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River forking
by
Stephen W. Draper
Can rivers divide (fork, split, diverge) and come out in quite different
places in the sea? We are used to them converging, not diverging. However
there is an example in Scotland where a small river splits, as was pointed out
in a recent New Scientist.
Anne and Alex Gray
offer a discussion of why this river divides,
complete with photographs. This web page offers a critique of their ideas.
Note however that most of the difference is about what angle to view this
issue from. My main reply, appended at the end, is addressed to the original
query in New Scientist, and is about what in general makes forking rivers very
rare but not impossible. Their argument is about the possibility that in this
particular case in Wester Ross, it could have been created artificially to
please Queen Victoria. A third angle is simply that going to see it is (and
for me was) an excuse for a great holiday, and an agreeable problem to think
about while tramping the hills in the burning Scottish sunshine, and to
discuss with other visitors and guesthouse owners while there.
From that third viewpoint I should say it brought me luck. While
sitting at the river fork contemplating it
(NG884676), the first otter I have
ever definitely seen approached to within two arms-lengths of me, before
finally noticing that his habitat was disfigured by a large pink Englishman
(and then promptly leaving in disgust). Besides investigating the forking
river (which is real), the visitor may like to another longer walk to
investigate what the map makes it look is a lake with two exits: Locahan Fada
[ref. OS Landranger no.19, 1:50,000, NH053695]. This is not real. The western
exit is real, the eastern one is (at present) an inflow. Loch an Sgeireach is
in fact the highest in the apparent chain. Currently (July 2000) it flows out
into Lochan Fada, but there is a clear channel at its other, southern, end to
Loch Gleann na Muice which has seen flow pretty recently; it looks as if after
heavy rain, the loch's level has only to rise about 3 feet for it to flow out
in two different directions, into two different lochs, and down two different
steep valleys. For a very agreeable place to stay, complete with a resident
geologist who will at least humour a discussion of these things, look
here.
(Other refs to check out.)
A river forks (divides) at the following place: OS Landranger no.19,
1:50,000, NG884676, on both the map and when visited on the ground in July
2000. The river is the outflow from Loch na h-Oidhche, and divides about a
kilometre downstream from it. The right hand fork flows into Loch Garbhaig,
which in turn feeds the "Victoria falls" flowing into Loch Maree, which in turn
flows into the sea at Poolewe on Loch Ewe. The left hand fork flows into Loch
Bad an Sgalaig, which in turn flows into the sea at Loch Gairloch. At the fork
the watercourses are 4-8 feet wide. The ground is much flatter than the
surrounding valley and hill sides, but sloping enough that the rivers are
everywhere "chuckling". The slope is also enough that a temporary dam or
blockage 100 or even 50 yards downstream in either fork would not change
matters: a pool would form but the overflow would go round or over such a
blockage without backing up to the fork. Only a change at or very close to the
fork would make a difference. It is hard for a layman to imagine erosion
taking place on a human timescale: the stones look too hard to dissolve or
abrade much, and too big to move in such small streams. However locals say
that boulders move from year to year in similar streams. Of course, observers
are much less likely to be on the spot during heavy rainfall, when perhaps the
flow rates triple or more.
The ground looks like glacial rubble: rounded pebbles and boulders, and
outside the streambeds held together by sand and mud, with some topsoil and a
cover of vegetation. The streams are cut down about 4 feet below the
vegetation. However the fork divides around a small hill (Meall Lochan na
Geala) which is solid rock, not rubble, and which apparently the glaciers never
flattened: that is, the river fork was presumably prefigured by a glacial
divide.
A description of the forking river together with a theory of its origin
is given, together with photos,
here.
(Or via this.)
Good points made there include:
- From the viewpoint of human history, it certainly could be artificial. Other
such things were done for Queen Victoria. It would only take a few
person-hours with a space or crowbar to change the flow now; and digging a
channel to make such a connection in the first place would at most be a few
person-days of work.
- They say that old maps do not show the river fork. This is an important
(if not totally decisive) line of evidence: excellent idea to be pursued
further. However (having pursued it a little) it turns out that the first OS
survey was not until after Victoria's visit, and I haven't found an earlier
map with a big enough scale that it would show anyway.
Bad points include:
- The estate owners (the present ones have owned it since before Queen
Victoria's visit) should be asked about this. If it was human intervention it
seems likely they have some record of it.
- As they say, there is a partial line of stones at the fork that doesn't look
natural (though it could be). However, exactly contrary to their argument, the
line partly blocks the right hand fork to the falls, NOT the other fork.
Furthermore, even though they look a bit suspicious, this is just where
material should be deposited: since the stream divides at the fork, its power
for carrying material will halve there, and the biggest lumps should drop right
there at the fork. Since momentum would have them travelling in a straight
line, it would take them into the right hand fork (the straight one at that
point) and drop them just where in fact they have been dropped.
Gilbert Scott asks how can a river divide (New Scientist no.2245, 1 July, 2000,
p.109). My thanks to him for prompting a great holiday looking into this.
(Other replies to him appeared in: New Scientist no.2254, 2 Sept, 2000,
p.97.)
Should we be surprised? No. Everyday examples of water flow dividing include
two or more taps continuing to provide water from a single house tank (or
municipal reservoir), river deltas, river islands, water dividing into broad
sprays from hoses, water flowing down a window pane or spreading out on a table
from a spill, and a simple experiment with water flowing down a gutter and a
finger thrust down into the flow causes the water to divide around it. If we
believe Newton, then water's movement will depend on its existing velocity and
on the net sum of forces acting on it, which will include not just gravity but
the net pressure in the surrounding water, viscosity, friction against
surfaces, surface tension, and so on: the direction of steepest descent is not
the only factor. The spirit of the question however is macroscopic: about the
rarity and stability of river division.
There is a suggestion
(www.annegray.com/gairloch/guide/river/)
that the Scottish example was artificially created to increase the water to the
"Victoria falls" for the late queen's benefit. However there is a bigger South
American example of a dividing river: the Orinoco divides into the lower
Orinoco and the Casiquiare which flows via the Negro into the Amazon. Unlike
deltas, this is like the Scottish case with the branches reaching the ocean in
widely separate places. Deltas are unstable due to deposition (silting up)
causing the channels to switch about; the Mississippi delta is thought to be
about to show a major shift (wasn't there a bit in New Scientist about this
recently?). Geographers argue that true river division (in places upstream
where erosion not deposition dominates) is also unstable in the long run, as
one branch will erode deeper, this depth will erode back upstream to the fork,
and capture the whole flow.
However perhaps we should say more. We should be able to say something about
the (very low) frequency of observing such permanent partings in rivers. River
islands occur when rivers divide; but their length is limited by a greater
tendency to re-converge. In the Scottish river in question, the map shows
islands further down the left hand (western) fork. In the upper stretch I
investigated, there is an island just a few metres down the left fork and
itself is a few metres long; a bigger island above the fork where the river
leaves the loch; and an island further down the left hand fork perhaps 50m
long. In very round figures, I saw 4 divergences in 4km (3 islands and a
fork), and 3 re-convergences in 80m (the lengths of the islands): so
re-convergent factors outweigh divergent factors by 40:1 give or take an order
of magnitude. Experience of the frequency of islands elsewhere (outside
deltas) is of this order too. What is special about the fork is that it
happened in a place where re-convergence failed to occur. Elsewhere, this
river like others flows in a valley. Any division soon runs into a valley wall
higher than the point of division, so re-convergence is inevitable whatever
random path the river takes. But the relative shortness of river islands shows
that in any case reconvergence happens much more easily than division does
(otherwise almost all stretches of river would have islands, and valley walls
would just ensure the multiple channels stayed near each other). Many rivers
have no points not enclosed by high ground with a single exit.
Permanent division (forking) can only happen when:
- there is a point with more than one exit over low ground
- a division happens there (but divisions only happen
about once per kilometre i.e. relatively infrequently)
- there hasn't been time for the geographers' argument about
erosional capture to take effect.
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