Yeah, technically speaking cfs is not measured. Its calculated from flow meter (speed) and depth measurements taken at intervals across the river. The USGS realtime site’s cfs values are interpolated from the gage height, which is directly measured on a continuous basis. The flow measurements are re-taken a few times a year, and the interpolated values on the website can get back-corrected, sometimes by as much as 25%.
If you go to the individual webpage for a particular USGS gauge that records both stage and discharge (flow), the graph of discharge versus time will periodically have a small red asterisk that indicates a time at which discharge was directly measured. It is usually quite close to the derived flow estimate but often not quite exactly on the line.
you get a feel for this, estimating runnable levels as you look at gauges and paddle streams. Virtual gauges (american ww uses some of these) are streams with no actual gauge but estimates based on another stream’s gauge. If you really want to get a particular stream “dialed in” the way to do it is to paddle it multiple times, note the level, and write down what it was like. Many gauges are just visual, meaning they have lines someone has painted on rocks or bridge abuttment. This info is’nt usually shared online. Sometimes an arrangement is made with a nearby resident to check the level. In some cases folks have installed a camera that feeds online to show what a stream is like.
I get a bit wary when any stream goes below 250 cfs. Often narrow streams/creeks become unrunnable below this level and wider streams may require even more flow to be runnable.
In the old days we listened to a few dial up gauges that had clicks and beeps but mostly we just drove to rivers and looked at them. If the stream we were looking at was too low, we headed to a bigger stream. If it was too high we went to a smaller tributary.
The idea of minimum level, maximum level, and primo can very from one individual to another. I’ve gotten a chuckle or two when someone tells me my beta is off compared to american whitewater. In some cases I’m the guy who set the level (on aw). Gauges that read in cfs get recalibrated and often change after floods which alter the stream bed.
Yesterday I had one individual who wasn’t happy with the level. We paddled the bottom of the cherry into the crupperneck section of the gauley. The cherry was dropping out and there were a lot of rocks to dodge and scrape over. I joke, “I like a little water to go with my rocks”. Too much water can be a problem as well. Locally, I’ve got a short list of streams I’ll do when everything is blown out. Those streams tend to be lower gradient, smaller but wide enough to not catch wood.
Good idea to post a link, for those that are unfamiliar with the USGS sites. Here are 2 I use fairly often. The first is the Chattooga at Burrels Ford where I fly fish for trout, and the Second is the Enoree River where I often paddle.
If gauge height is much above 2.0 we don’t fish as wading is difficult and the water clarity not as good. We know this from experience over time.
Flood stage is about 20.0 gauge height on the Enoree. We have paddled at 900 cfs, I have seen it above 5,000 cfs. It can go up rapidly with heavy rain as can be seen from the graphs of cfs and gauge height. Realize this data is constantly changing with time. So if in the future you read this you may not see the sudden rise I talked about. Today is Sunday 8-22 at 8:30 AM.
As has been said, stream beds do change over time. The accuracy of the discharge level (flow rate) that is derived from the stage (height of the water level at the gauge site) will depend on having an accurate value for the cross sectional area of the stream bed at any and all stages at the site of the gauge.
If a flood scours out the banks of the stream bed the cross sectional area at the gauge site gets bigger. In that event, until the gauge is recalibrated for discharge, the discharge level given for any given stage will be greater than is was before the high water event. Conversely, if a lot of gravel or silt gets deposited at the site of the gauge, the cross sectional area of the stream bed becomes less for any given stage. In this event, the discharge will be overestimated for any given stage until the gauge is recalibrated.
USGS personnel cannot directly measure discharge at every possible stage so even if the gauge was recently calibrated for flow, the discharge reading might not be accurate for stages much higher or lower than the norm.
Most of us who have paddled a given stream or streams over a long period of time have seen a big change in just how much flow relates to a given stage reading over the years. An example is the low water bridge at Ponca on the Buffalo National River. Before there was a USGS gauge at Ponca paddlers used to judge flow based on the “air space” under the center of the bridge, i.e. the vertical distance measured with a yardstick between the underside of the concrete bridge and the top of the water. I am told in the old days of yore, you could paddle the upper Buffalo below Ponca with as much as 40 inches of air space. Over the years gravel got deposited at the bridge which meant that the minimum level for a decent run became fewer and fewer inches of air space. Today it would not even be possible to have 40" of air space under that bridge. The bottom of the stream bed is less than 40 inches below the bottom of the bridge.
Pblanc identified a concern I mentioned about unseen dangers that many tubers may not realize. The explanation are fantastic. It’s interesting to learn that resources are available that substitute for make-shift “home remedies,” and how the absence of the makeshift remedies can be resolved through formal resources.
I know some of you criss over to open water and are intimately familiar with waves. I mention a very detailed description of waves - measurment, impact of fetch, current and tide, boat length . . . Anyone without personal experience would be well served to read it. I included the first page for reference. This was a good primer. Thanks to all posts.
You make a good point. Since I fish and paddle these places often their data has been consistent and reliable over time. The Enroee floods fairly often with the major changes so far being blow downs and changes to the to log jams. Sometimes it floods several times a year. I get to see these after effects. It is hard to see the changes underwater until you paddle at low water. The sandbars tend to stay the same so far. Local knowledge as you demonstrated with your example is important.
I had the job title of Surface Water Hydrologist for many years.
Flow or Discharge (Q) is typically now measured by the USGS on most streams large enough to carry a boat. Streams and rivers are still gauged with a meter once in awhile, and for storm events, but by now most rivers have a hydrograph. There is a relationship for each location on a river between the stage (river height) and the discharge (flow in cfs). So if the stage is 4 feet on the Truckee River at Floristan, then we know that the flow will be about 2,800 cfs. The discharge is the cross sectional area times the velocity.
For most rivers we need around 800-900 cfs or so for a raft or drift boat. Canoes and kayaks can get down rivers with flow down around 400 cfs or a little less. The width of the channel and the gradient have all kinds of effects on minimum navigable flows.
Rivers are typically pool and drop, like the Grand Canyon controlled by geology, or they have consistent gradients like Carson River. It is turbulence that makes rivers so interesting usually created by rocks, and woody debris like logs and downed trees. Bends and twists in the river also create turbulence. Reading water and understanding hydrology are the first skills river runners need to learn.
Gradient is the wild card in figuring out whether a run is suitable. There are plenty of guidebooks now for the popular rivers, but some more obscure ones are much harder to get information on. Gradient typicallly increases with the discharge. Nearly all rivers have maximum navigable flows beyond which they become dangerous. On smaller river, 5,000 cfs might put in the trees, but on big rivers like the Snake it might take 40,000 cfs or more. Most rivers start to feel pretty fast above a gradient above about 25 feet per miles. Most canoes are comfortable somewhere around 12-15 feet per mile or less. Short reaches can have gradients more like 100 feet per mile. On remote rivers look carefully at topo lines to detect any falls, ledges or drops.
Low head dams are a serious hazard in the front country. There are lots of irrigation diversions to watch out for and some need to be portaged.
I urge all river runners to spend some time looking at flow data published by the USFS. The season flow varies a lot on the annual hydrograph. In the West the good flows start with snow melt in March. They often end by June, except for the large rivers which can be run all summer. All watershed respond to precip events. Early river trips after snowmelt require appropriate attire to account for cold water. Always wear PFDs, practice rescues, go in a group and get familiar with the USFS hydrology data.
I like to navigate to my rivers from the main page’s national map because its interesting to see flooding and drought from another POV. For instance, right now Henry is having a big impact. https://waterdata.usgs.gov/nwis/rt
I also compare tide chart info to the attitude of bouys and crab pot floats and to the direction sea grass points. It shows me more than current reports posted on my GPS.
