45 



proved them to possess the greatest efficacy in protecting 

 the fish from the harmful influence of winter storms. 

 These refuges are always dug upon the windward side 

 of a valle. 



Apart from the intermittent factors which from time 

 to time work havoc on a grand scale there appear to be 

 a number of causes at work tending towards a gradual 

 deterioration of the lao-oon as a fish-rearino- establish- 

 ment. Fifteen centuries and more have elapsed since 

 man began to build dykes to limit the free access of the 

 sea to this great river backwater and to hold up water in 

 the marshes of this delta. The passage of a period of 

 this length, short though it be in geological history, is 

 long enough to produce effects of importance under the 

 conditions found at Comacchio. Deltaic accumulations 

 may be slow in certain cases, but the change of estuarine 

 swamps into low alluvial land is none the less inevitable. 

 Thus we find that the depth of water in the inner 

 sections of the lagoon, those margining the western shore, 

 is gradually becoming less, as silt brought down during 

 rains by the channels that drain the adjoining land, 

 gradually accumulates upon the bottom. As a conse- 

 quence the water capacity of these sections becomes 

 reduced and when an unusually hot and dry summer 

 supervenes, the consequent loss of water due to evapora- 

 tion entails much more serious results than in former 

 times — with a reduced volume of water at the beginning 

 of the season, evaporation produces a salinity much 

 higher than it would, were the orio-inal volume o-realer. 

 So far has this silting of portions of the landward 

 sections proceeded, that extensive reclamations have 

 been made. Valle Gallare and Valle Voile, two of the 

 largest valli after Mezzano, have been wholly reclaimed 

 since Coste's visit in the middle of last century and 

 extensive sections of the Mezzano and of Fossadiporto 

 have also been turned into agricultural land within recent 

 years. 



Supplies of fresh water to correct the tendency of the 

 enclosed waters to acquire too great a degree of salinity, 

 are in the main derived from the two rivers borderino- the 

 lagoon, the Reno and the Volano. These unfortunately 

 contribute less water than in former days, partly because 

 increased cultivation in their basins takes up greater 



