Physical Features 25 



and connected with the river-system at Duke's Lock. The country to the 

 east is drained by minor streams as Bayswater Brook, Northfield Brook, 

 Bullingdon Brook, the channels of which are cut as deep trenches 6-ia ft. 

 deep, and as many wide. These may carry a full stream in flood-time, on 

 impervious clays ; but are dry or dwindle to the merest runnels in extreme 

 dry summers (1921). 



The I sis Lock-system. For purposes of navigation, more particularly 

 by barge, the Thames is held up throughout its course to the sea at 47 locks 

 and weirs, of which 6 fall within the Oxford district (King's Weir, Godstow 

 Lock, Medley Weir, Osney, Iffley, and Sandford Locks). A collateral branch 

 of the Isis, taking off just above King's Weir, and rounding Pixey Mead to 

 Wolvercote Bridge, is uncontrolled, as is also the Hinksey stream taking off 

 at Hagley Pool, and running past Wytham : the latter is connected with an 

 extensive system of ditches in the alluvial area below the town, joining up 

 again with the main river at the Railway Bridge below Iffley. Even after 

 locking, the river, though liable to be blocked and banked up by weeds, 

 also tended to run out in summer, and barges might lie up for 3-4 weeks. 1 

 The lock-system was initiated in the reign of James I, and the mechanism 

 remains much the same to the present day. 2 The first barge, following the 

 completion of the lock at Iffley, reached Oxford in August, 1635. 



The navigable stream is seldom more than loft, deep, except where 

 gravel has been taken out, leaving deep pits (Old Stream, Cold Arbour, 

 and Isis at ' The Gut ') ; a depth of 5 ft. in the channel is sufficient for 

 navigation, a minimum of 3ft. 9 in. being maintained above Iffley. The 

 object of the Lock-system is to maintain the water at each lock at a maximum 

 level (' Head Water'), a datum-line marked empirically for each lock, as the 

 highest level to which the water can be kept without unduly flooding the 

 land above. The distances above sea-level are : 



King's Weir . . . (H.W.) 193-55 ft. miles between 



Godstow .... 191-02 i -io 



Medley .... 187-31 1-48 



Osney .... 185-89 -99 



Iffley i79'72 2-40 



Sandford .... 176-92 1-38. 



The total distance from King's Weir to Sandford is 7^ miles, the fall 

 14-10 ft, and the average gradient 2-25 ft. per mile. Tail-water at the lock- 

 gates also gives empirical data based on the requirements of navigation, and 

 records of such data are taken by the Thames Conservancy three times 

 a day ; so that the Thames is one of the best observed rivers in the 

 country. 



The total rainfall of the Thames basin normally ranges between 

 21-32 inches (1887) and 40-87 (1903), the average of 36 years (1883- 

 1918) being 28-32. The Oxford data (Magdalen Laboratory) give 25-16, 

 and the Radcliffe Observatory 26-02. Of this fall, less than half finds its 

 way into the river-system, or 40-3 % in the winter months, and some 21-5 % 

 during the summer ; the larger proportion being returned to the atmosphere 

 by evaporation and transpiration ; while there remains a certain leakage 

 in the form of subterranean streams which also pass beyond the local area. 

 Or, for 28-3 inches (average) of rain per annum, only 31-8 % (varying 19-44), 



1 Plot (1705) p. 239 (the text being written, first edit., 1675); p. 29, 'Onr Watermen here in 

 these shallow Rivers, praying not so much for Rain to fill them when low, as that \Veeds may also 

 grow to help to keep the Waters when they have them, which will otherwise too soon glide away to 

 their no small detriment.' 



* Plot, loc. cit., p. 238, described in considerable detail. The original 'Lock', with rimers and 

 keys worked by levers, is seen at Medley, with only a slight fall. What is commonly termed a 

 lock', with a double set of gates turned by a beam, was originally a ' turnpike '. 



