14 SILTING UP OP THE CREEKS OF FALMOUTH HAVEN. 



Sir Henry de la Becbe assumes in his Geology of Cornwall, 

 Devon, and West Somerset, a minimum estimate of one cubic 

 foot of alluvial deposit per day, from eacli square mile, and 

 this for a watershed of 29 sq. miles, with an area of joths of a 

 square mile of creek, will he equal to 1 foot accumulation in 

 depth over the whole river bed above Malpas in 1050 years. I 

 am, however, of opinion, that this estimate is a very low one, 

 and as Sir Henry gives no data on which he founded it, I have en- 

 deavoured by experimental enquiry to ascertain the proportion 

 of silt brought down by our rivers in flood. 



On February 7th, 1881, after a heavy rainfall, I took a sample 

 of the Eiver Allen in flood, and by a careful analysis, conducted 

 in the Laboratory of our Institution, I found that the water was 

 charged with one part in 1000 in volume of silt in suspension. 



Adopting this proportion for flood water, I have calculated 

 from the registered rainfall, and my own observations on the 

 proportion of water run off the soil to the rain, that in the year 

 1880, 367,700 cubic feet of mud were carried down by the streams 

 into Tresillian Creek from its watershed. 



The whole of this alluvial detritus will not be deposited above 

 Malpas, owing to the tidal flow leaving the creek bare, during 

 which the turbid water is confined to the channel, and does not 

 flow over the mud banks. 



Taking the average time per tide during which silt could 

 settle on the banks above Malpas at 4 hours, one third of the 

 above mentioned quantity of mud would be deposited, or 122,566 

 cubic feet, the remainder settling in the tideway below. Assum- 

 ing this to be the yearly average, the creek, with its area of ^^gths 

 of a square mile, is now silting up at the rate of about a foot in 

 a hundred years, and this I shall assume as being the normal 

 rate of increment of mud from alluvial deposit, although I can 

 only offer it as an attempt to approximate to the solution of an 

 interesting problem. 



Truro Creek, with Calenick creek, receives three streams. The 

 E,iver Allen having a drainage area of 7070 acres, discharging 

 on the average about 1100 cubic feet per minute ; the Kenwyn 

 stream having a watershed of about 5070 acres, and an average 

 flow of about 800 cubic feet per minute ; and the Calenick stream 

 (graining about 4690 acres, and discharging an average of about 



