34 



a valle by several mouths instead of by a single large 

 one. It is at the inner or valle mouth of a covola that 

 the tressa palisading is erected. Usually it takes the 

 form of a large rectangular enclosure projecting well out 

 into the water of the valle. When constructed at the end 

 of the stocking season, the palisading is complete on all 

 sides except that facing the covola. A simpler form 

 sometimes used is where the tressa has a V-shaped form, 

 being constructed of two palisades built out from the 

 mouth of a covola and inclined inwards or towards one 

 another so that they eventually meet in an acute angle. 



OH T.mnii-rrTTTTTimill.imP 



COVOLA 



QUADRANGLE 



OF 



TRESSA 



Pt'it.ii i i i n li.l nil llllT 



Fig. 5. — Plan of a true tressa. 



PSEUOO TRESSA 



VALLE 



Fig. 6. — Flan of a pseudo tressa. 

 Earthwork shown by dotted shading. 



Both these forms are designed to afford a very large 

 surface for percolation, just as the screens used in fresh 



