6io 



Yearboo^ of Agriculture 1949 



fore they have a chance to build up to 

 destructive potentials in the channels. 



Floods are caused by the excess rain 

 water that is not absorbed and tem- 

 porarily stored in the soil or contained 

 within defined watercourses. The dam- 

 age caused by these overflows and by 

 the sediment they carry and deposit 

 may occur in fields adjacent to upland 

 watercourses, in bottom lands along 

 small tributary streams, and in the 

 flood plains of great rivers. 



Programs to improve watersheds are 

 designed to work with nature by retard- 

 ing runoff and reducing soil losses. 

 That is accomplished by managing the 

 soil and protective plant cover and by 

 stabilizing gullies, watercourses, and 

 stream banks to help control the move- 

 ment of water from the time it falls 

 as rain or snow until it enters the rivers 

 and waterways. Such an approach 

 builds and conserves the soil so that it 

 will function to best advantage as a 

 natural reservoir and at the same time 

 become more productive. 



A highly important, lasting effect of 

 stabilizing and improving the soil and 

 vegetation on a watershed is the grad- 

 ual restoration by natural processes of 

 a better balance between stream-chan- 

 nel conditions and the runoff they 

 carry. The amount, rates, and quality 

 of stream flow reflect the character- 

 istics of a watershed during any given 

 period, so any betterment in these char- 

 acteristics in time brings a correspond- 

 ing favorable adjustment in the habits 

 of the stream. 



Certain intense or prolonged rains 

 produce so much water that even 

 watersheds in the best of condition can 

 modify the occurrence of high flood 

 flows only to a limited extent. Proper 

 watershed measures carefully installed 

 and maintained can lower the fre- 

 quency of floods, especially the smaller 

 ones, greatly lessen the chances of oc- 

 currence of swift flash floods in the 

 smaller valleys, and materially reduce 

 the silt load that adds to both the flood 

 volume and the amount of damage. 



The natural processes whereby the 

 soil and plant cover of a watershed 



operate to reduce flood flows and sedi- 

 mentation are described earlier in this 

 chapter. Let us consider briefly here 

 the role of the land in the operation of 

 the phase of the hydrologic cycle that 

 pertains to flood discharges. 



Watershed lands influence flood 

 flows and sedimentation by the manner 

 in which they dispose of rain and 

 snow melt. Flood runoff from the land 

 occurs when rain falls or snow melts 

 faster than the soil can absorb it. The 

 ability of the soil to take in and hold 

 back water is affected in turn by the 

 kind and condition of the vegetative 

 cover, as well as by the structure and 

 depth of the soil. Surface runoff is the 

 most destructive. It is a highly impor- 

 tant factor in sheet and gully erosion 

 and in the rapid formation of flood 

 peaks. 



Rapid subsurface discharge from 

 pervious shallow soils or soils with an 

 impervious layer near the surface can 

 also contribute to flood flows. Because 

 the storage capacity of such soils is 

 limited, it is soon used up, and any ad- 

 ditional water entering the soil quickly 

 drains off into channels. Subsurface 

 discharge, however, is less destructive 

 than surface runoff, in that it seldom 

 causes erosion. 



The basic principle involved in 

 watershed flood control is to increase 

 the ability of the soil to absorb water 

 and temporarily keep it from enter- 

 ing stream channels and to control the 

 runoff movement of water so that it 

 causes a minimum of damage. 



That is accomplished ( 1 ) by restor- 

 ing and retaining a good cover of 

 vegetation and litter to protect the soil 

 against compaction and erosion and to 

 increase its intake and storage capacity 

 and (2) by stabilizing gullies, water- 

 ways, and tributary stream channels. 

 During the growing season, vegetation, 

 especially heavy forest growth, can also 

 increase the storage capacity of the 

 soil by rapid and heavy transpiration. 



THE FLOOD CONTROL ACT of 1936 

 provides the basis for current Federal 

 flood-control activities. It is an out- 



