I am curious if I can look at river A and say it is 450cfs and then look at river B that is 900cfs. Is river B flowing twice as fast as river A? If not, is there a way to compare the two river speeds and thus plan better for how many miles I will cover on river B knowing my performance on river A?
Thanks!
No consistent relationship
Nope, you can’t tell a thing from gauge readings alone. The bigger the river, the greater the flow, but obviously big rivers are not universally faster than small rivers.
For example, in one of my common paddling areas, there are two rivers that are equally difficult on which to paddle upstream, and one is flowing at 700 cfs and the other at 11,500 cfs. It’s no mystery which of these two readings belongs to the bigger river, but which one is faster? I don’t know. If there’s a difference in current speed, it isn’t much.
Some newer gauges measure the current velocity, but again, this may not be a good indication of overall current speed on other parts of the iver. It will only tell you the speed at the gauge location, which is commonly an area of constricted flow (= faster current) such as under a bridge, because such locations allow easy calculation of cross-sectional area of the channel.
No and No
Unfortunately, water volume is just one aspect that affects speed. Gradient and width/depth of river being two more, but likely there are more. So basically, a doubling of CFS will likely lead to faster water, but not double the speed. And I don’t know any way to figure out what that speed difference would be.
For a real application - does the Mississippi River (which a quick Google search says ranges from 200,000 CFS to over a million CFS) go 450-1000 times faster than your 450 CFS river?
CFS
Cubic feet per second is a volume measurement.
It is applicable only to a single river and not
any use for comparing rivers…unless you know the detailed topography of the compared rivers.
A big deep, wide river can have a huge cfs and a slow velocity while a narrow shallower river can be all speeding whitewater while still maintaining a low (relative to the big river) cfs.
People track on cfs because it is useful for predicting river conditions (especially when comparing the numbers with previous trips on the same river)
USGS data for velocity and conductivity
I daily check river flows for both upcoming paddle planning and pure curiousity since I love hydrology and I have recently found that the USGS gages sometimes have a real-time velocity chart. Very interesting. MY QUESTION to those with better understanding than I ( and I have a BS degree with hydrology as a minor) IS: when a river is in flood stage, the conductivity drops! WHY? FLOW and Conductivity seem to be inversely proportional. It seems to me that clear low-flow water would have lower conductivity than muddy turbulent flood stage water. I keep looking at all these graphs and it always show a huge drop in conductivity as a river is spiking during a flood!
Volume flow rate
– Last Updated: May-16-13 11:44 PM EST –
Flow in CFS is volume flow rate, usually called Q in fluid mechanics. Volume flow is the product of mean velocity and cross sectional area of the channel.
Q = A * V
So in principle, if you knew (or could estimate) the cross-sectional area of a given river (say, 75 feet wide by 3 feet deep), then for Q = 450 CFS, the mean velocity of the river would be
V = Q/A = (450 ft^3/s)/(225 ft^2) = 2 ft/s
Keep in mind that this is mean velocity - center-line velocity will be higher, velocity at the banks will be lower. It's not really practical to know the area of a given river, especially since it depends on the depth on a given day, so the flow rates reported are really useful for comparison with respect to a given river that you are familiar with. It's numerical data, but is only useful as anecdotal info unless you know the detailed river topography (unlikely).