THE FLOW OF WATER IN IRRIGATION CHANNELS. 39 
No. 173, Expt. S-80, central main canal, California Development Co., California. 
This test was made on a long reach of straight canal. The banks were nearly vertical 
as left by cleaning of silt with a bucket dredge. The bottom is very hard and quite 
regular, despite this method of cleaning. The velocity was retarded for about I foot 
from each bank by a growth of tules. The silt-laden waters form slick banks. Plate 
XII, figure 3, shows the reach tested and the portable rating car in action. If freed 
from growth at all times, which is impracticable in this region, the value of n would be 
under 0.020. For small canals in this region see Nos. 162 and 215. Coefficient 
n=0.0221. 
No. 174, Expt. H-20, Billings Land & Irrigation Co., Montana. This canal was 
originally constructed in varied strata having an earth surface underlaid with a stratum 
of gravel, while the bed of the canal was in Benton shale. This has now been covered 
in places with graded gravel. In general the upper end of the reach tested had a 
smaller sectional area, consequently a higher velocity, and the gravel was scoured 
(-lean, while the lower end of the reach had a lower velocity, and the gravel influence 
had been reduced by the deposit of silt. Coefficient n=0.0221. 
No. 175, Expt. S-84, Salt River Valley Canal, Salt River project, United States 
Reclamation Service, Arizona. This test was made on a straight reach of canal origi- 
nally constructed in graded gravel underlying silty loam soil. The high velocity 
encountered (mean 3.12 feet per second) scoured the bed of the canal, exposing hard- 
packed small gravel, while near the sides a slick deposit of silt formed a surface with 
but little retarding action on the water. The fringe of grass and small roots at the ex- 
treme edges (PL XIII, fig. 1), influenced but a very small portion of the flow. Coeffi- 
cient n= 0.0222. 
No. 178, Expt. H-2, Big Ditch near Billings, Mont. This test was made on a canal 
originally excavated in Billings loam, which tends to be clayey. The bed of the canal 
is iii the original earth with a slight deposit of sand which undercuts beneath the feet 
in wading, showing that the mean velocity, 2.09 feet per second, was almost sufficient 
to cause scouring of sand deposit. Fine mud has been deposited at the sides where the 
velocities are low. Coefficient ti=0.0225. 
No. 179, Expt. H-38, Bitter Root Valley Irrigation Co., Montana. The first half 
of this reach is on tangent, while the second half is on a 20° curve around a gravelly 
point. On the tangent the bed of the canal is covered with fine sand in serrations from 
1 to 2 feet longitudinally with the canal and about 6 inches deep. In the second half 
of the reach the sand covers the middle portion of the bed, while gravel up to cobble 
size forms the edges. The value of n found, 0.0226, is lower than is to be expected on 
this type of channel. 
No. 180, Expt. S-40, Lateral No. 10, Orland project, United States Reclamation 
Service, California. Many of the conditions holding for this test are clearly shown in 
Plate XIII, figure 2. The gravel, most of which is under hen's-egg size, is well com- 
pacted in the bed of the ditch, while a few scattered patches of moss have a retarding 
influence. There were about two patches, each 5 feet in diameter, in each 100 feet of 
length down the ditch. Coefficient n=0.0228. 
No. 184, Expt. S-39, main canal, South Orland project, United States Reclamation 
Service, California. This reach of earth channel (PL IV, fig. 3) lies just above the 
lined section tested in No. 51. It is straight, originally constructed in a yellow clay 
which is very slick when wet. A darker deposit of silt now covers much of this clay. 
A value of n of about 0.017 might be expected but for patches of moss and water grasses 
occupying about 20 per cent of the bottom of the channel. This influence brings about 
a value of 0.0231 for n. 
No. 186, Expt, S-36, River Branch Canal, Sacramento Valley Irrigation Co., Cali- 
fornia. This ditch was originally excavated in Sacramento silty clay loam, which 
breaks into very hard small clods (PL XIII, fig. 3). The bed of the ditch was very 
slick and hard. A few scattered soft lumps of mud and a fringe of grass retarded the 
