through a pit. This difference depends on the size of the pit and the amount 

 of mixing. A pit could facilitate deposition of suspended and bed load ma- 

 terials if flows are through a pit and velocity is decreased. 



The aquatic biota of pits differ depending on whether there is an 

 opportunity for exchange between the pit and the active channel. Those pits 

 that are separated (e.g., Tanana River-Downstream) or have little potential 

 for exchange (Dietrich R i ver-Upstream) typically are unproductive. The 

 Tanana River-Downstream pit is situated in the middle of an island and is 

 completely surrounded by a broad undisturbed (except for an access road) 

 timbered buffer zone. The likelihood for injection of nutrients and organ- 

 isms into this pit is remote, except during high flows. The aquatic surveys 

 reflected this. The occurrence of a few fish suggests that overflow may 

 occur at irregular intervals. The Dietrich R i ver-Upstream pit, on the other 

 hand, is connected by its outlet to the channel. A spring, exposed during 

 excavation, floods the pit and exits through a channel. The pit system has 

 been used by overwintering fish but the pit itself is relatively unpro- 

 duct i ve. 



All other pits studied were highly productive and the diversity of the 

 fish community was usually increased over that in the river channel. All of 

 these pits were connected to the river channel through either inlets or 

 outlets and thus exchange was possible between the two systems. The still 

 waters in the pit, which are warmer than the river water, provided con- 

 ditions more suitable for primary and secondary productivity. Fish such as 

 Arctic grayling entered presumably to utilize the pit as a feeding area. 

 This situation is particularly good for feeding by fish of younger age 

 classes because of the greater supply of food available and the lack of a 

 current. 



Fish we I I suited to a sti I I water environment, such as northern pike 

 and burbot, also did we I I in some of these pits and, being piscivorous, had 

 an abundance of young age classes of other fish to feed upon as they entered 

 the pits to feed and rear. Northern pike also utilized two of the pits as 

 spawning areas. The potential for the pits to provide a more diversified 



556 



