422 XXIII. LEGUMINOSAE 



importance compared with that of the floods. The Indus river, which is fed 

 by the Himalayan snows and receives during its coiu-se through the Punjab 

 plains the drainage of the four other large snow-fed rivers of that province, 

 annually floods a large stretch of country on either bank. The flood season 

 commences about the beginning of May with the melting of the snows and 

 continues, with occasional interruptions, until July and sometimes later, the 

 river thereafter subsiding until it reaches its winter level about the end of 

 October. The extent and duration of the river floods vary greatly from year 

 to year, and this variation has a marked influence on the conditions affecting 

 forest growth. 



The powerful river action causes the formation of new land in the shape 

 of fresh alluvial deposits, and the loss of much existing land through erosion. 

 The new alluvial river deposits (known locally as kadui ground) become 

 densely covered in one or two years with a growth of Tanmrix dioica, and 

 T. Troupii from water-borne seed, and of ku7ih grass {Saccharum. spontaneum). 

 Where the grass is not too dense, seedlings of Acacia arahica soon make their 

 appearance amongst the tamarisk, the seed being dropped by cattle or con- 

 veyed by water : the tamarisk undoubtedly assists the babul to establish 

 itself by protecting it from frost. The level of this new ground is gradually 

 raised in com^se of time by subsequent alluvial deposits along its riverain 

 edge, but as long as the land is subject to annual inundations the babul 

 continues to flourish, other conditions being favourable. If, however, the 

 flood water is shut out or the ground becomes elevated above the reach of 

 ordinary floods the babul commences to languish, and unless there is sufflcient 

 subsoil moistiu-e by percolation from the river or from perennial canals the 

 trees die off if the flood water is withheld for three or four years : sufficient 

 percolation from the river, however, is exceptional in ground not subject to 

 floods. It has been estimated that the minimum depth of surface flooding 

 necessary for good babul growth varies from 2 inches on the stiffest to 8 inches 

 on the most porous soils. Deeper flooding is common, and the babul 

 benefits rather than otherwise provided the water drains off at the end of 

 the flood season, for prolonged inundation on low-lying ground is apt to kill 

 off young plants which have been submerged. 



Although the most flourishing babul forests are those which are now in 

 riverain situations subject to annual floods, there exist also forests situated 

 some miles from the river, the majority of which formerly received annual flood 

 water. Within the past forty years, however, embankments constructed by the 

 Irrigation Department along both sides of the Indus have shut off the forests 

 from their annual supply of flood water. To a large extent this has resulted 

 in the disappearance of the babul, its place being taken by the deep-rooted 

 and hardier Prosopis spicigera. Where the babul has survived it is in a weakly 

 condition, except where it obtains a more or less adventitious supply of water 

 from the surplus drainage from neighbouring cultivation or from occasional 

 breaches in the river embankments. Some of the forests are also irrigated 

 from canals. For some years past steps have been taken to remedy this state 

 of affairs by the construction of sluices in the river embankments, connecting 

 with distributary channels leading into the forests, and by the embankment 

 of forest boundaries to head up water. Whether or not these works will prove 



