377

and probably it commenced long before;
but it naturally would not be much noticed
until its results began to tell upon the
navigation of the Harbour.

The practical effect of the change was
first felt upwards of a year ago, when
the Aparima grounded on the growing
end of the spit, where not long before
the channel had been perfectly safe. Soon
after a more serious accident resulted
from it.

The growth of the spit continually
shifted the channel, and consequently
the run of the tide, closer to the shore
near the vicinity of the Maori Kainga.
This encroached away the sand near the
beach, and gradually denuded a small
patch of rocks which, at the time of Mr.
Thomson’s survey, were in shallow water,
nearly buried in sand, but which, by the
change I have described, came to be
painted rocks, standing up quite in the
fairway of the channel. On one of these
the Guiding Star steamer struck and was
lost.

In the course of a few days the barque
Eucalyptus, and soon after her another
vessel touched these rocks, which, being
now first brought into the channel, seemed
dangerous to many persons as if they had been
suddenly planted. The channel ever since
has been forced more and more over
towards the land, until now the only
passage for vessels is between or inshore
of these rocks; and the inshore channel,
which alone is navigable over the bar,
is almost cut off from the main course of
the tide and from the upper part of the
harbour. The depth of water at this
point is only twelve or thirteen feet at
low water springs, so that vessels of con-
siderable draught can only cross it about
high water.

This, then, being the change which has
taken place, it becomes necessary to con-
sider:

1st. Its causes.

2nd. The probable result from the
continued action of the same causes.

3rd. Whether any artificial means
can be employed to vary them;
or to avoid any further injurious consequences, or to expedite any beneficial ones, which
may result from them.

The whole of the shoals and the
northern and western shores at the mouth
of the Harbour consist of perfectly clean
and loose quartzose sand, having a specific
gravity not exceeding 2.

Such material must possess a high
degree of mobility, and it may be laid
down as a principle that the banks formed
of it can have no stability beyond that
of the influences which act upon them.
Their form is, in fact, an immediate result
of the motion of the bodies of water
passing through and over them, that
motion being very slightly modified only
by the inertia of the sand, and that
form will promptly alter as soon as any
changes take place in the direction,
rapidity, or volume of the currents, unless
the equilibrium is such that the flood
tide exactly brings back and replaces all
the sand carried down by the ebb, or else
a supply is perpetually brought forward
to make good the portion removed.

The size of the channel under such
circumstances must be in direct propor-
tion to the quantity of water which
passes in and out during ebb and flow,
and its depth must depend upon whether
the circumstances tend to contract it in
breadth or unduly to expand it laterally.

Now, the amount of water in the New
River is very large, quite sufficient under
favourable circumstances to scoop out a
channel deep and wide enough to answer
all the purposes of the most extended
commerce.

The section of the Oreti River above
the junction of the Makarewa is at its
lowest summer level about four hundred
square feet, and its velocity on the
surface not less than two-and-a-half miles
per hour. Its mean section during the
three past months has been full six
hundred square feet, and its velocity over
three miles per hour. The Makarewa
may be taken at one-third of the volume
of the Oreti; and the Waikiwi, Waihopai,
and a number of smaller streams must
make up in the aggregate a volume of
not less than one hundred and ten thou-
sand cubic feet per minute in summer,
and two hundred thousand in seasons of
flood. But this great constant discharge
is a mere trifle compared with the tidal
waters flowing through the Heads. The
estuary covers an area of about eighteen
and a-half square miles, of which it may
be roughly estimated that one half is
covered at low water, and consequently
receives and discharges in each tide the
full depth of the rise and fall at the
Heads, and the remainder about one-third
of that depth.

This great discharge of water neces-
sarily supposes a very large orifice of
discharge, and in fact between the Horse-
shoe Channel and the Bombay Rock, the
channel at low water has a breadth of