Physical Features 27 



by the grooves in the contour-lines of the map, but are now running 

 brooks of no great size, apt to dry up in hot summers, or left as mere 

 runnels and ditches along the sides of fields, uniting to form such streams 

 as the Bullingdon Brook, Bayswater Brook (Cherwell), Hinksey streams, 

 Northfield Brook (Isis), Baldon Brook, Holton Brook (Thame), which may 

 also run in considerable volume in times of winter flood, but in summer 

 months are almost negligible. 



Flood : One of the most spectacular features of the Oxford Valley 

 is its tendency to flood at any time in the winter months. Both the 

 Thames and the Cherwell bring water from beyond the district ; the winter 

 delivery of the Thames is several times that of the summer months, thus 

 filling the channel. When wastage of evaporation and transpiration of 

 vegetation is checked, and the average monthly rainfall increased, water 

 tends to accumulate, the more as the predominant clays are impervious ; 

 and on the alluvial flats the water runs off with difficulty. The control 

 of the main stream of the Isis for purposes of navigation pinches the stream 

 at locks and weirs, older water-courses may become filled up, and water 

 may be intentionally held up in the upper reaches of the river in order 

 to reduce the excessive flooding of tracts farther down stream. Though 

 filling the alluvial basin in older times, improved control of the stream 

 by deepening the channel and the regulation of weirs has considerably 

 lessened both the extent and the duration of the flood, and further diminu- 

 tion is probable ; but when Port Meadow, Osney Fields, Iffley Fields, and 

 the Cherwell Meadows are under water, the aspect of the country returns 

 to its primary winter condition, with hamlets spaced along the flood-margin 

 and the town on its gravel bank, appearing from the surrounding hills as 

 more or less isolated in a broad lake. 



Above the locks flooding follows as soon as the stream runs much over 

 Head Water, and below the locks where the land is exposed to the delivery 

 of uncontrolled streams. The meadows below Iffley flood from ' The Weirs * 

 and the Hinksey Stream ; Osney fields from the Wytham stream, and Port 

 Meadow from Medley and the mill-stream above Godstow. Medley Weir is 

 wholly drawn in winter flood : Osney and Sandford Weirs were reconstructed 

 1884 : Iffley Weir is in process of reconstruction (1922). 



The last great flood (Nov. 1894) was the expression of excessive autumnal 



rainfall. Floods at the melting of the snow in spring are less usual (Feb. 1 900). 



More rarely excessive rainfall may produce floods in May and June (1903). 



Medley Weir was wholly drawn, and meadows of the Upper Cherwell under 



water in August 1922. The record flood of 1894, at Iffley Lock, rose to 



a maximum of 2 ft. 9 in. above Head Water, or 9 in. over the lock-edge and 



towpath. Tail-water 9 ft. implies up to 3 ft. over the lower meadows for a wide area. 



Ecologically the flood, as falling within the period of winter-peren- 



nation, is now of minor importance, and tends to be less the more it is 



subject to control. A rushing torrent of water scours the bottom and sides 



of the stream, washing out rhizomes of Nuphar, Acortts, Sparganium, etc. ; 



these with quantities of stems, fruits and seeds of aquatics and subaquatics 



may be left as the water recedes as a conspicuous debris-line, extending 



in special cases (as above the locks) to the margin of the old alluvium. 



Spring floods may do considerable damage to crops in allotment-areas 



(Osney), and to the hay-harvest (June, 1903) ; the water rising after the 



grass was cut, and the crop left to blacken on the ground (Iffley fields). 



In Aug. 1 92 2 the field below the lock was cut with 2 inches of water standing 



in the bottom grass. The relation of the pasture flora to flood -distribution 



(e. g. Rumex, sp.) is open to investigation, as also the amount of silt in the 



flood-water as determining the fertility of the flood-meadows : but the 



general tendency is to eliminate all flooding as much as possible. 



