"Eric" wrote in message
...
Here's something to think about - and I hope someone has some good
answers or can point me to a good source of info...
. . .
First question - does anyone have a good explanation for what river
level is supposed to represent and a good way to explain some of the
things above without degenerating into a technical hydrodynamic
discussion?

"Degenerate?" It's for explaining relationships like you describe that
"technical hydrodynamic discussions" were invented. The hydrodynamic
relationship you're looking for relates the 'stage' (which, with the profile
of the stream bed, determines the cross sectional flow area) with the flow
velocity or the total volumetric flow rate. Without going into all of the
hand waving, the velocity will vary roughly as the square root of the depth
in a broad shallow stream. You can degenerate as far as you'd you like if
you feed 'streamflow' or 'Manning equation' into Google.

Better than that, you can construct the relationship directly from the real
world flow data on a particular stream you're interested in.

Second (and partially related question) - I'm probably missing
something obvious here but I can't see the forest for the trees. I'll
use a specific example and we can generalize from there.

Looking at the James River (VA) gauge at Buchanan, you see that the
river is at a particular level and CFS - let's say, for example, 4.75
feet and around 3800 CFS, give or take. Looking downstream, we see that
the Maury River flows into the James at Glasgow, adding its flow to the
James at that point. We see that the Maury River at Buena Vista (a ways
upstream from the confluence) is running at 4.5 and around 2800 CFS.
Now moving downstream on the James, we look at the James River at
Holcomb Rock and the gauge reads about 7.25 feet and about 6000 CFS and
change. Common sense would say (river features like dams excepted) that
part of the extra CFS would be from the water coming from the Maury.
However, the ratio of level / CFS from Buchanan to Holcomb Rock seems
to be pretty continuous, regardless of the level / CFS being added by
the Maury.

Forget trying to work with stages. First, their zeros are completely
arbitrary. Second, the relationship between stage and CFS can be minutely
dependent on the particular transverse profile and bottom characteristics of
the stream on which they're reported.

So what gives? Is this a situation that is particular to this area or
am I missing something here that is true across the board? How does the
Mississippi change when the Ohio flows in?

Combining stages would be meaningless. The CFSs, though, had better add up,
unless it's raining or it's VERY, VERY dry.

hth,
Fred Klingener