The Mystery of River Gauges

The Mystery of River Gauges

By Mitch Lloyd

A Carolina Paddler article

-Editor’s note: Mitch Lloyd is the President of the Lumber River Canoe Club and often writes about coastal river hydrology and culture.  Mitch gave his permission to use this recent article.  In a later interview, Mitch offered more insights.  Lara Chapman, a Raleigh based USGS hydrologic technician, has been featured in other Carolina Paddler articles about river gages.  She read Mitch’s article and provided comments.

One further note.  The spelling “Gage” is used by the USGS. Other services and individuals use “Gauge.”

The Mystery of River Gauges by Mitch Lloyd, from the Lumber River Paddling Club Facebook page.

I’ve been watching river gauges for years to learn what the best and worst levels are for paddling my favorite waters. I’ve found a few answers that you might like to know about.

I’ve always looked at the United States Geological Survey (USGS) river gauge website, https://waterdata.usgs.gov/nc/nwis/current/?type=flow&group_key=basin_cd to find my favorite spots.  This can be set up to search by major river basins for your choices.

There is another set of river gauges that are recorded by the National Oceanic & Atmospheric Administration (NOAA) which runs the National Weather Service (NWS). Their gauges can be found at https://water.noaa.gov/ and are different gauges and measuring points from USGS.

A third set of river gauges are administered by the State of North Carolina. They are the Flood Inundation Mapping & Alert Network (FIMAN), found at https://fiman.nc.gov/ .  If you are looking at another state, substitute the state’s two letter abbreviation for nc.

What has always confounded me is that for rivers, the gauges are arbitrary numbers, that for any given water level on a stable river, adjacent gauges will provide vastly different numbers.  Lake levels are all recorded in relation to sea level but not river gauges.

I have learned that each gauge is set to a river gauge datum.  This is magically determined value that represents some stable point found below the bottom of the river, such that if the river were to erode around the gauge the bottom definition of the gauge would still be valid.  So imagine that a gauge was established 50 years ago and the bottom of the gauge was defined as hard rock bottom 6 inches below the sand. Time goes by and the river silts up and now the river “bottom” is 6 feet of mud and sand above the original hard rock. So now, an essentially dry river would still be a reading of 6.5 on the gauge.  The next gauge downstream didn’t silt up due to topography changes and it’s dry river value is 1.5 ft on the gauge.  Each gauge has a determined starting point, or datum, defined by an educated guess of where to start measuring at a stable stratum.

So on my favorite river, the Lumber River, low water conditions or bottom-dragging-dry is 6 feet at one gauge, 7 feet at the next gauge and 1.5 at the next gauge.  You just have to watch your favorite gauges and learn what values work best for you.

Flood levels are even more fun.  Most gauges seem to be set at choke points in river flow, where rising water will show quick rises. The narrow high banked channel of the passageway makes it easier to calculate the flow rate of the moving waters.  If the gauge is located in a narrow ravine, a certain volume of water will cause a noticeable rise in water level.  Downstream that same volume of water flows into a swampy floodplain and spreads out into a wide body of water (just like a lake) and the rise in river level is very much less. Thus, flood levels at each gauge are also unique and arbitrary to that measuring point.

I got interested because I’m currently involved with a disaster response to flooding in Lumberton NC. I kept watching the USGS gauges which showed a river peak at around 18.5 in Lumberton but the National Weather Service kept issuing flood warnings giving values of over 20 feet. I learned that over a 10 mile distance there were 4 different gauges being reported by two different agencies.

So it’s up to you, the avid paddler, to figure it out and chose your own reference.  Happy viewing.

More on interpreting river gauges.

Notes from Carolina Paddler interviews with Mitch Lloyd and Lara Chapman.  Mitch and Lara were interviewed separately but their comments are grouped here according to topic.

Carolina Paddler:  Do you have a good handle on gauge readings for the Lumber River?

Mitch:  Yes on the Lumber and the lower part of the Black River.  I do a lot of paddling on Three Sisters swamp.

CP: Do you have a go-to gauge for the Lumber River?

Mitch: It depends. I compare various ones because of the strange issue of river gauge datum, where they assign the bottom point, there’s so much variability with that. It’s kind of hard to figure out what’s a good level. Basically, it’s a lot of years of seeing what the gauges are and going down and looking at the river and seeing what’s it’s doing.  Going to Lumberton and 17 ft. is dry and going to Boardman, twenty miles downstream and one foot is dry. It’s a matter of having visual data, and experience along with gauge data.

Editor’s note: The discrepancy Mitch was finding was largely based on the different vertical datum used by these services.  FIMAN and NOAA generally use NAVD 88 or the alternate “mean sea level.”.  NAVD 88 stands for the North American Vertical Datum of 1988. This is a geopositioning standard used in the US for mapmaking and surveying.  USGS uses this in some cases but commonly uses local vertical datums for most rivers. Local vertical datums are periodically updated. Most of the USGS gages CCC paddlers rely on for river information work on these assigned river datums, not NAVD88 or mean sea level.  The subject of datum, the assigned arbitrary measurement point, is very complex, hence Mitch’s comments on incorporating personal experience.

Lara Chapman:   First and foremost, reading Mitch’s article was incredibly eye-opening! I never realized how difficult it may be for the public to determine paddling conditions based on gage-height values at various gages. As you and I discussed previously, those river datums are arbitrarily assigned, even though they are additionally tied into NAVD88 vertical datum. A lot of sites around the South Atlantic Water Science Center report both gage height (relative to river datum) as well as stream level (relative to sea level).

As a whole, Mitch is correct! 50 years ago, a 6 ft. gage height may mean you are paddling down an elevated river, whereas today you are dragging your kayak along the streambed. My entire job is to develop the relationship between gage height and discharge. These relationships certainly change over time!

Going off that, discharge might be a more consistent metric to use for determining the best paddling conditions across multiple gages. However, 10,000 CFS could look drastically different from one gage to the next based on channel geometry.

CP:  Do you start with the USGS gage?

Mitch: The USGS was the only gage I was aware of until recently. I keep getting notices on my phone, from a weather service, that the river was topping out at 20ft. and every gauge I looked at mentioned 18 ft.  Where are these readings coming from?  I learned The Weather Channel is using NOAA gauges and I started looking for them.  NOAA has a lot more gauges in towns, it seemed, not out in the woods.  They had three or four gauges in Lumberton that I wasn’t even aware of.

FIMAN, Flood Inundation Mapping & Alert Network, started putting gauges in the gaps that the other services left. Because they monitor flood inundation, they put the gauges where it will do the most good.  They put a gauge on the South River where there had never been one. The South contributes greatly to the flow on the Black River. This gauge was to help with the recent major floods we’ve had.

What I’ve noticed about the Piedmont, the river valleys are more sharply defined, you see a much quicker rise and a much quicker fall than in the coastal plains so there is a big concern over people being injured or having damage.  There are a lot more gauges around major metropolitan areas than out in the country where I live.

CP:  You’ve found gauges are often located at “choke points,” places where the river narrows.

Mitch:  -You see a road, the road is often built at the shortest place to cross a river.  Someone has to maintain the bridge, so they naturally put gages where the road bridges are.  If you put a gage in the middle of Three Sisters and it takes a tremendous amount of water to raise the level.  In a narrower point, flooding can take place much quicker.  Now sometime the very road, the bridge across a river can contribute to being a choke point, a place where water backs up.

Lara: Yes, we do gage at what most would call “choke points”. This is for the ease of making discharge measurements with our equipment. For example, at the pedestrian bridge in Bynum, the Haw is split into numerous channels around islands of rocks and trees. If I were forced to place a stream gage there, it would be incredibly difficult, if not impossible, to get discharge data. That’s why our gage is further downstream, at the “choke” where these multiple channels become one again.

There are a lot of factors that go into determining the best placement for a streamgage, and having a single channel is typically one of those criteria.

CP:  The USGS will consider locating a streamgage at a wider point that doesn’t have obstructions like islands or large rocks provided the river is not braided into many channels.

Mitch: Coastal plains are different. The Black River has one gage on it that’s useful and it’s about 20 miles upriver.  The Black River can be falling but if we have locally heavy rainfalls that will actually raise the lower Black River to a much higher level when the gauge says it is falling.  Like in the Piedmont, you have to know where the gauge is and keep in mind how many feeder streams are coming in, that need to be monitored to really know how much water is coming in.  That takes a lot of talking to people and learning from experience to know when you can paddle.

One final note on gauges.

There’s yet another set of gauges for CCC paddlers to consider, the classic “Paddler’s Gauge.”  When whitewater paddling first became commonplace many of the gauging services now used were not available or easily accessible. Paddlers devised their own system by painting marks on bridge pilings close to river access points.  Several standards were used but the most popular seems to be designating the “O” or zero level as the minimum water level for decent paddling.  Paddling gauges are still in use and provide an additional cross-reference for a paddler’s knowledge of river levels.