The New Orleans daily Democrat., 1880-04-04
THE OUTLET SYSTEM.
What the Mississippi River COM
mission Think or It.

Secretary Uf War, but was compelled, BY
want Of space, TO omit that portion of the re-
port relating TO the "0utlet System" The
Views OF the Commission on that subject are
exceedingly interesting, and seem to us 10 be
unanswerable. We give them, therefore. In
full
It has been supposed by many persona that
because the Immediate effect of a orevasse
during aood II the reduction of the height oI
the rivers surface ia the vicinity Of the Gre
se r y wbe ood waefs o the
river are drawn off and conveyed through a
shprter route 70 the sea. tend to prevent the
recurrence of destructive Guods, by supplying
additional avenues for their escape. This
method would undoubtedly be effective II the
hood wate rs Of the Mississippi were not highly
charged with sedimentary matters, which are
held In Suspension In the water by the current
To support this immense macs OF earth and
eand IN suspenston,, apd thus insure Its traits,
portal Ion to the gulf, the velocity OF the current
must be sustained. Without stopping to deter
mine or even diseuss the character of the rela
tiun which exipta between the various velocities
OF current. and the propottionate quantities Of
sediment which such veioeities are eapable OF
carrying in ,suepeneien. the fact seems to, be
ite natural flow. during nvods. a deposit of
sediment will occur. Shoals are found In the
river Immediately below. crevaeees, which It Ia
difficult to refer to any other cause than the loss
OF current velocity, which takes place below the
crevasse. .As a portion OF the volume Of the
~ Y tbs he
low the crevasse can then bs ss rapid as 1t was
before its occurrence. Being less rapid II Ia
linable TO sustain the whole quantity OF matter
held Ia Suspension by the more rapid current
above the outlet. and consequently its surplus
sediment falls to the bottom below the orevasse.
This deposition continues until the eise Of the
river below the crevasee has been 80 reduced
by the ehoaliug that the current ie again re-
etored through the ehort dstanee Ia witch the,
the channel diminished. Ii the crevasse re-
matned open, however. fol, several years. 1t is
he he r ebs s redasaa veocty w
still exiat In the river below the shoal. III the
crevasee be kent open indefinitely. the shoaling
certain other Injurious effects are produced
which will be presently referred to-.
Il is well es abished law OF hydraulics that
the ratio, OF fictional resistance, per unit Of
volume increases II the sectional area be di-
inlshed mnps a,a 1hs E. vonms, sS,a Of the river
counter more frictional resistance than II met
with while flowing in q single channel. Hence
.ne hronehb A. hsse % = would he
sediment, the sluggish current would oauss q
deposit Ia the channels, which would frat begin
mppf ba aa aaa woaa Wa h
eteepened that the current would attain a ve.
loclty capable OF carrying The suspended sedi
IL the two channels were uf nearly euual length
and eise, they would probably remain perma-
nent, aud the alope of the river surface In food
them than above and below where the volume
lows n eingle channel, Ii one OF the two
sSs othsr s e 81 he pssr to than
crease the steepness of the longer chapne
would be, eboruye. because Its slope would he
e y ing on saoser one vos sa9a 10 the
be built up TO eueh height by the depositing ac
tton OF the sluggish water In II as analy TO shut
ha IN 10 md nf
such channel would promote the deposition Of |
sediment at that end to such an extent as to |
nmid snn d y up also and THUG oompIetey separate
river, In the mean time the shorter channel |
would have enlarged 30 as to accommo odate the |
entire river Ihe Longer channel in would In this
akes which are seen on map OF the alluvial
basin Of the river Being removed from the In
tnence Of ovetfows these lakes remain deep
md clear for many centuries. The phenome-
non Just described Invariably accompanies the
formation OF CUTOFF When one of these oo.
,ur. the volume OF the river ts at first divided
INTO two channels OF unequal length, an Island
beiug left between them.
In the case of q crevasse an island ts also
ormed. having the main body of the river on
he other side, As the volume flowing Ia the
main channel below crevasae hne been de-
creased BY the amount drawn of through it, q
sleeper slape In the main river, II the crevasse
DO kept permanently open, becomes inevitable.
because the shoal below the outlet, as 1t grows
n length down stream from the deposition Of
successive foods. gradually increase ses the frie
ional resistance OF the volume flowing through
hat diminished channel, aud this tebds TO
'hees the current Of the river above the ere,
md the raising of the food line above the site
u the'outlet ensue rs secondary and perma-
II is In this way that silt bearing streams.
lowing through alluvial deposits, have the
ABILITY to increase or steepen their surface
slopes. and thus recover the velocity OF their
and adjust them TO the work of trnna-
porting the sedimentary matter with which the
iood waters are charged. 80 that this matter
" the correctness OF these views. and of their
ull accordance with we'll established hydraulic
avs. we have the evidence OF this relation be-
lomena Of silt bearing streams all over the
eorld. Wherever such streams flow through
nsa ohsr esndiGps ms being me
reatest; and, conversely, the slope is found TO
9 Invariably increased as the volume is d.
iiuyial basin. not only In Its main trunk. but ,
q all of its outlets. presents no exception to
his peculiar feature. Among numerous illus !
rations Of this law, the following examples !
The fall OF the Atchafalaya is abou, six 10 ches
er mile from i!s head and the gulf level, while :
J less than two inches per mile. The volume :
the Atchafalaya Is only about one twelfth as :
reat as that Of the Missiseippi when they
nches per mile while that of the Southwest G
ASS is but two inches per mile. The vou M I
the South Pass is only about one quarter as {
lrge as that OF the Southwest Pass
AS water selects the line of least resistance In
owing from a higher to a lower level. 1t follows
aat nasmuch as that portion of the Missis- {
lppi floods which enters the Atchafalaya seeks I
le gut level through a route not half so long I
s that which follows the main river. and as it q
as q descent three-fold greater than the por-
on that flows In the main river. the resistance 9
the shorter and sleeper route of the Atcha- {
alaya must be so much greater, that these ele-
ients. which tend to increase the current. are q
D far neutraliHed as to produce In both routes Y
, the sea that rate Of current which is capable I
transporting the sediment without loss or 9
ain TO the gulf level and thus coalition of :,
luilibrium is established between these two q
,utes TO the sea. 1t seems unnecessary to 2
ate that the ratio Of ftietional resistance to d
olume of water. resulting from the smaller a
re OF the Atchafalaya. is SO much greater than y
lat IN the main river that this condition of r
auilihrium. or regimen of the Two channels is s
le result. Anything which will tend to increase v
le flow permanently through, either roule 4
ouid, III unchecked. have q tendency to cause "
le entire river to find Its. way ultimately Is
trough that route to Il g sea by lessening In 1t,
1t en- arged the ratio of frictional resis stanceto
e volume of water flowing In It The sub. n