OF THE EAST NORFOLK RIVERS. 4II 
cultics are due to the fact that it is impossible to distinguish 
localities as fresh- or brackish-water without regard to the 
state of the tide.^since, at 'any given spot one may find the 
water fresh at one time and salt at another. Also, though 
one can distinguish certain species which are decidedly fresh- 
water or marine, others are difficult to place, and also it is 
often impossible to say whether a species taken in water of 
a certain salinity is really capable of prolonged exposure to 
such a medium or is only submitting to it for a time. 
The following remarks deal chiefly with the River Bure 
and its tributaries the Ant and Thurne River, but observations 
made and species found on the Yare and Waveney will also 
be mentioned. I intend also to mention one or two species 
which are not Crustacea. 
The three rivers Bure, Yare and Waveney have a common 
opening to the sea, and have a general similarity in their 
slight fall and velocity and in their tidal nature. In the Yare 
and Waveney thejdeepcr channel, and the absence of con- 
siderable Broads, are probably the reasons why the tide is 
greater in these two ^rivers 'than it is in the Bure. In the 
Yare the tidal oscillation is very considerable even at Norwich, 
and in the Waveney there is a rise and fall in Oulton Broad 
of about 18 inches. In the Bure, on the other hand, the 
oscillation diminishes very rapidly above Acle, and is quite 
insignificant above Horning. 
There is, however, a very necessary distinction to be made 
between tidal oscillation and the range of salt water. The 
range of salt water up the Yare and Bure, in spite of the 
difference in the tidal oscillation, is very much the same. 
That is to say, in ordinary circumstances, I believe that salt 
water cannot be traced further up the Yare than Cantley — 
about 12 miles from Yarmouth. In the Waveney it certainly 
reaches as far as Oulton Broad, but probably not much 
further. In the Bure, in normal circumstances, there is no 
“ Chemical Tide,” if I may call it so, further than Acle Bridge, 
which is also about 12 miles from Yarmouth. Even at the 
period of Spring tides, and those among the highest of the 
year (July 4-ioth, 1906), there was no difference appreciable 
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