THE NAUTILUS. 



67 



The snails which were present were found only under fair-sized 

 rocks where they were well protected; they did not occur out in 

 the open stream as had been the case when the creek was more 

 densely populated with snails. 



In April 1915, on the 22d and 24th, Waller Creek was swept 

 by three scouring floods which devastated the entire bottoms. 

 The water rose higher during the night than it had at any 

 time since 1886; bridges were washed away and much damage 

 done. The bottom of the creek was again washed clean except 

 under the larger stones and in deep holes in the bed. Snails 

 were not observed in any numbers following this flood during 

 the rest of that year. Early in January 1916, however, Physa 

 had again appeared fairly abundantly where they had formerly 

 been very thick, in the region just above the University; later 

 in the spring they became quite numerous here. In January 

 of this year the first Planorbis were found that had been seen 

 in the creek since the autumn floods of 1913. Between Fifteenth 

 and Sixteenth Streets in a pocket containing good-sized stones 

 over which the water flows rather swiftly a number of speci- 

 mens were taken, although none were found above or below 

 this locality. It is noted that below this region the creek is 

 frequently covered with oily scum and that it receives the refuse 

 from the adjoining properties. Except after high water which 

 would clean it out, the creek in this region would hardly be 

 expected to support much snail life. It must be supposed either 

 that the Planorbis had made their way up to Sixteenth Street 

 during the short time following the floods before the water be- 

 came badly contaminated, or else that in this particular place a 

 few specimens from the previous years had withstood the floods 

 and reproduced themselves in sufficient numbers to be notice- 

 able in January. Of these two suppositions the latter seems 

 much the more rational. 



After the flood of 1915 the water subsided very quickly so 

 that a new layer of humus and algae was not deposited, but 

 that the bed was again restored to its former condition of a clean 

 smooth rock bottom. Upon the return to this condition the 

 snail population increased very rapidly, and at the time when 

 the last observation was made in the early summer of 1916, 

 seemed in a fair way to return to the condition of 1912. 



