where grades are 

 steep and the precipitation heavy. This detritus is deposited in the 

 stream bed with the decrease in the velocity of the water, the coarser, 

 heavier parts dropping near the mouth of the canyon, Ijuilding up a 

 debris cone, while the finer parts travel with the stream even to the 

 point where the waters are finally discharged into the ocean. xA.s a 

 result, during very heavy storms the harbors are badly silted, and even 

 during normal years considerable debris is deposited in the channels 

 between the canyon mouths and the sea. This filling gradually in- 

 creases the height of the stream bed and causes frequent shiftings in 

 its location, the new channels often being through fertile, valuable 

 lands, while the transportation systems, highways, and municipalities 

 are badly damaged. 



A large proportion of the erosion damage is caused by the silting 

 of these lower channels, resulting in a decrease in their carrying ca- 

 pacity. As much of this debris is brought from the mountains, any- 

 thing which would keep this soil in place aids greatly in the solution of 

 the flood problem, for by restraining the water in its upper reaches 

 the velocity is held below the point of excessive erosion. To this end 

 check dams are used. 



Check dams are small obstructions usually built of stone so placed 

 across the channel that the water, though able to percolate through 

 them to some extent, collects in a basin behind the dam and then 

 falls vertically, or nearly so, over its front face. If the height of this 



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