lERIGATION IN NORTHERN COLORADO. 7 



The wide seasonal variation is due to the spring flood produced by 

 rapid melting of snow in the hills. Its duration and intensity de- 

 pend on the amount of snow to be melted and the temperature, a con- 

 tinued high temperature producing a rapid rise, a high crest early in 

 the season, and a subsequent rapid fall, and a low temperature pro- 

 ducing a more gradual rise and fall with comparatively low and 

 late crest. The average date of the crest is June 10, but it has come 

 as early as May IT and as late as June 28. The discharge at the 

 crest has varied from a mean of 1,550 second-feet on June 19, 1888, 

 to a mean of 5,800 second-feet on June 23, 1917. The lowest dis- 

 charge is about 30 second-feet and occurs in winter after severe cold 

 weather has frozen most of the stream. 



The daily rise and fall is pronounced only during the spring flood, 

 when alternate freezing and thawing of snow on the high slopes of 

 the basin produce a variation of several hundred second-feet at the 

 gaging station, the maximum recorded being 1,500 second-feet. Sud- 

 den floods, due to storms, are common and there are records showing 

 a rise of more than 5 feet in less than 30 minutes, caused by cloud- 

 bursts in narrow branch canyons with steep slopes. 



Records of discharge of the river from 1884 to 1917, inclusive, are 

 given in Table 2. During the winter months ice conditions at the 

 gaging station are such that automatic records are of little value, 

 and such as were available were discarded. Estimates of the flow 

 from November to March, inclusive, were furnished by John Arm- 

 strong, water commissioner for the stream, who has handled the di- 

 vision of the winter flow for over 25 years. To arrive at the annual 

 discharge, Mr. Armstrong's estimates for the winter months were 

 combined with available figures for other months, and then, if April 

 or October records were partly missing, they were interpolated in 

 the proportion of the percentages shown in the table. The winter 

 flow is so small that this method of estimating could not produce an 

 error of as much as 5 per cent. Data included in the table are from 

 the original records of the Colorado Experiment Station and from 

 reports of the State engineer. In considering this table it should be 

 noted that the North Poudre and Poudre Valley Canals divert about 

 40,000 acre-feet above the gaging station annually, and that part of 

 the water passing the station is foreign water from other drainage 

 basins. 



