percent greater than the estimated $165 millicn. From the infor- 
mation available it. was not possible to present downstream losses 
by types of damage or geographic aress. 
In the upstream areas damage to crcps and pasture constitutes 
over 50 percent of the tetal, and other agricultural damage about 
20 percent. Flood-plain damage by scour, streambank erosion, 
gullying and valley trenching, infertile overwash and swamping, 
though not so important monetary-wise, are importent from the 
standpoint of our agricultural resources, At lesst 5 nercerit. 
of our total agricultural land lies in the alluvial ficod olains 
of tributary valleys. In general, this lanc is the most produc- 
tive that we have. If given flood pretection it will remain 
productive for a long time. Some land damage is temporary, as 
full preductivity cen be restored within a few years. On other 
land the damage is more nearly permanent, and productivity is 
impaired for future generations. Deposition of sediment may 
actually be beneficial to some bottcomlands. However, this bene- 
ficial effect is slight in upstream areas, 
Sediment damage occurs more often in cultivated farmland, es= 
pecially in geographic areas 2 and 3 (Table 3h), Lands under 
cultivation are usually most subject to washing and to other 
forces of erosion. Ercsion contributes large amounts of sediment 
to the headwater tributaries and conseqently to downstream areas. 
Such sediment fills channels, is deposited on the highly produce 
tive flood plains, anc is carried into major reservoirs. Damages 
from sedimentation terd to be cumlative. Sediment that is 
brought down from the top of the hills into tributarv and major 
valleys will cause damage in future years through decreasing the 
channel capacities, gradual destruction of the productive bottom 
lance, and swamping of the bottomland with consequerit raising of 
ground-water levels, 
In most upstream tributary valleys, a large share of the total 
loss from flocds is caused by short intensive storms which cover 
relatively smell areas, Such storms occur far more often than 
do those of longer duration over large areas, which are required 
to produce a main stem flood. Because of the thousands of head- 
water streams that drain our vast agricultural regions, the 
damage caused each year by many frequent floods is enormous, 
At 1951 prices, it is estimated that the annual agricultural 
and nonagricultural floodwater damage in the major river valleys 
is about $500 million. b/ At 191:2—51 prices the figure would 
be about $410 million, of which $165 million is agricultural 
damage. In addition, annuel downstream sediment damage will 
arvroximate $28 to $30 million. These sediment damages, unlike 
those in the headwaters of streams, are predominately nonagricul- 
tural, 
l/ Pick, Lewis A. 1952. Flecd control. Military Eng. bh(301): 323. 
3 IGS 
