18 BULLETIN 304, TJ. S. DEPARTMENT OF AGRICULTURE. 
is necessary to keep the plant operating; at other times the low- 
water flow of the ditch will be sufficient to warrant the plant operating 
only a small portion of the time. This feature will be discussed in 
more detail under the heading, " Necessary capacity of pumping 
machinery." 
The greatest amount of sediment is likely to be washed into the 
main ditches during a heavy storm when the ditches are bank full. 
It should be noticed that at such a time there is an important differ- 
ence between the condition of these ditches and that existing in main 
ditches of gravity drainage systems. In the latter, when more than 
normally full, the velocity is a maximum, and hence there is the least 
possible tendency toward the deposition of sediment hi the ditches. 
But in a pumping district, on the contrary, when the main ditches 
are unusually full, the velocity will be low, and hence sediment may 
be easily deposited. As a rule, the velocity in these ditches will 
always be so low that they are not self-cleaning. On this accoimt, as 
well as on account of the drawing down of the level in the ditches by 
the pumps, such ditches should be of unusually generous dimensions. 
The engineer will wisely give particular attention to the designing of 
the ditches to insure ample depth and breadth to provide the requisite 
capacity. In general, it is probable that the main ditches should be 
dug from 8 to 10 feet deep. Figure 1 of Plate V is a view of the main 
ditch, Coal Creek Levee and Drainage District, Illinois, which was 
constructed in 1909 with a dipper dredge having side spuds. The 
view was taken before the ditch was completed at the lower end. 
The water level as shown is about 4 feet below the ground surface 
and about 4 feet above the normal level after the ditch was com- 
pleted. Figure 2 of Plate V is a view looking up Branch A of east 
lateral, from near the outlet of tile drain No. 7. An inspection of 
these ditches at the end of 1914 showed them to be in very good con- 
dition and remarkably free from deposits of silt. This very desirable 
condition of the ditches is largely due to the complete diversion of 
silt-bearing waters from the hills. Other districts already in opera- 
tion would do well to investigate the advisability of treating their 
hill drainage in a similar manner. 
Experience shows that to put bottom land into perfect agricul- 
tural condition considerable tile drainage is necessary. This has 
been demonstrated on all the districts along the Illinois and Mis- 
sissippi Rivers, and landowners who have already been put to con- 
siderable expense for the mam drainage of the district can not afford 
to have their land in any but the most fertile condition; it is, there- 
fore, money exceptionally well invested to go to the additional 
expense involved in the necessary tiling. Tiling in such lands should 
be done with the same careful attention to the slope and to the 
nature of the soil and subsoil as is necessary in getting the desired 
