CFS
CFS or cubic feet per second is meaningless unless one knows the typical depth and width of the river in question. Since rivers always vary in depth and width throughout its length, CFS has little meaning except to those familiar with the section of river in question and how challenging that section is given a particular CFS. In other words it takes observation of CFS over time, for any given section of a river in order to judge difficulty. This is why local paddlers on a particular river are the best source of info concerning CFS and run-ability. I’ve been on sections of the Mississippi River at 1,000,000 CFS that was barely moving and creeks that were ripping at 300.
Your illustrations are good, but if I’m
generally familiar with regional rivers, I don’t need details about width and depth of a particular section to make good use of cfs to tell me if a stream will carry my boat. As a rule of thumb, if a particular stream is running about 300 cfs, it’s good to go.
Of course, some very large streams may need 600 to be tolerable. But in my years of exploring north Georgia streams, under 300 cfs is going to get painful.
In line with your observations, when I paddle LA bayous, 50 cfs may be ample. But there aren’t a lot of shoals and shallow ledges down there.
The USGS site shows the Median flow.
Just Google it: USGS Real Time and the town name. Local knowledge can be wrong.
True, but…
It depends upon how similar the floatable streams are in your area. Like I said above, any of the floatable Ozark streams are somewhat similar in size and width…in other words, on the smallest floatable streams a riffle might be 20 feet wide, on some of the larger ones it might be 50 feet wide, but only the very biggest ones have riffles that are hundreds of feet wide. Those are going to always be floatable anyway. So on any stream that might NOT always be floatable, the figures I gave, while valid only for Ozark streams because they all have similar characteristics, will hold true. On a typical riffle/pool stream with gravel and cobble riffles like nearly all MO Ozark streams, anything over 100 cfs is easily floatable with very little scraping bottom, anything under 75 cfs will mean scraping bottom and possible dragging the boat over some riffles. Furthermore, the jetboats that are one of the banes of canoeing Ozark streams pretty much need a minimum of 200 or so cfs, so if you want to avoid them you have to stick to streams that are flowing less than that, although if it’s a big stream that’s exceptionally low, even 500 cfs might be a little too thin for running a jetboat.
And gauges can be wrong, too.
There are a few Georgia gauges that are invalid for low flows, and a couple that are invalid for all flows. It depends on who maintains and recalibrates the gauges.
Median flow tells one whether a stream might have enough water to paddle, much of the time. Most streams of interest to me are too low nearly all of the time, so I have to watch for spikes in flow that may last only a day or two.
With all due respect::
To answer your question; for any given section of a particular river a CFS number in and of itself means nothing. You cannot know what to expect on a section of the South Llano from knowing the CFS number (in this case 78) alone. Only a paddler who has been tracking CFS on this section for a few years can tell you what it is like at 78 CFS. There is no such thing as a standard CFS number for floating. If possible, find a reliable individual who has been tracking the CFS on the section in question on the USGS website and has experienced that section at 78 CFS. Absolute numbers such as 300 CFS, or any other number are meaningless without prior history.
Without going into a lot of boring calculations just, know that CFS is generally a figure used in the Slope Area Method of measuring flow in open stream channels. It ultimately is a function of Velocity times Area. Velocity is a function of Cross-Sectional Area, Hydraulic Gradient, Wetted Perimeter, and Roughness Coefficient. It is generally calculated along a river from one major gradient change to the next. This is why river with pool drop characteristics, such as the Chattooga exist. A section with 100,000 CFS might be at a virtual standstill, while another with say 100 CFS might be a big rapid.
Stream flows
Stream flows are measured in the US by discharge in cubic feet per second (cfs). Stage is measured by height on a staff gauge in feet. A hydrograph can be developed so that there is a correlation between dishcarge in cfs and height on the gauge. Look at the gauge and know the discharge.
Some rules of thumb apply for navigable rivers. Usually around 250-300 cfs is required to float a loaded canoe and avoid lots of walking and dragging. Gradient and the composition of the bottom have some affect. For rafts good minimum flows are more like 700-800 cfs.
… width also has a huge effect
As others have said, CFS – with nothing more – is meaningless
When I canoed the Dolores’ Slickrock
Canyon, mild gradient, some rocks and easy rapids, the flow was about 700 cfs. Canoes were having no trouble, but the rafts were finding it getting a bit picky.
streamflow
Streamflow is always important and often the only information we can get. To say it is meaningless is obtuse. It is always an advantage to know rivers at different stages, but general rules work almost all the time.
For instance, I check a river flow and trend in the spring and it is consisently above 800 cfs. The trip is a go.
If the flow is “meaningless” how do you make a decision?
You’ve added additional information
"For instance, I check a river flow and trend in the spring and it is consisently above 800 cfs. The trip is a go."
You’ve added two very important additional components: you’re checking it consistently and checking it in the spring. I’d call that a go too.
Ppine, all I’m saying is
that the number alone is meaningless. CFS is significant when one knows the past history of that section and what its characteristics are at different CFS. As Clarion points out, consistency is the key. Let’s say you paddle Creek “A” 5 times a year and each time you checked the CFS and took note of the flow. Was it ripping fast or too low? Then later checking the CFS online will yield some idea of what the flow will be like before planning your trip. Then CFS has some meaning vis-a-vis Creek 'A". Without that history the CFS of a given stream on a given day means nothing to the paddler who is wondering what to expect.
flow
We paddle new rivers all the time. They are remote. There is no one to check with, maybe a remote stilling well with telemetry. That is it.
flow
Compared to no information, flow data are perfect and all you need.
I agree
new rivers never paddled and no local paddlers with info are a “just go and try it”. Flow data is no help without prior history. Some streams have no data. There’s usually some local guage or USGS guage and you can get a good idea from those. Some local might point to a bridge piling and offer that when the water level is up to the third rivet on the bridge the rapids will be big, or river will be too low,or etc. You can usually find out something in the area.
not f&%&% much
78 sounds low or its a tiny river. Think of CFS as = to the amount of water the size of a basketball floating by in 1 second. So 78 would be equal to 78 basketballs to give you a visual reverence.
Not exactly…
You don’t need to know the river and watch it at various flows to get good info from the cfs number alone…IF you have some idea of the river’s characteristics, and can compare those characteristics to those of rivers you do know, or that you can find flow information on.
Like I’ve already pointed out, I can look at gauge info on any river in the Ozarks, and have a pretty good idea of whether it’s floatable or not. I don’t have to have been on that river before, all I need to know is that it’s similar to other streams in the Ozarks in the characteristics of its riffles and rapids. And since nearly all Ozark rivers ARE similar in the characteristics of their riffles and rapids, I can be pretty certain that I need about 100 cfs to comfortably float one, under 75 cfs and it’s going to be bony, over 150 and it will be very pleasant floating with little or no scraping bottom or getting out of the canoe.
Now…can I relate that info to streams outside the Ozarks? Maybe, with a couple other pieces of info. One is median flow, which is also found on the gauge. If the 78 cfs on the river in the original post is somewhere close to the median flow for that date on that river, I can assume that the river is not abnormally low. If it’s not abnormally low at 78 cs, it’s probably not a very wide river, so the riffles shouldn’t be extremely wide and shallow.
The other piece of info I need to get elsewhere, and that’s gradient. Most floatable Ozark streams drop at a rate of 2-10 feet per mile. If a river has less gradient than that, it’s more likely to be floatable at low flows like 78 cfs. If it has much more gradient than that, it’s likely to have rock garden rapids that require quite a bit more volume to be floatable. Anything within that 2-10 fpm range, and it should be similar to the Ozark streams I know.
And…one more thing that will complete my evaluation. Go to
Google Earth, or even just the satellite views on Google Maps, and actually see what the river looks like. I can do that and get a definitive answer on how wide the riffles are, and thus be more sure that my Ozark figures will or will not translate to this stream.
Al
I like your post. Lots of pool and drop rivers in the West, so 250 cfs is a lot more maganeable in a canoe. Many higher gradients also.
It depends on the river
– Last Updated: Sep-16-13 1:37 PM EST –
Knowing the flow without knowing the size of the river is more or less meaningless. To illustrate this... imagine a riverbed 1/4 mile wide vs a creekbed 5 feet wide. 100 cfs would be literally a drop in a bucket in one and probably a raging torrent in the other (depending on gradient).
If you have a full years data then SOMETHING could be gleaned from just the CFS ( say the river varies seasonally from 100-1000 cfs), now you could determine if its at least at high or low flow for the year and make an educated guess as to what to expect, but to really know what a river is doing you need to know 3 things..
Flow
Width
Gradient
If its a whitewater type river then one also needs to know how the gradient is arranged, smoothly or in steps. To take this to extremes... imagine two rivers, both 100 feet per mile gradient, the one with the even gradient is a swift moving river, the other is more or less 2 still pools, with a 100 foot waterfall in the middle. Both might be fun, but it's nice to know what to expect!
If you go to the USGS gauge page
and find the “location” data for a particular gauge, it will tell you the watershed area for that gauge. For a particular area, that is an indication of size.
A map will have appeared, and clicking on the map will allow inspection of the stream gradient on a topo. Also available is a view from space where the more significant rapids are visible, as well as trees in the river.