372 



AMERICAN FORESTRY 



It is February, and there have been sev- 

 eral weeks of cold weather; the ground, 

 which is bare of snow, is frozen solid. 

 There comes a heavy fall of snow, or per- 

 haps several, bringing the snow cover to 

 a depth of 12 to 18 inches. Within a few 

 days the weather moderates and a pro- 

 longed rainy period follows. The first rain 

 is caught by the snow and little escapes. 

 But as it continues to come, it melts the 

 snow so that the water from the snowfall 

 and the rainfall is concentrated upon the 

 surface at one time. If the ground were 

 not frozen, much of it could be absorbed 

 and distributed downward, but in the case 

 we have assumed the earth is frozen solid, 

 so that no water, or at least but little, can 

 be absorbed. The whole volume is liberat- 

 ed on the surface and races away from 

 the smaller streams to the larger, and at 

 last, gathering in immense quantity, it 

 overtaxes the channel of the main river 

 and inundates the entire valley. It is this 

 condition of rain, snow, and temperature 

 operating together w'hich brings about 

 some of the greatest floods. A flood result- 

 ing from such a cause is not likely to occur 

 more than once a year in any stream, and 

 several years may pass without flood on a 

 given watershed from this conbination of 

 causes. It can not be said that floods from 

 this cause show any progressive change, 

 and consequently we can not ascribe to this 

 cause the increased flood conditions in the 

 rivers now under consideration. 



TOPOGRAPHY AND GEOLOGY 



A region's topography largely influences 

 its run-off. Steep slopes and sharp ridges 

 shed water quickly. A flat surface causes 

 it to flow away more slowly. 



Likewise, geology has a great deal to do 

 with the drainage of anj^ region. When 

 rain has fallen and has passed through the 

 soil cover and the soil it comes in contact 

 v/ith the rocks which lie beneath. In obe- 

 dience to gravity it will penetrate as deep- 

 ly as possible. Considering the depth to 

 which it can go and the quantity in which 

 it can be stored, the earth forms a great 

 storage reservoir which tends on most 

 watersheds to have a strong influence to- 

 ward steadiness in the flow of streams. 

 While topography and geology are impor- 

 tant factors in considering the amount and 

 character of run-off when one stream is 

 compared with another, they are of no con- 

 sequence when one is considering the con- 

 dition in the same stream during two dif- 

 ferent periods, because they are subject to 

 no appreciable variation. Both the topog- 

 raphy and the geology of any given stream 

 may be said to be constant, and so are not 

 of importance in considering what has 

 caused a changed condition of flow in any 

 particular stream." 



The effect of natural and artificial reser- 

 voirs, which gather and hold immense vol- 

 umes of water, such as are found on some 



river systems, notably the Merrimac in 

 New England, is noted. And on this point 

 the circular says: 



"If at the headwaters of the rivers under 

 consideration in this paper there had been 

 great lakes or swamps which had been 

 drained during the period under considera- 

 tion, it might well be said that the influ- 

 ence of that action would be sufficient to 

 cause a difference in the run-off of the 

 streams. But, except in the case of the 

 Wabash, there have been no changes of this 

 character. There are on these streams no 

 important natural lakes, and no reservoirs 

 of great extent have been developed. Con- 

 sequently, this factor also is insignificant." 



Soil and ground cover are next discussed 

 as follows: "Though the topography and 

 geology of a watershed change too slowly 

 to be readily observed, there are other fac- 

 tors in which changes may be rapid, radi- 

 cal, and of great importance. One such 

 factor is soil. The soil, when considered as 

 a factor in controlling run-off, is a com- 

 plicated and delicate apparatus. It works 

 admirably when in good condition, but it 

 is easily deranged and is liable to severe 

 and fundamental injury, by which its ac- 

 tion on storm water may be lessened or 

 almost entirely destroyed. Mistreatment 

 frequently produces results of a serious 

 nature in the drainage of a watershed. 



"The mineral soil is composed of disin- 

 tegrating rock that has broken down by 

 the slow process of weathering. Deep min- 

 eral soils, if they have not accumulated 

 from transported materials, represent a 

 long period of rest, during which the sur- 

 lace has been free from erosion, while the 

 disintegrating rock beneath has added lit- 

 tle by little to the depth. Upon the min- 

 eral base, and more or less mixed with it, 

 is the humus, which owes its existence to 

 organic matter mostly vegetable, which has 

 decayed and added the accumulated rem- 

 nants to the mineral soil. Humus vastly 

 increases a soil's capacity to absorb and 

 store water. 



"The important work which soil performs 

 in regulating the run-off for a region is 

 easily understood. If the soil is present 

 in sufficient quantity and good condition, 

 that is, is porous and well supplied with 

 humus, it readily absorbs storm water at 

 the surface and passes it on downward 

 through the underlying rocks and strata. 

 Although these rocks themselves change 

 l-.ut little, their storage of water is regu- 

 lated by the soil and varies with its 

 changes. If the soil is hardened or re- 

 moved, the amount of water which can be 

 taken in is correspondingly reduced and 

 the amount which is thrown off over the 

 t^urface into the streams correspondingly 

 increased. A watershed without soil or 

 with a soil which does not readily perform 

 its normal functions, results in erratic 

 streams, which are usually agents of de- 

 struction rather than of use. The consid- 



