Central Water and Power Research Station, Poona-3.
A graphical examination of 49 years (1913—61) annual highest water-levels attained on the Brahmaputra at Dibrugarh shows a small uniform rising trend of the data all through. On account of the large variations of the year to year different flood flow rates being confounded with the simultaneously occurring different aggradations or degradations of the river-bed, any apparent changes in the long term behaviour of the annual observed high water-levels alone do not afford the correct media for reflecting the separate similar behaviours or continuity of trends of either or both of the two intermixed variations.
An examination of the annual lowest water-levels data revealed on the other hand their different trends holding over different stretches of years. As the smaller daily flow rates ruling at this time of the season can be reasonably expected as not sensibly different from year to year, any observed low water-level variations from year to year probably reflect better truly the time to time exact changes occurring in the only other dominant variable factor, namely on the river-bed. The causes of the occasional flashy rises apparent in the annual low water-level variations have then been traced to the respective earthquakes preceding such rises.
On account of two earthquakes following very close one after the other in 1947 and 1950 the debris carried by them to the river-bed probably did not have time enough to be washed or scoured down the river. Thus the very steep rise observed of the low water-level in 1951 reflects only the superposed accumulation of the debris thrown up by both the earthquakes. The high level so attained of the bed has been maintained through more than ten years following; it is surmised only on account of the spilling of the annual flood flows over the river banks and thus not keeping their full intensity to the channel course to scour the aggraded bed. It is concluded, therefore, that the future annual high water-levels may not fall far below their 1955—61 mean lie until the bed-level and the annual low waters line drop back to the pre-1951 level.
