The above observat i ons indicate that after November the outlet froze, 

 then the side channel of the Tanana River adjacent to the pit started flow- 

 ing through gravel into the upper pit, opened the connection between the two 

 pits and flowed back into the side channel through an intergravel pathway. 

 The raising of the surface of the lower pit appeared to have been caused by 

 overflow on top of the existing ice and snow. Oxygen depletion was a poten- 

 tial problem at the upper pit because of the dense stands of aquatic vege- 

 tation (the March 1978 dissolved oxygen was 3.2 ppm) but these were absent 

 in the lower pit and the dissolved oxygen was consistently higher than that 

 of the upper pit. The net effect was the creation of one and possibly two 

 overwintering areas, depending on the minimum winter oxygen levels at the 

 upper pit. 



Assuming an adequate water depth, the main factor determining the 

 suitability of a pit as an overwintering area is an adequate level of dis- 

 solved oxygen through the winter. A pit with sufficient depth for over- 

 wintering but with an extensive, heavily-vegetated littoral area may ex- 

 perience an anoxic period following the initial snow cover. Barcia and 

 Mathias (1979) found that winterkill in eutrophic prairie lakes was closely 

 correlated to the mean depth of a lake and developed a method to estimate 

 the potential for winterkill based on the initial oxygen storage, rate of 

 oxygen depletion and the mean depth. The critical mean depth for the lakes 

 studied was approximately 2.0-2.5 m. Lakes with an average depth less than 

 2.0 m experienced regular winterkill, lakes 2.0-2.5 m experienced occasional 

 winterkill, and lakes with an average depth greater than 2.5 m generally did 

 not experience winterkill. The indications were that a productive pit with 

 an average depth of less than 2.5 m may have marginal uti I ity as an over- 

 wintering area, especially during years of early heavy snowfall. 



The upper Tanana R i ver-Upstream and West Fork Tolovana River pits had 

 the characteristics to fit this type of pit (Table 14). The 6 m deep area in 

 the latter pit may have provided sufficient volume to maintain a suitable 

 dissolved oxygen level, but both of these pits should be considered marginal 

 overwintering areas. Intergravel flow from the adjoining river, however, 

 adding a continual supply of oxygenated water, could maintain sufficient 



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