366 golze. PROBLEMS OF IRRIGATION CANALS [Ch. 21 



Deposits of silt provide a fertile base for weeds and water plants. 

 Irrigation canals are abundant sources of noxious weeds which, with 

 a little nourishment, flourish prolifically. Fine soil material, which 

 comprises the sedimentation found in many irrigation canals and de- 

 posits on the banks of canals, particularly along the water line, pro- 

 vides excellent seed beds. Weed seeds float down with the irrigation 

 water, sometimes coming from remote places in the watershed, or are 

 blown in from adjacent fields and are deposited in this fine material. 

 With moisture from the canals the seeds sprout, and a weed infesta- 

 tion on both the canal and adjacent farms is started. 



Water weeds are of two types: submerged and emergent. Examples 

 of the first are sago pondweed, horned pondweed, waterweed, and 

 coontail; examples of the second are cattails, tules, and watercress. 

 For support these plants depend in large measure on soil in the side 

 or bottom of the canals. Silted areas of the canals are an ideal 

 place for these plants to live in. 



Removal of silt to restore the canal to its original hydraulic proper- 

 ties and to destroy the weeds and water plants it supports is expensive. 

 The cost of removing silt represents a portion of maintenance costs 

 directly chargeable to sedimentation. Other costs, such as deprecia- 

 tion of equipment and possible crop reductions or crop losses due to 

 inability to deliver adequate water, are less tangible costs but are 

 also involved. A further discussion of costs is given in subsequent 

 paragraphs. 



On the favorable side, sealing of canals by silt reduces seepage 

 losses. Many canals are built through soils of high porosity; that is, 

 sufficient water can seep through the wetted perimeter to cause a 

 measurable loss of valuable irrigation water. Deposits of silt in the 

 bottom and sides of the canal often reduce these losses to negligible 

 amounts. Modern irrigation engineering, however, requires that seal- 

 ing of porous areas by silting should be done as a planned engineering 

 operation rather than be allowed to occur as an act of nature with 

 no control over its rate or period of action. 



Silt carried in irrigation water, not deposited in the canals, usually 

 passes on to be deposited on the farms. Test measurements show 

 that the portion of silt carried through the farm turnout may be as 

 high as 50 percent of the silt content at the head of the canal system. 

 On the farms, with the reduction in velocity, silt is deposited at the 

 head ditches, causing sedimentation problems there. Where not 

 trapped in the head ditches, the silt is carried on to the crop land it- 

 self, where it acts as a fertilizer to the crops being grown. 



