8 BULLETIN 832, U.S. DEPARTMENT OF AGRICULTURE. 
It was found that abrupt changes in the size of a channel that are 
repeated at short intervals are often alone responsible for the high 
values of n obtained. 
Figures 1, 3, 5, 7, 9, and 11 show the average cross sections for all 
of the channels. From these a general idea can be obtained as to 
the size and shape of the various channels. 
SIDE SLOPES AND BOTTOM. 
Irregularities in the perimeter of the channel are discussed under 
the two headings, side slopes and bottom, for the reason that the 
bottom alone is often the chief retarding factor during low stages. 
Irregularities in the perimeters of channels have a very decided effect 
upon the value of n. For instance, irregularities in the side slopes 
of the South Forked Deer River near Jackson, Tenn., were largely 
responsible for the higher values of n Wisse for ae course than 
were obtained for the same channel near Roberts, Tenn., where the 
side slopes were much more regular (Table 3). The irregularities 
in the channel at Jackson were due to rough finishing work with the 
dredge at the time of construction. To smoothness of perimeter 
may be attributed in part the low values of nm obtained for the 
Allen and Willow Creek channels in western Iowa (Table 4). 
SOIL. 
Under this heading descriptions of the different types of soils found 
in the various channels are given. However, no noticeable difference 
in the retarding effect of the different soils was detected. All of the 
soils are of an alluvial nature, the bottoms through which the ditches 
were dug having been built up by deposits of silt brought down from 
the hills during heavy rains. 
CONDITION. 
An effort has been made to describe accurately the condition of the 
channels at the time the measurements were made. Mention is made 
of all sorts of growth, obstructions, and débris—factors which are in 
many instances responsible for the high values of n obtained. The 
pronounced effect of vegetation, such as grass, in a channel is clearly 
shown by the experiments made on three of the channels in North 
Carolina (Table 5), of the effect of small growth, such as weeds, 
bushes, and saplings by the experiments on Old Town Creek (Table 
1), and of the effect of large obstructions, such as logs, fallen trees, 
and débris by the experiments on the South Forked Deer River at 
Campbell’s levee, near Jackson, Tenn. (Table 3). Whether or not 
much erosion has taken place in the channel is also mentioned under 
this heading. 7 
a 
