36 The Control of Mosquitoes 



and the formation of pot-holes frequently result, 

 often making it difficult to keep a stream properly 

 trained, and unless controlled, larvae will be found 

 at its edges where there is least current velocity. 

 Where branch streams occur, debris, sand, etc., are 

 deposited at their junction with the main stream, 



Table No. 9.— 



evaporatvm — Canal Ztme, years 1912, 1913, and averages. 

 (Value In Incbes.] 



Note.— Averagea at Rio Grande and Brawa Brook are based on four years' record; Ancpn five years' 

 record; and Gatun two years' record, 



Evaporalion meo-siiremcnla are from Qoating pana 4 foct In diameter and 10 Inchca deep at Kto Grande, 

 Oatun and Bra7X}3 Droolc, and insulated tonic 10 Incliea in diameter at Ancon. 



For monthly evaporation during past years, see provioua annua) reports. 



and afford collecting places for larvae brought 

 down-stream. 



Various accumulations of vegetable matter, 

 twigs, branches, etc., are found at the edges of 

 rivers and make suitable hiding places, often 

 inaccessible to the fish that prey upon mosquito 

 larvae. At some points, seepage outcrops occur 

 on the stream banks and are constant sources of 

 larvae throughout the year. It was not at all 



