DISCUSSION 



In an experiment of this type^ it is difficult to eliminate all 

 influences which might have an effect on the final outcome. Results of 

 the experiment would have been more conclusive, had it been possible to 

 alternate the position of the two ladders. However, the importance of 

 position is minimized to a degree at least by two conditions: (1) the 

 small flow requirement for each of the ladders as compared to the total 

 flow over the dair gave both ladders a relatively equal attraction to 

 salmon approaching from either shore ;, and (2) despite the fact that sal- 

 mon are sometimes known to follow the shorelines in their upstream migra- 

 tion^, our experience at other damSj such as Tumwater Dam further upstream^ 

 indicates that the greatest numbers of fish do not always use the entrance 

 situated nearest the shore. 



Another question which arisen is v:hether or not the pool-type ladder 

 at Dryden Dam was a good example of that type. That is, were the pools of 

 adequate dimensions and was the entrance most suitably located with respect 

 to the downstream face of the dam? Also, might not the pool-type ladder 

 have been more effective had it been possible to provide submerged orifices 

 in the weirs? Short ladder pools ( 6 feet) precluded the use of submerged 

 orifices, //hile the pool-type ladder at Dryden Dam is not suggested as 

 being the best exam.ple of that type ladder, it is a typical example of 

 the ladders constructed at low-head dams on the tributary streams through- 

 out the Columbia River basin. 



It appeared from observations made in connection with the experiment 

 that the foremost advantage of the Denil-type ladder is the attractive 

 entrance condition created by the nozzle-like flow from the ladder entrance. 

 In the case of the pool-type ladder, the effect of the nappe of flow over 

 the entrance weir is quickly dissipated as it plunges into the deeper water 

 below the dam, whereas the effect of the jet of flow from the Denil-type 

 ladder extends downstream a considerable distance from the ladder entrance 

 horizontally along the water surface. This fact, it was concluded, was the 

 chief reason why most of the fish selected the Denil-type over the pool- 

 type ladder. 



In arldition to providing what appears to be a better attraction 

 to fish, the Denil-type proved to be superior with regard to facilitation 

 of fish passage. Salmon and fish of several other species, including 

 squawfis'h, suckers and certain less active fish, appeared to arrive at 

 the upper end of the ladder with little effort. The ease with which 

 fish appeared to negotiate the ladder is attributed to the dissipation of 

 energy by the baffles. Part of this dissipation of energy produces up- 

 ward currents which /jere demonstrated by Furuskog in a model, as shown in 

 Figure 2. 



It was not always possible to see fish entering the two ladders, 

 hence the length of time it took fish to negotiate the two types was 

 not measured. However, there were a few occasions when the water was 



13 



