migration blockage was present at sites, including Oregon Creek and 

 Washington Creek, where the entire active channel was scraped. Because of 

 the known complexity of fish movements throughout arctic watersheds, migra- 

 tion blockages can have a significant, but as yet unstudied, effect on 

 popu I at i ons. 



Potential Entrapment Areas 



The potential for fish entrapment was high at areas with extensive 

 backwater, as was found at newly inundated areas (Dietrich River-Downstream, 

 Middle Fork Koyukuk R i ver-Ups tream) and areas with increased braiding (many 

 sites, including Sinuk River, Kuparuk River, Sagavan i rktok River, Ivishak 

 River, and Middle Fork Koyukuk River-Downstream). At these sites, areas of 

 ponded water became isolated from the active channel as the water level 

 dropped, trapping fish and invertebrates that had moved or been carried into 

 these depressions during the high water. Mortality of stranded fish and 

 invertebrates is assumed to be high because they are subjected to high 

 summer water temperatures, low dissolved oxygen, increased predation from 

 terrestrial predators, winter freezing, and total loss of aquatic habi- 

 tat as the isolated pools often dry up if the river continues to drop. 



EFFECTS OF INUNDATED PIT FORMATION ON THE ASSOCIATED RIVER BIOTA 



The direct effects of pit excavation on the river biota were difficult 

 to assess because the river habitat was not directly affected; inundated 

 pits were created from previously terrestrial habitat. Because of this, the 

 pits represented a new habitat and the fauna inhabiting the pits was con- 

 siderably different from that inhabiting the associated river. 



Summer Utilization by F i sh 



Two of the pits, Dietrich Ri ver-Upstream and Tanana River-Downstream, 

 were deep clear water pits with low productivity and fish utilization. At 

 Tanana River-Downstream this low utilization was easily explained because 

 there was no connection to the river and immigration into the pit occurred 



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