Rapids classes and CFS flows

Can someone tell me how to interpret the CFS river flows? In Texas right now, for example, the South Llano River is at 78 cfs, whereas quite a few are at 0! What can I expect at 78 cfs?

It depends on the kind of river.
In the piedmont around Atlanta, 78 cfs is way too low. I try to wait for 300, though some smaller streams are tolerable at 250.

Down in the bayous, where stream gradient is low and water moves slowly in the streams, 78 cfs might be enough for a good day of paddling.

In GA piedmont streams, 78 cfs is too little to cover the gravel bars and little rock ledges.

In Louisiana, rivers and bayous may have deep channels, with 78 cfs moving slowly past the river gauge.

You have to acquire knowledge about your own region. Study the USGS flows on rivers you know, and extend that to similar rivers you don’t know yet.

River flows
Thanks for the advice. I want to fish while I kayak, so I don’t want too fast of a flow. I’ve got a couple of small rivers I want to get on.

Local knowlege
You need to talk to folks who are familiar with the rivers in question.

All rivers and even sections of the same river are different. Each has it’s own characteristics. 1000 cfs could be a raging flood in one and too low to float in another.

American Whitewater
Go to http://www.americanwhitewater.org/ and search for your local creek. If what you have is worth any notice, there will be explanation.

In very general terms, cfs stands for cubic feet per second, or a measure of water volume flow per second. Obviously, the more flow you have, the higher river level is going to be. As mentioned before, it can drastically change water features on the river. Sometimes, higher flows imply more danger, sometimes higher flows imply nothing at all, it all depends on what the river looks like.

In the most general case, high cfs - fast flows, quick water, any small misstep has potential to create adrenaline rush, or worse.

Good luck

Well Tommy, local knowledge is best, if
it’s available. But I’ve been locating unknown streams in the SE since 1972, using topo maps, with good success and no bad surprises. Now that I have space photography and USGS gauge info, it’s much easier.

Some local knowledge is not knowledge at all. A stream was posted on the AW database with a statement that it was runnable at 100 cfs and above. I ran it at 200 and found it unsatisfactory. I caught it at 280, and it was just fine. I’ve seen many other examples where “knowledge” will give the unwary a bad paddling day.

I must say that I don’t know of a stream in north Georgia that would be too low at 1000 cfs. Even the Flint and the upper Ocmulgee are quite tolerable at that level. Local knowledge, that’s me.

CFS as an indicator
CFS can act as an indicator of a river’s floating ability, but in my experience only if I can compare the floatability of the same river at differing CFS levels over time.

Even then it is just an indicator. Rivers can change dramatically even over just a few years. I can think of one local river that was floatable at 180 CFS just a few years ago, now it is barely floatable at 250 CFS.

Gradient is an interacting factor.
In Georgia, I’ve found that if stream gradient approaches ten feet per mile, one can expect some class 1 and maybe class 2 rapids.

But this is also affected by stream size and flow.

A stream with high gradient, say 60 feet per mile, will need more rain, more water to get those rapids covered. A stream with low gradient might be negotiable throughout spring and early summer.

On stream size, Five Falls on the Chattooga has a local gradient around 40 feet per mile. But it’s a pool/drop stream, so the five “falls” can be run even at low levels. I explored a tiny stream with 60 foot per mile gradient, and while most rapids were just tiny ledges, that stream is almost never runnable. It’s just too tiny and it takes spectacular rain to cover the rocks.

The South Llano River
is one of the few Texas streams with enough water to canoe or kayak during our present drought. It is located near Junction, TX, and is a beautiful small Texas Hill Country river that I have paddled at least 100 times over the past 20+ years. It is runnable even at very low flow levels, although you will have to keep a sharp eye out for channels through many of the rocky riffles. And you should expect to drag your boat through some of them. At 78 cfs all accessible sections from Telegraph Crossing (known locally as “Second Crossing”) to the City Park in Junction are definitely Class I. At levels above 150 cfs some of the rapids will become low Class IIs. At high levels the river can become quite dangerous, so don’t run it when it is flooding; that’s just common sense. There are numerous long flatwater pools that are popular with fishermen. One section of the river flows through the South Llano River State Park. The stretch from the State Park to Flat Rock Crossing ( 6 miles) is a designated Texas Paddling Trail. There are several canoe/kayak liveries in the Junction area, my favorite being South Llano Kayak and Canoe Rental, 325-446-2220. Call them or send me an email for further information. Southwestpaddler.com has great descriptions of the river.

a couple of things…
One useful thing is to compare present flow to the median flow for that stream at this time. Median is represented in the little triangles on the graph, and also is given in the table of flow on each site. It is a fair representation of “normal” flow for any given time of year. So if your stream is normally floatable this time of year and the flow is near that median, it will probably be floatable.

As others have said, you can’t go just by a number. But if you understand the streams of a given region, you can tell a lot from that number. In the Ozarks, basically if the stream is flowing less than 75 cfs it will be very bony, and require some dragging and a lot of scraping bottom in the riffles. If it’s flowing over 100 cfs it’s usually floatable without much dragging and scraping. Of course, th

is varies somewhat with the width of the stream…75 cfs on the upper Jacks Fork, a small stream with narrow riffles, is reasonably floatable, but 75 cfs on the lower Buffalo, a much wider stream, will mean walking quite a few riffles. I float to fish, and don’t mind the dragging and scraping, so I routinely float streams that are flowing less than 30 cfs in the summer, but if you want easy paddling in the Ozarks you need at least 100 cfs.

Local info…
Sometimes good, sometimes not so much. But that’s not always the whole problem. In some cases (around here, at least) there are multiple agencies with river gauges on the same river - that will give entirely different flow numbers. If the local you are talking to is looking at a different source of reference than you are, he may not be much help at all - even if his info is correct.

Talking to local sources is a place to start, but don’t be surprised if it doesn’t work out. The only way to really know is through experience or direct knowledge from someone (with experience) you know you can trust. Once you’ve been on a river at some different flows, you can use that gauge for future reference.

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.