sites are listed in Table 10. They range from 0.09 m /s at McManus Creek 

 to 540 m /s at Tanana River-Downstream. 



Flood frequency curves show the expected frequency of occurrence of 

 different magnitude floods at a specific point on a river. The frequency of 

 occurrence is commonly referred to by the recurrence interval of the flood, 

 which is the average number of years between floods of that magnitude. The 

 reciprocal of the recurrence interval is the probability of occurrence of a 

 given magnitude flood in any year. Flood frequency curves were developed for 

 each of the study sites. Discharge values corresponding to selected fre- 

 quencies of occurrence are shown in Table II. 



Changes Due to Gravel Mining 



Hydrologic characteristics are, to a large extent, governed by basin- 

 wide parameters such as climate and geology. Gravel removal operations did 

 not have a significant effect on these characteristics. However, local 

 changes in the ratio between surface flow and subsurface flow occurred at 

 several sites. The local changes were not measured; quantitative ratings 

 shown in Table 12 were assigned based on a subjective evaluation. A local 

 reduction in mean annual flow occurred at the upper Washington Creek and 

 upper Aufeis Creek sites as a result of a loss of surface flow to inter- 

 gravel flow. At Washington Creek, the flow entered the gravel removal area 

 and spread out through loose, uncompacted gravel; a large percentage reduc- 

 tion in surface flow resulted at low flows. This intergravel flow component 

 was sti I I evident in the site 13 years after the site was worked. The rela- 

 tive effect of the loss of surface flow during flood events was likely 

 minimal. At Aufeis Creek, surface flow appeared to cease entirely for a 

 period of 2 years, although continuous surveillance was not available to 

 verify this. Thus, the mean annual flow of Aufeis Creek in this local region 

 was reduced to near zero for 2 years. The effect on flood flows was unknown. 



Two other sites, the upper Oregon Creek and Penny River sites, had 

 a potential for a similar, but not as extensive, decrease of surface flow 

 lost to intergravel flow. No observations or measurements were available 



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