SURFACE CONDITIONS AND STREAM FLOW 



A delayed publication of the Forest Serv- 

 ice has recently appeared — Circular 176 on 

 Surface Conditions and Stream Flow, by 

 William L. Hall, assistant forester, and 

 Hu Maxwell, expert. This report was ready 

 for publication when the chief of the 

 Weather Bureau, Mr. Moore, brought out 

 as a congressional document through the 

 House Committee on Agriculture the re- 

 markable report which was so much dis- 

 cussed at the time bearing on the same 

 subject. Both reports had been held up 

 by the Secretary of Agriculture until the 

 position of the Department could be deter- 

 mined. Now the work of Mr. Hall and Mr. 

 Maxwell is given publicity through the 

 regular department channels. 



The circular notes the popular belief that 

 floods in many of our rivers are increas- 

 ing in frequency and duration, the interest 

 taken by scientific bureaus of the gov- 

 ernment in the question, and says that both 

 the Forest Service and the Geological Sur- 

 vey have secured data warranting the 

 statement that "unmistakably, floods are 

 steadily on the increase in some of our 

 most important rivers," particularly those 

 rising in the eastern mountains and where 

 surface conditions of the watersheds have 

 been most changed. Specific mention is 

 made of such streams as the Ohio, Cumber- 

 land, Wateree, and Santee, where the for- 

 est has been destroyed. Streams on which 

 floods have decreased and low waters im- 

 proved, show conditions which also seem to 

 prove the rule "that there is the closest 

 kind of relationship between the surface 

 conditions of a watershed and the flow of 

 water through the stream which drains it." 

 A table follows giving the records of the 

 Potomac, Monongahela. Ohio, Cumberland, 

 Wateree. Savannah, Tennessee and Alle- 

 gheny rivers for periods ranging from 

 eight to seventeen years, showing that 

 the high water and low water stages have 

 been intensified in these streams without 

 a corresponding change in precipitation. 

 Figures for the Wabash and Red rivers 

 show that floods have decreased during 

 periods of eight and nine years. After a 

 discussion of these tabulated figures, the 

 circular notes that — 



"While the record is given for only eight 

 streams which show increased floods, it must 

 not be understood that these are the only 

 streams which show this tendency; rather, 

 they are examples — particularly they are 

 examples of streams having their source in 



the Appalachian Mountains. They have 

 been given because their records are more 

 complete and longer than those for other 

 streams. The records of most other streams 

 of the region, so far as they are sufficient 

 to show a tendency, indicate the same 

 progressive change toward increased floods. 

 For example, this is true of the Alabama 

 River, of the Connecticut, of the Muskin- 

 gum, and of the Congaree. ' 



Possible causes for the conditions shown 

 by these records are then discussed. The 

 factors affecting streamflow, climate, topog- 

 raphy, natural or artificial reservoirs, soil 

 and ground cover, are each considered, clim- 

 ate being understood as embracing precipi- 

 tation and evaporation with whatever in- 

 fluences them, as wind, humidity, tempera- 

 ture, and altitude. 



It is observed that precipitation, which 

 must modify stream flow in a vital way, 

 has not increased over the Appalachian 

 mountain region in any degree correspond- 

 ing to the high water figures, which "show 

 that on only two of the rivers mentioned 

 in the table has there been any increase 

 and there by small amount while in the 

 other six there has been a decrease, in the 

 case of one stream, the Tennessee, amount- 

 ing to an average of six inches. The real 

 tendency of the precipitation over these 

 basins would therefore be to decrease flood 

 conditions. Consequently we must con- 

 clude that the increase in flood conditions 

 are in no degree due to precipitation." 



Since more than halt of the precipitation 

 goes off through evaporation, this is an 

 important factor in the problem and it is 

 fundamentally influenced by temperature, 

 air pressure, wind and humidity. 



"It is during the prolonged heated pe- 

 riods, which are often dry periods, that 

 evaporation has greatest influence. It un- 

 doubtedly affects as much as any other fac- 

 tor the low water stages of the streams, 

 yet by its very nature it cannot much in- 

 fluence floods, because they rise quickly 

 and are never of many days duration. 

 There is not time for much water to evap- 

 orate, hence we can put aside this climatic 

 Influence just as in this case we have put 

 aside precipitation. We must look else- 

 where for the cause." 



"Temperature, while influencing stream 

 flow indirectly, through evaporation, also 

 influences it directly. For example the 

 upper watershed of the Monongahela River, 

 in West Virginia, may be considered: 



371 



