16 BULLETIN 304, U. S. DEPARTMENT OF AGRICULTURE. 
hills in the background show the nature of the country drained by 
Coal Creek. The last-built ditch and its reservoirs have now been 
in use for six years and an inspection made in the latter part of 1914 
indicated that they would still be effective for a number of years. 
No other district has installed so complete a system of silting basins 
in connection with diversion ditches. The Eldred and the Crane 
Creek Districts on the Illinois River and the EHsberry and the Des 
Moines County No. 1 Districts on the Mississippi River have con- 
structed diversions for the hill drainage, though not all of them were 
able to divert the hill drainage in its entirety. The locations of these 
districts and the details of then improvements will be found in the 
summary of all districts given at the end of this bulletin. 
In constructing diversion ditches the experience of the Coal Creek 
District should be kept in mind and ample provision made for the 
deposition of silt before it reaches the diversion channel proper, 
otherwise the usefulness of the latter will be of very short duration. 
Even when silting basins are provided it should be remembered that 
they will eventually become filled, and they should therefore be so 
located that other basins can later be constructed to receive the silt 
from the same stream without incurring needless expense due to the 
improper location of the first basin. Local conditions will deter- 
mine the plan to be followed, but the subject merits the most careful 
study. While in this country work of this nature is just starting, in 
England and Holland diversion ditches to prevent drainage water 
from flowing from higher lands onto lower areas, to the great damage 
of the latter, have been in systematic use for a long time. 
If the levees are properly constructed, as heretofore explained, the 
amount of seepage water passing under them is ordinarily so small 
as to be negligible, but if the soil or the subsoil of the district be verv 
sandy, or if the levees have been carelessly constructed upon an 
improper foundation, the seepage may accumulate in sufficient quan- 
tity to be troublesome. Ordinarily, if land near the levee is injured 
by seepage, the difficulty may be corrected by laying a line of tile 
parallel with the levee and 50 to 100 feet from the toe of the slope. 
The determination of the proper size and arrangement of the 
interior drainage ditches for a pumping district is not fundamentally 
different from the planning of a gravity ditch system for an ordinary 
district. The same hydraulic principles apply to the one as to the 
other, but in some details essential modifications are necessary. 
Ordinarily river-bottom land has a somewhat uneven surface, vary- 
ing in elevation by several feet, and lies in long ridges and depressions, 
which latter bear local names as lakes, sloughs, or creeks. The 
ridges may be quite sandy, while the lakes contain a heavy black 
soil. To fit all the land for cultivation the drainage system should 
be planned to keep the ordinary ground-water level at least 3 feet 
