What is the 20-year flood potential of this watershed? 



INTRODUCTION 



Foresters and other land managers have long considered water an important product 

 of forest lands. This product yields great benefits when its flow is orderly, timely, 

 and is confined within the banks of a stream. On the other hand, water appearing as a 

 flood is a potentially destructive force that land managers should consider when plan- 

 ning future forest operations. 



The effects of clearcutting operations on the hydrologic regime of a watershed are 

 complex, and the magnitude of these effects is highly variable. Nevertheless, we do 

 know that a clearcut watershed yields more water (Hibbert 1967) , and that the pattern 

 of runoff distribution is changed, including an increase in base flow and peak flow 

 (Bethlahmy 1971). Thus, foresters and other land managers who contemplate clearcutting 

 operations should consider the associated problem of future changes in the hydrologic 

 regime. 



Our research has shown that on a unit area basis the long-term average water yield 

 of a watershed is a good index to the magnitude of its expected peak flow. The greater 

 the average yield per unit area, the greater the expected peak flow for period of record. 

 This relation is readily perceived from table 1 that shows data for 11 rivers in Idaho 

 which have good or excellent streamflow records (U.S. Department of Interior 1964). 

 Notice the regular decrease in values of both average annual flow and associated peak 

 flow. Although peak flow per unit area is related to unit drainage yield, any discus- 

 sion of peak flow should include the element of chance. A record of many years of data 

 will probably include- greater peak flow values than a short record. Hence, a comparison 

 of peak flows for different areas should be based not only on the watershed's average 

 yields but also on the element of time; how long is the record, and how often can one 

 expect a peak flow of a given magnitude? Such considerations are especially important 

 to engineers and economists who are concerned with investments in structures having an 

 economic life expectancy. 



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