326 HORTICULTURE, FORESTRY, FLORICULTURE 



underlying rocks, and the general climate, as well as the forest itself, 

 that we shall probably never be able to measure quantitatively the 

 influence of forests on the flow of streams by the comparison of 

 forested and nonforested regions. Catchment areas differ so greatly 

 in the features mentioned above that our most conservative and 

 able investigators have been forced to the conclusion that "in respect 

 to run-off, each stream is a law unto itself." Although the above is 

 probably in the main true, yet, by the careful selection of small 

 catchment basins for comparison, it appears that the influence of 

 the forest in diminishing the surface run-off can be determined with 

 a fair degree of accuracy. When the catchment areas compared are 

 in the same region, are influenced by the same or nearly the same 

 climate and precipitation and by the same storms, have approxi- 

 mately the same configuration and area, and have a similar mineral 

 soil and underlying rocks, the effect of these various factors on the 

 run-off can be ignored, and the differences in the behavior of the 

 stream flow on the forested and nonforested areas can be assigned 

 to the influence of the forest. 



General Discussion. When a farmer in Ohio or Indiana has 

 a piece of woods on fairly level ground he has no good reason for 

 keeping it as woods unless he considers the wood growing on the 

 land to be as valuable as any other crop he might raise a matter 

 often difficult to decide. A farmer in Mississippi, on the other hand, 

 may be induced to leave a patch of forest on a hillside, not because 

 he cares much for the wood, but because, if the forest is cut away, 

 the land will wash into a labyrinth of deep gullies and soon become 

 utterly worthless for any purpose. In the former case the forest is 

 merely for the crop it yields ; in the latter it is for both crop and 

 protection, and this case is far more common than is usually sup- 

 posed. But while the farmer in Mississippi may use the forest to 

 keep a piece of land from gullying, and thus use it as a protection 

 against erosion, he cares little as to how this forest affects the flow 

 of water or the climate, for he has ample rain and does not utilize 

 the creek or stream. With the farmer in Gallatin Valley, Montana, 

 this is quite different. He takes what seems to be a desert gravel 

 bar and by the use of 1 inch (miner's) of water per acre he converts 

 this arid ground into a farm and raises as high as 90 bushels of a 

 superior quality of oats per acre. To him the little mountain stream 

 is everything. Here the forest takes on another function; it holds 

 the soil of the neighboring mountains and keeps it more pervious, 

 and thus it regulates the flow in these important streams. The man- 

 ner in which it does this will be clearer from the following: Sup- 

 pose we take a table and tilt it several inches, so that its top rep- 

 resents a slanting surface. If we sprinkle water on this surface, it 

 is clear that the water runs off about as fast as it strikes the table. 

 If the table is now covered with a layer of soil about 3 inches thick, 

 and the sprinkling is renewed, some of the water runs off from the 

 surface and some soaks into the layer of soil, so that if, after a time, 

 we quit sprinkling there will still be water running off from the 

 table for hours. We have here then a "surface run-off" and an "un- 



