736 MR. E. D. OLDHAM OJiT THE LAW THAT ao VEENS 



cause a sudden check in the Telocity of the current, and a large 

 portion of the debris will be deposited as a bar across the reach. 

 This bar, both by the actual resistance it offers and by its influence 

 in causing the waters to spread out at a point higher up stream than 

 before, will continually increase by the deposit of debris on its up- 

 stream side ; the lower fan will consequently encroach on the reach 

 above it, and we have the compensation required for the encroach- 

 ment of the reach on the fan. 



These encroachments of fan on reach and reach on fan necessitate 

 some modification of what has gone before. Eoth reach and fan 

 have been regarded as regions of transport pure and simple, without 

 either erosion or deposition, but it is evident that the gradual growth 

 of the bar at the foot of the reach must react upwards and induce a 

 certain amount of deposition in the reach, while the erosion at the 

 foot of the fan must similarly react upwards and cause a certain 

 amount of erosion on the surface of the fan. 



These considerations lead us to the following definition of the 

 normal condition of equilibrium which a stream tends to assume : — 

 that it will consist of alternate fans and reaches in which the stream 

 is respectively split up into several shallow ill-defined channels, and 

 collected into a single deep well-defijied channel ; that the former 

 will be a region mainly of transport, but subsidiarily of erosion, 

 while the latter will be a region, mainly of transport, but subsidiarily 

 of deposition ; that fan will pass into reach by a short stretch of 

 maximum gradient where erosion is at its maximum, and reach into 

 fan by a short stretch of minimum gradient where deposition is at 

 its maximum ; and that the sum total of erosion will equal that of 

 deposition. 



Probably no stream is a purely transporting agent, but, unless the 

 deposition or erosion are excessive, the above definition will apply 

 with the exception of the last clause ; there will be an excess of 

 erosion or deposition according as the stream is on the whole eroding 

 or depositing. 



The only actual levels, taken under circumstances that will test 

 the above hypothesis, which are accessible to me are those taken 

 in connexion with the Ganges Canal and recorded in the late Sir P. 

 B. Cautley's account of that work ; but before referring to them there 

 are some general considerations bearing on the hypothesis. 



Since framing the definition given above I have travelled largely, 

 to a great extent in country where the streams flow at the general 

 level of the surface and where, owing to their drying up completely 

 for half the year, the form of their channels can be very favourably 

 studied ; and in every case where the stream does not flow in an 

 excavated channel, but at the general surface-level of the alluvial 

 deposits, wherever, in fact, there are no interfering influences, 

 I have found that the stream-bed exhibits those features which it 

 should according to my hypothesis. The stream-bed alternately 

 spreads out into a broad ill-defined stretch of sand or gravel, and 

 contracts into a narrow well-defined channel ; moreover the broad 

 stretches always rise up to the general surface, often the gravel 



