It has been asserted in various quarters that less water runs in English rivers now than was probably the case centuries ago, when the abundant forests caused a greater rainfall. This may be so, but, on the other hand, a number of witnesses examined before the Select Committee of 1877 expressed the belief that the water flowing into the rivers had increased of recent years, owing to the improved land drainage, which drained off rapidly and sent down to the sea much rain water that previously would have passed into the air again by evaporation. In the matter of high tides, "Rees' Cyclopædia" (1819) says that the tide "often" rises at the mouth of the Wye {152}to a height of 40 ft.; while "Chambers' Encyclopædia" gives 47 ft. above low-water mark as the height to which the tide has been known to rise in the same river at Chepstow. Of the floods in the Yorkshire Ouse Rodolph De Salis says in "Bradshaw's Canals and Navigable Rivers of England" (1904): "The non-tidal portion of the river above Naburn Locks is liable to floods, which at York often reach a height of 12 ft., and have been known to attain a height of 16 ft. 6 in. above summer level." The liability of English rivers to a shortage of water would seem to be as great as their liability to excess of it. In Archdeacon Plymley's "General View of the Agriculture of Shropshire" (1803) there is published a table, compiled by Telford, giving the heights reached by the Severn between 1789 and 1800. It shows that, as against some very serious floods and inundations, the river often, during the dates mentioned, ran for considerable periods with a stream of no more than sixteen inches of water; that it frequently had less than a foot of water; and that in times of extreme drought the depth of water had been reduced to nine inches. In 1796, the period during which barges could be navigated even down-stream with a paying load did not exceed two months, and "this interruption," it is stated, "was severely felt by the coal-masters, the manufacturers of iron, and the county in general." The navigation of the Trent is declared in "Rees' Cyclopædia" to be "of vast importance to the country"; yet the authority of John Smeaton, who had examined the river in 1761, is given for the statement that in several places the ordinary depth of water did not exceed eight inches. In the upper part of the river there were, in 1765, more than twenty shallows over which boats could not pass in dry weather without flushes of water. The inadequate depth of water may be due, not alone to drought, but to the formation of shoals or shallows owing to the rapid fall of the river, its excessive width, or the amount of sediment brought down from the hill-sides or washed from the bed over which it flows. Alternatively, much silting-up may be caused by the sand brought into the river by incoming tides, and not always washed out again by out-going tides.