12 



Appendices to Twenty-eighth Annual Report 



getting them up is secured — but the best chance for the pass proper is when 

 the river is at the level at which the fish freely entered the pool. There 

 are even cases in which the possibility of planning the course of the pass 

 so as to secure a. well-protected upper end may fairly influence the position 

 of the entrance, but in no case should this imperil the real value of the 

 entrance, or cause it to be moved far from what is considered the best 

 position. An instance of this is seen at the Tummel Falls pas?. 



(2) Level. — From what has just been said, it is apparent that when 

 dealing with waterfalls the question of water level bears a close relation to 

 the question of the selection of the entrance. I consider that it should 

 first be determined in what stale of the river fish freely enter the fall pool; 

 in other words, the level of the fall pool should be marked when fish are 

 running, their natural " lie " ascertained, and the entrance selected as close 

 as possible to the place where they congregate. Tf allowance be then made 

 for the depth of water in the pass when working at its best, the position of 

 the actual floor of the pass at the entrance can at once be ascertained and 

 measured from any selected datum point. An estimate of the line of the 

 pass may then be laid out and the position of the upper end selected. This, 

 in my view, is a much surer method than, as is sometimes done, com- 

 mencing by selecting the upper end and then working downwards. When 

 the two ends of the pass have been approximately fixed, it is well to erect 

 two water-gauges and note the rise and fall with changes of volume in the 

 river. It not infrequently happens that owing to the constriction of the rocks 

 at the fall, the rise and fall of water is by no means the same above and 

 below the obstruction. A knowledge of this is very useful when the 

 details of the pass come to be adjusted. For want of it curious discrep- 

 ancies are apt to occur. 



It may now be instructive to describe a pass in which the two vital 

 considerations of suitable entrance and proper water level have been 

 disregarded — a useless pass. I may select as an instance the Mugiemoss 

 Dyke Pass on the river Don. The dyke is elliptical in shape, and has a 

 steepish slope on the down-stream face. The pass is a single chute 45 feet 

 long in the centre of the dyke. The entrance to the pass is certainly 

 where, under ordinary conditions, fish would naturally find it, but in the 

 Mugiemoss dyke the area between the two arms of the ellipse, projecting 

 down stream on either bank of the river, has been filled up with masonry 

 or concrete, the surface being a flat table-land of smooth cement. Im- 

 mediately below this is a deep pool. The water descending the pass (which 

 it does with undue velocity — the gradient being 1 in 5) flies over the flat 

 area of smooth cement before it reaches the deep water where fish can 

 alone lie. When the water level is best suited for fish ascending the pass, 

 the smooth cement flat has but a thin sheet of fast water, so that fish 

 cannot remain upon it. When the water-level has risen sufficiently to 

 enable fish ascending from the pool to swim over the flat area, the pass has 

 become a raging torrent which no fish can take. As a matter of fact, the 

 fish which manage to negotiate this bad dyke do so in high conditions of 

 water by disregarding the pass altogether, making the ascent over the 

 surface of the weir close to the left bank. To render the present pass of 

 value it is necessary to get a suitable depth of water on the surface of the 

 flat area of cement. This flat area, from the foot of the chute to the edge 

 of the deep water, is about 60 feet. The height of the weir above it is 

 9 feet. There is room, on the surface of the flat area, for two pools of 30 

 feet, which could, without great difficulty, be constructed by building two 

 subsidiary dykes across the flat area from bank to bank. If the lower of 

 the tw T o dykes were 3 feet high and the upper 6 feet high, the result would 

 be a two-pool pass with three rises of 3 feet each. The two subsidiary 

 dykes would each have a short gap and pass, arranged so that the inflow 

 and outflow of water is not in a straight line, and the level of water in the 



