424 
THE TUSCAN MAREMMA. 
apprehension. Between the beginning of the fifteenth century 
and the year 17G1, thirty-one destructive floods of the Arno 
are recorded ; between 1761, when the principal streams of the 
Yal di Chiana were diverted into that river, and 1835, not 
one.* 
Improvements in the Tuscan Alaremme. 
In the improvements of the Tuscan Maremma, more formi¬ 
dable difficulties have been encountered. The territory to be 
reclaimed was more extensive; the salubrious places of retreat 
for laborers and inspectors were more remote; the courses ol 
the rivers to be controlled were longer and their natural in¬ 
clination less rapid; some of them, rising in wooded regions, 
transported comparatively little earthy matter,f and above all, 
* Arrian observes that at the junction of the Hydaspes and the Acesines, 
both of which are described as wide streams, “ one very narrow river is 
formed of two confluents, and its current is very swift.”— Arrian, Alex. 
Anab ., vi, 4. 
A like example is observed in the Anapus near Syracuse, which, below 
the junction of its two branches, is narrower, though swifter than either 
of them, and such cases are by no means unfrequent. The immediate 
effect of the confluence of two rivers upon the current below depends 
upon local circumstances, and especially upon the angle of incidence. 
If the two nearly coincide in direction, so as to include a small angle, the 
joint current will have a greater velocity than the slower confluent, per- 
haps even than either of them. If the two rivers run in transverse, still 
more if they flow in more or less opposite directions, the velocity of the 
principal branch will be retarded both above and below the junction, and 
at high water it may even set back the current of the affluent. 
On the other hand, the diversion of a considerable branch from a river 
retards its velocity below the point of separation, and here a deposit of 
earth in its channel immediately begins, which has a tendency to turn the 
whole stream into the new bed. “ Theory and the authority of all hydro- 
graphical writers combine to show that the channels of rivers undergo an 
elevation of bed below a canal of diversion.”—Letter of Fossombeoni, in 
Salvagnoli, Raccolta di Documenti , p. 32. See the early authorities and dis¬ 
cussions on the principle stated in the text, in Frisi, Del modo di regolare i 
Fiumi e i Torrenti , libro iii, capit. i. 
t This difficulty has been remedied as to one important river of the 
Maremma, the Pecora, by clearings recently executed along its upper 
