STABLE AND MIGRATORY PLANT FORMATIONS 1 37 



In their lower courses rivers form wide alluvial plains of 

 loams or silts, subject to flooding at seasons depending upon 

 peculiarities in the environment of the river basins. Where 

 the rivers become icebound in winter, the flooding in spring 

 is accompanied by ice rafts, effectually preventing tree 

 growth (13). Or the flooding may be due to the melting of 

 the snows in early summer, or to autumnal rains. Unlike 

 the subalpine alluvial terraces, the lowland flood plains 

 remain subject to flooding for comparatively long periods, 

 and owing to the sluggish nature of the widespread current 

 are apt to accumulate silt over those parts nearest the river 

 channel These consequently become raised as natural em- 

 bankments, hindering drainage from the lower marginal 

 parts of the alluvial flats, which form swamps or marshes. 

 The rivers also cut laterally and form oxbow windings, 

 which, in course of time, often become deserted by the river 

 waters, owing to short-circuiting of the main stream by 

 further erosion. The deserted oxbows then often form 

 natural ponds, and where beyond the sphere of the river 

 floods, become filled up only through the growth and decay 

 of vegetation and accumulation of humus. 



Unfortunately for British plant ecologists, the natural 

 vegetation of the alluvial flood plains of our own rivers has 

 been largely replaced by cultivation and artificially treated 

 pasture. Our rivers are tamed and managed and only occa- 

 sionally get beyond control. The banks are artificially 

 raised, the oxbow windings are replaced by cut canals, and 

 the formerly flooded tracts have been drained by pumping. 

 The natural head and rise and fall of the waters has also 

 been altered by drainage at their sources, and erosion is 

 minimised by breakwaters, locks, floodgates, and mill-dams. 



Lately formed alluvial deposits generally have a greater 

 surface soil fertility than the areas occupied by stable forma- 

 tions. This is due to their filtering action on the soluble 

 salts or silts supplied b}' the river waters. The climatic 

 effects of leaching of the surface layers of the soil, which is 

 so marked a factor in our stable formations, is of little con- 

 sequence in alluvial soils until they have been for some time 

 deserted by the river waters. This surface soil fertility 

 favours surface rooting species, and especially grasses, the 



