THE RIVER BURE AND ITS TRIBUTARIES. 
' 221 
entering the river drives back the fresher water previously 
occupying the seaward section of the river, and this effect 
is repeated higher and higher up. There is, therefore, an 
oscillation backwards and forwards of bodies of water, though, 
owing to the ebb lasting longer than the flow, there is a slow 
progress of all the water to the sea. The same water that 
occupies the river at any given point at slack of the ebb 
will be driven upwards and come down again with the next 
ebb to a point a little befow that at which it started. It 
has been calculated that in the Thames at Barking water 
descends the river at the rate of one-third of a mile per day. 
Wheeler (ibid. p. 103) states that sea water runs 15 miles 
up the Bure at normal tides and 19 on high tides. It has, 
however, been abundantly proved both by figures given by 
Mr. Innes and by others in our possession as well as by the 
evidence of the fauna, that in normal circumstances sea water 
cannot be traced beyond Acle Bridge. With exceptionally 
high tides the salt water probably penetrates more than 
19 miles up the river, while in opposite conditions the water 
may be nearly fresh and may contain living fresh-water 
Crustacea as far down the river as the Yacht Quay at 
Yarmouth. On September 15th, 1910, the fauna was that 
of fresh- water right down to the Yacht Quay, and the salinity 
there at low tide only 34 grs. Chlorine per gallon. 
The Tidal Wave. 
As has been said above (p. 218), the tide in a river rises 
quicker than it falls, so that a shorter time elapses between 
low water and high water than between high water and low 
water. This is very well shown by the charts which have 
been taken on the Bure. For instance, at Muckfleet on 
June 7th, 1906 (Fig. 1), the water rose nearly 8 inches in the 
first 45 minutes, or at the rate of about .18 inch per minute, 
while it took /\\ hours to rise the remaining 8 inches of the 
complete tide. The whole period of the rise was, therefore, 
5J: hours. From high water to low water the period was 
about 7 hours. 
