Thoughts on volume (CFS) and river levels and such (sort of rambling)
 

Thank you, Mr. Kanubic Travesty, this is the sort of answer that I was
looking for when I posed my "poorly-worded" question. Your reply was
certainly well put together and I hope other folks benefit as well. I
also appreciate the other replies that have been posted.

Oci-One Kanubi wrote:

Alright, Eric; here are some more monkey wrenches in yer werks, but
let me say the most important thing first: go out and buy Roger
Corbett's "Virginia Whitewater". Difficulty ratings, as you know, are
subjective (and I think Corbett overrates by about 1.5 classes) but
Roger's ratings are absolutely consistent. Roger was also an expert
amateur hydrologist, and I think you might find some good layman's
hydrology in his book, though I don't really remember for sure. Okay,
now:

You know that level (known to the USGS as "stage") is, for our
practical purposes, based upon an arbitrary zero, and that the river
can only be said to be "low" or "high" with reference to actual
experience. Over time, with experience on a given river at various
stages, a paddler can make his determination as to what stages are
"low" and what stages are "high". Of course, this is subjective and
will vary with the type of craft being paddled and with the skill of
the paddler. So, anyone you explain this to will need to understand
that difficulty as a function of stage is different for every river,
and can only be determined by direct experience (though, as we shall
see, there are some good predictive indicators).

Next, you know that volume, or "flow", is NOT arbitrary, and that
conclusions drawn from volume on one river may help you predict the
difficulty of another. But volume is only actually measured when it
originates with a dam release. On the James, for example, volume at
each gauging station is calculated as a function of the stage. The
parameters of this calculation are (1) stage, which is measured
directly, (2) cross-section, which was surveyed some time in the past,
and (3) velocity, which was measured at some time in the past at
different points on the cross-section. From cross-section at stage
(in square feet) and average velocity at stage (in feet per second) a
table was generated that shows cubic feet per second by stage.

UNFORTUNATELY (for gauging purposes) the river is not a static entity;
it changes. The cross section changes, bars (ephemeral natural dams)
build up and wash away, the bottom may change as sand, pebbles, or
cobbles are deposited or removed. A result of this is the phenomenon
of a paddler saying, "yuh know, I'm really getting better; this used
to be very difficult for me at 5.2'," or "geez, I'm getting old; this
used to be really easy at 2.8'." Well, maybe. But quite often, as
the riverbed changes, a given stage reflects a greater or lesser
volume than it once did, and difficulty is more rightly a function of
volume than of stage. Because flow is not measured, but calculated
from stage, these changes are not necessarily obvious to a paddler who
doesn't know the river intimately.

Another result of this is that, in yer case, for e.g., the Maury, or
the Buffalo, or the Tye may have changed its bed so that for a given
measured stage it is delivering a very different volume, whereas the
reverbed at the gauge below the confluence has NOT changed, so THAT
gauge continues to report flow very accurately. Hence (beyond the
water travel-time, groundwater seepage, ungauged tributary, etc.,
issues mentioned by others), there is oftentimes a disjunct between a
reported volume on the main stem and the sum of the reported volumes
on the tributaries.

Okay, so that is a major caveat wrt volume, or flow. Nevertheless, in
the short term, we as paddlers can use reported flow the same way we
use reported stage based upon an arbitrary datum. It doesn't really
matter (on a given river) if the USGS reports a flow of 700 cfs when
the actual flow is 1,000 and reports a flow of 1,000 when the actual
is 1,500, as long as we, from our experience, know that we can be
comfortable at (a reported) 700 but not at (a reported) 1,000.

But, beyond that, it is also fairly safe to say that the reported flow
is rarely GROSSLY misstated at the levels we normally boat -- i.e.,
above boating zero and below flood stage. If the reported volume is
approximately in the right ballpark, we can use our experience on one
river to make some reasonable assumptions about a river we have never
seen. But volume alone is never enough. The next most important
factor (in my personal view) is gradient. As volume goes up,
difficulty goes up, and as gradient goes up, difficulty goes up.
Thus, the Upper Yough at 600 cfs is approximately the difficulty of
the Upper Gauley at 2800 cfs, because the gradient of the Yough is so
much greater. The next most important factors in difficulty are
probably streambed width and morphology. The effects of streambed
width can be seen on various rivers with a rapid named "The Narrows".
Volume and gradient may be the same as the wider spots above and
below, but the difficulty increases in the narrows. Likewise is
morphology an issue: a river with constant gradient and few surface
rocks is usually a lot easier (though sometimes a more dangerous place
to swim) than a pool-drop river with complicated technical rapids,
other things (volume, gradient, width) being the same.

The upshot of this is, you can make some predictions about the
difficulty of an unfamiliar river based upon (reported approximate)
volume and measured gradient, compared to familiar rivers of similar
volume and gradient, and you can make even better predictions if you
have been able to learn something about the bed-width and morphology
ot the unfamiliar river. You also ought to look at Monte Smith's book
("Southeastern Whitewater", I think it is called, for an excellent
discussion of these four, and several other, factors that affect the
difficulty of a river.

Finally, however, there is the matter of more-or-less permanent
dangerous features; undercuts and boulder sieves, for e.g., which are
entirely independent of volume, gradient, or bed-width, though they
are obviously a function of morphology. These can sometimes be
discovered by low-water scouting, but are usually only known through
paddler lore - experience passed on from one boater to another.

The upshot is, you can explain it as well as you can to non-boaters,
but river difficulty- and danger-assessment is highly subjective,
dependant upon a number of inter-related attributes of the river, of
the boat, and of the boater, highly variable with weather, and has
never been subjected to rigorous analysis (I mean, 300 million
Americans are interested in the weather, and that has not yet yielded
to rigorous analysis, so who's gonna solve an equally difficult
problem for 15,000 whitewater boaters?) The judgment of a trusted
experienced paddler who knows yer skill level and that of yer group,
is still yer best tool for river safety.

Dunno if this is what you were looking for, exactly...

--
-Richard, His Kanubic Travesty

PS, ignore Tooley; he's cranky (i.e., he's not always a crank, but
sometimes...)

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Richard Hopley, Winston-Salem, NC, USA
rhopley[at]earthlink[dot]net 1-301-775-0471
Nothing really matters except Boats, Sex, and Rock'n'Roll.
rhople[at]wfubmc[dot]edu 1-336-713-5077
OK, OK; computer programming for scientific research also matters.
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