'Backdoor Drainage', an Anti-Malarial Measure Designed to Meet a Particular Physiographical Situation in Sylhet District, Assam

Physiography of Sancherra garden A river has a regimen like that of a pendulum; in its course it oscillates from side to side, and the consequence is that it moves in a series of curves, these having an amplitude depending on the gradient and a wave-length depending on the width of the river. The phenomenon however is influenced in its manifestations by such static factors as the heterogeneity of the earth?for instance if the banks of a stream that would naturally swing in curves be rigidly protected it is compelled to follow a straight course?or it is biased by such a dynamic matter as the deposition of alluvium by another stream approaching it?and so when one observes that two streams pursue independent courses for some distance down a valley plain, one may be sure that the oscillation of each has been biased by the resistance offered by the alluviation of the other, the resultant course of the streams being the state of equilibrium between the apposing forces.* An example on a grand scale of the influence of a stream on the course of a second one, even when the latter is the greater of the two, is seen in the Ganges Valley, where the great river has been for various reasons pushed over to the south against the peninsula by the deposits of its tributaries the Gogra, Gandak, and others; while on a small scale such factors doubtless have determined the courses of the Dholoi River and the Legata River in the Dholoi Valley of South Sylhet, the scene of this report, the two rivers running down the valley independently for some distance before joining (see map 1). Now not only do two apposing streams influence the oscillation of each other, but the result may even proceed to the length of the larger stream choking, as it were, the smaller, if this indeed cannot escape elsewhere, for it stands to reason that if the alluviation of a stream be powerful enough to influence another in its course, the deposit may go so far as to invade the latter grossly, the first effect of the choking of the lesser stream* by the deposition of silt in its bed being that the stream is raised,