20 
Colorado Experiment Station 
vegetation. Consequently, in many respects, a ditch bank 
usually presents an early stage in plant succession, with its 
large number of species. 
SEASONAL RELATIONS 
When water is first turned into the ditches in the spring, 
there is an enormous number of weed seeds and debris of all 
kinds carried into the stream. During the irrigation season 
many weed seeds sink in transit and become lodged in the 
mud, and during the non-irrigation season there is a heavy 
accumulation of seeds in the soil of ditch bottoms and sides 
which are blown there by the wind. The first flush of water 
in the spring washes these seeds down with the current. 
The greatest number of seeds comes with the first flush 
of water in the spring — reference is here made to samples col- 
lected in May and June. This being true, it would be advisable, 
when practicable, to avoid turning water into the field laterals 
and rows, until considerable water has been run through the 
ditches. 
The seed-shedding period of some weeds is readily de- 
tected, of course, in the collections from the seed traps. This 
is particularly noticeable in the case of weeds which shed their 
seeds quite completely within a short period, such as the 
dandelion. 
Following the first two or three weeks of flow in the spring 
there is a decrease in the amount of seeds and debris carried 
by the ditches. Reference is made to samples collected in 
July and August. 
The weed seeds carried in irrigation streams are by no 
means only from weeds which border them. Seeds are carried 
long distances by the wind, fall in the water, and are conveyed 
by the current. The prevailing direction of the wind and the 
relative position of streams and weed infested areas, as well 
as the velocity of the stream, are factors which affect the 
number of seeds carried. 
Seeds are carried long distances by streams. The distance 
any particular kind of seed will travel depends upon the ve- 
locity of the water, the character of the water surface, the 
specific gravity of the seed, and character of the seed coat. 
Some seeds, such as Indian mustard (Brassica juncea)^ Har'e’s- 
