INTRODUCTION. XIX 



The Thames is liable to heavy floods, which spread over the valley 

 on either side. ' The flood waters are sometimes out in winter for 

 weeks together. In summer, when extreme floods occasionally occur, 

 they are most injurious, spoiling the crops, and sometimes even 

 sweeping them bodily away.'* In extreme floods at Kingston the 

 water rises as much as 11 feet 10 inches above summer level, and 

 it very frequently rises from 4 to 6 feet above summer level.t 

 After heavy rains the flood water at Kingston is first felt from the 

 Mole, which drains the eastern half of Surrey. The up-country flood 

 is not felt till a little time afterwards. J 



To the winter floods may perhaps be attributed the introduction of 

 plants elsewhere characteristic of a calcareous soil into the parts of 

 Middlesex adjoining the Thames from the chalk districts to the west; 

 for example : — 



Cerastium arvense. Campanula glomerata. 



Spiraea Filipendula. Salvia Verbenaca. 



Scabiosa Columbaria. Calamintha Acinos. 



Picris hieracioides. Nepeta Cataria. 



Carduus acaulis. Avena pube&cens. 



Scilla autumnalis and Allium oleraceum are also confined to the Thames 

 bank, to which their bulbs may possibly have been carried by the stream. 



The depth of water in the river at summer level is very variable in 

 different places from the irregularity of the bed. According to one of 

 the witnesses examined before the Rivers Commission, ' it is all in hills 

 and holes. '§ The depth in the Ivingston reach varies from eighteen 

 inches to four or five feet ; or, where ballast has been dredged, to eight 

 or nine feet. As the water becomes shallow, weeds become more 

 abundant. Below Teddington the level of low water has been much 

 depressed by the removal of Old London Bridge, and the deepening of 

 the channel below Richmond. This allows the water to flow ofl" with 

 greater rapidity, leaving the upper part of the course comparatively 

 dry. The level of low water at Richmond Bridge has been shown 

 to have sunk three feet since its construction. || 



The principal tributaries of the Thames between Staines and 

 Blackwall, or that part of its course which bounds Middlesex on the 

 right or Surrey side, are the Wey, JMole, Hog's Mill River, and Wandle. 



The Wey drains the west of Surrey, and joins the main river at 

 Wey bridge ^ ; while the Mole, which joins it at East Molesey, drains 



* Report of Rivers Commission, p. 28. f Eindenre, p. 178. % Ibid. p. 184. 



§ Evidence, p. 18o. II Repo7-t of Royal Commission, p. 29. 



^ The naturaliseii Impaliensfulva has been carried from mid Surrey bj' tlie Wey into 

 the Thames, and theuce iu;o its Middlesex tributaries (see p. 71). It thus affords a 



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