CFM converted to MPH ?

Is there a conversion factor?

For instance if I am reading a flow gage of a river, is there a way of converting CFM at that particular gage to MPH ?



Not really
Cubic feet per minute refers to volume not speed.

A fast flowing stream could have a high CFM, a large river flowing slowly but has more volume could have the same CFM.


Apples and Oranges
Your trying to convert a measure of volume to a measure of speed. It’s like comparing apples to oranges. If you knew the width & average depth of the river at the gage you could calculate a rough approximation of the speed of flow. That’s way over simplified since the water in a river doesn’t flow at a constant speed from one shore to another but it’s the basic concept.

Visualize a square pipe 10ft x 10ft. if you moved 100 CFM past through the pipe you’d have a speed of one foot per minute.

I understand the above two replys, but
isn’t that gage that is being read in a particular spot whih given dimensions that could be converted to MPH ?

I realize that the river depth and width make the CFM never the same in any two places, but I am asking about the exact spot where the gage is located.


My guess is
That the gage is calibrated in such a way to take into consideration the cross section of the river/stream and then uses a base line of standard flow to calculate the CFM.

Is that a run on sentence?


local knowlege

– Last Updated: Sep-21-08 5:37 PM EST –

My local boulder infested river runs about the same speed no matter the volume (within reasonable limits). More water, higher level,same speed. Low level (not too low), the water is squeezed between the rocks, less volume but less area to flow, about the same speed, compliments of gravity and gradient. The way we determine speed is via dam releases and looking at various gages along a river, seeing when the "spike" from an upstream gage reaches the next gages, and conversely, when the "bubble" passes from gage to gage.

That part is true

– Last Updated: Sep-21-08 5:51 PM EST –

One of the things gauge operators do on a periodic basis is survey the river bottom at the gauge location to determine the cross-sectional area of the river at that point. Usually they do this at a bridge, so it's easier to know the cross-sectional area at a variety of different water levels. They also conduct a survey of current speed at multiple locations (including various depths) throughout that cross-sectional area, and this must also be done at a variety of different water levels since the speed changes with level. If you had all that data at your disposal, you could calculate the average current speed at that location based solely on the CFM value. That's asking a lot, though.

There's one gauge near my home that uses dopler sonar or radar to actually measure the current speed. In this particular case, that is valuable information because seche action within lake that this river runs into causes the current to periodically change speed and even reverse directions, so CFM can't be calculated without current-speed info. With fancier equipment being available all the time, current-speed sensors may become more common at normal gauge locations someday.

Jack, is it cold on your mountain?

No this was on
the Lumber River, and we all knew the flow reading, but were wonder what MPH the water was flowing at.



The answer to the question is a simple yes. The solution involves considerably more work. The key word in your question is “particular”. A chart can be developed by measuring the speed(mph) at each cfm at a particular gauge site. Then simply using the cfm go to the chart to find the mph. Every gauge site will need its own chart. In general because every site has different bottoms, sides and contours no single mathematical formula will work for all sites.

It would be interest problem using chart values to attempt to derive a formula a given site. The project would need to be highly empirical.

Measuring flow

– Last Updated: Sep-22-08 11:02 AM EST –

In Burch's "Kayak Navigation" he gives a way to figure out the current speed by dropping a bottle or float attached to your painter. You time it either from one end of kayak to other or until painter is fully stretched.

The neat trick (if I remember correctly) is that with the right length painter or kayak, the secs it takes is equal to the MPH because the conversion factors cancel out.

If you do not have a copy I can look it up and post the detailed info.

Of course this does not help with getting current speed from on-line guage readings, but it is a quick way to answer the "Hey guys, how fast is this current flowing?" question while you are on the water.


Simple answer
Get in your boat, turn GPS on, float, look at GPS.

GPS in dry bag, throw in river and chase
I woulc have been so much better at math if we would have used these scenarios for story problems.

that won’t get it inless you are …
floating by the gage.

The river will flow much faster at narrow places, and if it is up and over it’s banks will be slower.



All I know is …
that you and Nanci’s MPH was a lot higher than mine on Saturday.

Thanks again for all the advice and encouragement. I had a great time. BTW, I think I have my wife convinced to run pit for me next year for the Adirondack Classic.

another variable
as if there weren’t enough already, is that I believe current flows at different speeds at different depths, at least in the highly irregular rivers I generally paddle. I evidenced this a few weeks back when my local run, usually flowing around 600ish cfs, and enjoyed numerous times the past few months at 1200-2500cfs, was paddled after work by Aaron and I at 3500cfs. The surface was washed out, really too high for an exciting wave train and hole playing run, although what few holes were there were huge, scary huge. The thing I found was the flattish put in area appeared on the surface to be flowing calmly, but dipping a blade accelerated my OC-1 without even stroking. I was turning just by holding the blade in the water, practiced eddy turns and “backwards ferrying” (for lack of a better term) with blade placement. Very interesting run.

Sounds good, but what could be even
better is if she paddled a tandem with you.

Ease her into it, and once she gets hooked there will be no turning back.

You’ll be her hero, she’ll be your hero and life just gets better.

We have several tandems and your more than welcome to try one any time.

It’s even better when your not racing, with her in the bow taking all the fauna and flora pictures and you paddling the boat.

Any one who can paddle a Voyageur for the forty miles the way you did is ready for the ninty miler now!

Congrats on your first race, (long distance to boot) and your win in your class.



eddies, hydraulics, and waves, oh my!
Sorry I don’t have an answer, and I sure don’t want to discourage you, but I get dizzy just thinking of all the variables…

  • slow/fast/faster current on the inside/middle/outside of a bend.
  • eddies where current is either still or moving upstream.
  • waves/boils where water movement is vertical.
  • pulsating waves/eddies, here one minute and gone the next.
  • Surface vs. subsurface current.
  • banjo music, beavers, corps of engineers

    I swear I have seen some good whitewater rivers where there is more surface water moving upstream than downstream because the eddies are larger and more numerous than the drops.

    To simplify, I would get familiar with a stretch of river, find the fastest lines, and take several GPS readings at several levels.

    Good luck!

Re-read a couple of my posts above.
I am asking about the MPH right at the gage.



need area
What you need is the area of cross section of river at the gauge.

if v is speed of the flow, V is volume per second, and A is the area,


You can definitely estimate the area.

If V is in cubic ft per second, A is square feet,

v_mph = V/A/1.5

Good luck