RIVERS. 
sg6 
bed of theteanal below the horizontal level 
of the bottom of the sluice will serve to no 
manner of purpose, either for navigation, or 
for carrying off the back-watt-is, but will 
soon till up with mud, in spite of all means 
used to tlie contrary, except that of digging 
jt continually anew to no manner of purpose. 
Setting off from this determinate point, 
at the mouth of a river, or at the bottom of 
the last sluice upon a canal, which are to be 
cleaned and deepened; the work must be 
carried on, inconsequence uniformly through- 
out; their whole course backwards into the 
country as far as is found necessary for the 
purposes intended. This is to be done after 
the following manner: 
1st. One must dig up and carry away all 
irregularities in the bottom and sides of the 
bed, such as banks of sand and mud, rocks, 
stumps or trunks of trees, and whatever else 
may cause an obstacle to the regular motion 
of the water, and to the free passage of ves- 
sels upon it. 
2d!y. If the declivity of the bed should be 
stili too little to give a sufficient current to 
carry off the water as often and as fast as is 
necessary, the whole bed itself must lie regu- 
larly deepened, and what is dug out from the 
bottom must be laid upon the sides, to ren- 
der it narrower in proportion to its depth. 
3dly. Wherever the banks are too low to 
contain the stream in all its situations, they 
must be sufficiently raised; which may be 
conveniently done with what is dug out from 
the bed ; and the whole being covered with 
green turf will render these banks firm and 
solid against the corrosion of the water. It 
is proper at all times to lay upon the banks 
what is dug from the bed, by which they are 
continually strengthened against the force of 
fche current. 
4thly. It is often necessary to diminish the 
windings aiad sinuosities in the channel as 
much as possible, by making new cuts where- 
by its course may approach towards a right 
line. This is a great resource in flat coun- 
tries subject to inundations ; because there- 
by all the declivity of a great extent of the 
river, through its turns and windings, may 
be thrown into a small space by cutting a 
new channel in a straight line ; as may ge- 
nerally be done without obstacle in such 
countries as we are speaking of, and hereby 
the velocity of the current will be very greatly 
augmented, and the back-waters carried off 
to a surprising degree. 
Stilly. Wherever there is a confluence of 
rivers or canals, the angle of their junction 
must be made as acute as possible, or else 
the worst of consequences will arise from 
the corrosion of their respective streams ; 
what they carry off from the sides will be 
thrown into Irregular banks in the bottom of 
the bed. This acute angle of the junction 
may always be procured by taking the direc- 
tion at some distance from the point of con- 
fluence. 
Othly. Wherever the sides or banks of a 
river are liable to a more particular corro- 
sion, either from the confluence of streams, 
or from irremediable windings and turns in 
the channel, they must be secured against 
it as much as possible by weirs : for this cor- 
rosion not only destroys the banks, and alters 
by degrees the course of the river, but also 
nils up the bed, and produces all the bad 
effects we have spoken of above. 
7thly. But the principal and greatest at- > 
tendon in digging the beds of rivers and ca- 
nals must be had to the quantity and form of 
their declivity. This must be done uniformly 
throughout their whole extent, or so much 
of it as is necessary for the purposes in hand, 
according to the principles laid down. Con- 
formable thereto, the depths of their beds, 
and of the lloors of their sluices, at the mouths 
where they discharge their waters, being 
fixed, the depth of the rest of the beds, and 
the quantity of declivity, must be regulated 
in consequence thereof, so as to increase re- 
gularly the quantity of the declivity in equal 
spaces the further we recede from their 
mouths, and proceed towards their sources 
or to the part where the regular current is to 
take place. 
If the depth and volume of water in a river 
or canal is considerable, it will suffice, in the 
part next the mouth, to allow one foot per- 
pendicular of declivity through six, eight, or 
even, according to Desclrales, ten thousand 
feet in horizontal extent ; at most it must 
not be above one in six or seven thousand. 
Hence the quantity of declivity in equal 
spaces must slowly and gradually increase 
as far as the current is to be made fit for 
navigation ; but in such a manner, as that at 
this upper end there may not be above one 
foot of perpendicular declivity in four thou- 
sand feet of horizontal extent. It it is made 
greater than that in a regular bed containing 
a considerable volume of water, the current 
will be so strong as to be found very unfit for 
the purposes of navigation. 
Mr. Mann calls the centre of the current, or 
more properly, line of greatest current, that 
line which passes through all the sections of 
a river, in the point where the velocity of 
the current is the greatest of all. If the cur- 
rent of a river is regular, and in a right line, 
its centre or line of greatest velocity will be 
precisely in the centre of the sections ; but 
on the contrary, if the bed is irregular and 
full of turns and windings, the centre, or line 
of greatest current, will likewise be irregu- 
lar, and often change its distance and direc- 
tion with regard to the centres of the sec- 
tions through which the waters flow, ap- 
proaching successively, and more or less, to 
all parts of the bed, but always in proportion 
and conformably to the irregularities in the 
bed itself. 
This deviation of the line of greatest cur- 
rent from the centres of the sections through 
which it passes, is a cause of many and great 
changes in the beds of rivers, such as the 
following : 
1st. In a straight and regular bed, the 
greatest corrosion of the current will be in the 
middle of the bottom of the bed ; because it 
is that part which is nearest to the line of 
greatest current, and at the same time 
which is most acted upon by the perpen- 
dicular compression of the water. In this 
case, whatever matters are carried off 
from tine bottom will be thrown, by the 
force of the current, equally toward the two 
sides, where the velocity of the steam is 
the least in the whole section. 
2dly. If the bed is irregular and winding, 
the line of greatest current will be thrown 
towards one side of the river, where its great- 
est force will be exerted in proportion to the 
local causes which turn it aside: in short 
turns of a river there will be a gyration, or 
turning round of the stream, from its beating 
against the outer side of the angle ; tins part 
will be corroded away, and the bottom near 
it excavated to a great depth.. The matters 
so carried off will be thrown against the op- 
posite bank of the river where the current is 
the least, and produce a new ground called 
an alluvion. 
3dlv. Inequalities at the bottom of a river 
retain and diminish the velocity of the water, 
and sometimes may be so great as to make 
them reflow ; all these effects contribute to 
the subsiding of sand, earth, and other mat- 
ters, which cease not to augment the volume 
of the obstacles themselves, and produce 
shallows and banks in the channel. These 
in time, and by a continuance of the causes, 
may become islands, and so produce great 
and permanent changes and irregularities in 
the beds of rivers. 
4thly. The percussions of the centre of the 
current against the sides of the bed are so 
much the greater as they are made under a 
greater angle of incidence ; whence it follows, 
that the force of percussion, and the quantity 
of corrosion and detriment done to the banks 
and weirs of rivers, and to the walls of build- 
ings which are exposed to that percussion, 
are always in a direct compound proportion 
of the angle of incidence, of the greatness 
and depth of the section together, and of the 
quantity of velocity of the current. 
5tlily. It may happen in time, that the ex- 
cavation of the bottom, and the corrosion of 
the sides, will have so changed the form of 
the bed as to bring the force of percussion 
into equilibrium with the velocity and direc- 
tion of the current; in that case, all further 
corrosion and excavation of the bed ceases. 
6thly. This gives the reason why when 
one river falls into another almost in a per- 
pendicular direction, and makes with it too 
great an angle of incidence, this direction is 
changed in time, by corrosions and alluvions, 
into an angle much more acute, till the 
whole comes into equilibrium. 
7lhly. So great and such continued irre- 
gularities, from local causes, may happen in 
the motion of a river as will entirely change 
its antient bed, corrode through the banks 
where they are exposed to the greatest vio- 
lence of percussion of the stream, and open 
new beds in grounds lower than the old one 
is become. 
8thly. Hereupon the state of the old bed 
will entirely depend on the quantity of 
water, and on the velocity and direction of 
the current in the new one ; for immediately 
after this division of the v\ aters into two bed's 
is made, the velocity of the current in the old 
one will be diminished in proportion to its 
less depth. Inconsequence, the wat; rs will 
precipitate more of their mud, &c. in equal 
spaces than they did before ; which win more 
and more raise up the bottom, sometimes 
even till it becomes equal with the surface of 
the stream. In this case, all the water of the 
river will pass into the new bed, and the old 
one will remain entirely dry. It is well 
known that this has happened to the Rhine 
near Leyden, and to many other rivers. 
9thly.' Hence the cause of the formation 
hsf the new branches and mouth, by which 
great rivers discharge their waters into the 
sea. 
